Quercetin 101: What You Need to Know (2024)

In this article, we have covered quercetin with more than 100 studies and references that represent the best of science-backed information for quercetin.

Contents

1. What is Quercetin?

2. Benefits of Quercetin

3. Quercetin Dosage

4. Food Sources of Quercetin

5. Quercetin Supplement Types

6. Quercetin Comparison

7. Quercetin and Other Supplements (Zinc, Bromelain, Vitamin C)

8. What Not To Take With Quercetin

9. Quercetin Side Effects

10. Online Shopping Guide

1. What is Quercetin?

Quercetin is a pigment that belongs to a group of plant compounds called flavonoids (also known as bioflavonoids). Flavonoids are naturally present in fruits, grains, teas, and wine. Researchers have linked flavonoids to a variety of benefits, including a reduced risk of heart disease, cancer, and degenerative brain disorders.

When looking for and using quercetin supplements, it’s important to maintain a nutritious, balanced diet with fiber and healthy fats in it, as research shows that accumulation of quercetin in the body is dependent on “factors such as glucose moieties, solubility, human factor, vitamin C status and food matrix.”

Specifically, research also says that quercetin supplementation is best achieved when consumed with a fat-enriched diet.

a. What are other names for quercetin?

Quercetin goes by a number of different names. You might see quercetin supplements labeled as bioflavonoid concentrate, bioflavonoid extract, or citrus bioflavonoid, for example.

b. Where does quercetin come from?

Most supplement companies use various plant or vegetable-based sources of quercetin. Check the label.

c. Can you get citrus-free quercetin?

Some quercetin supplements are specifically marketed as citrus-free, making them ideal for those with citrus sensitivities. Read the label or official product website for your supplement to be sure that it is actually citrus-free.

d. Is quercetin non-GMO?

Quercetin supplements are generally labeled non-GMO. Check the label to verify your quercetin supplement is not made from genetically modified ingredients.

2. Benefits of Quercetin Supplements

As the health benefits of the supplement become more widely known, the market has grown rapidly (R). According to market research, quercetin market was worth $261.12 million in 2020 and is expected to reach $406.58 million in 2027.

Quercetin has been linked to the following health benefits:

• Possessing anti-inflammatory, antioxidant properties that can help combat certain diseases. One study found that 500 mg of quercetin a day could improve symptoms of rheumatoid arthritis.
• Helping prevent cancer cell growth from breast, colon, prostate, endometrial and lung.
• Counteracting allergic reactions and helping with immune responses.
• Helping to prevent osteoporosis
• Addressing cognitive decline, including Alzheimer’s and dementia.
• Assisting in control of blood pressure. A study in 2002 found that quercetin can have vasodilator effects.
• Potentially helping to lower blood sugar levels. One study found that dosages of 500 mg per day “significantly reduced” fasting glucose levels.

a. Benefits of Quercetin for Allergies

Quercetin is a natural antihistamine and anti-inflammatory plant pigment that boosts your immune system and may work to control viral replication, according to some research. It helps to couteract allergic reactions and helping with immune responses. It allows zinc to exert its proven antiviral properties.

i. How Does Quercetin Manage Allergy Symptoms?

Studies have shown that quercetin stabilizes mast cells that release histamine. Histamine is the principal mediator of reactions to pollen and other allergies. This makes quercetin a natural antihistamine. Many people use quercetin to treat symptoms of hay fever, including runny nose, watering eyes, and itching.

ii. Quercetin Benefits for Inflammation

Flavonoids (aka bioflavonoids) including quercetin, are key anti-inflammatories that act as antioxidants, which means they fight the natural process of oxidation that takes place over time as we age. Quercetin can help stop damaging particles in the body known as free radicals, which negatively impact how cells work. It can also reduce expression of inflammatory genes such as interleukin.

Research now shows that inflammation is the root of most diseases, including heart disease, cancer, cognitive decline, some mental disorders, and autoimmune disorders. At this time, practitioners and patients report using quercetin to effectively fight a variety of conditions related to inflammation, including:

• “Hardening of the arteries” (atherosclerosis)
• High cholesterol
• Heart disease and circulation problems
• Insulin resistance and diabetes
• Eye-related disorders, including cataracts
• Allergies, asthma, and hay fever
• Stomach ulcers
• Cognitive impairment
• Gout
• Cancer
• Viral infections
• Chronic fatigue syndrome
• Inflammation of the prostate, bladder, and ovaries
• Chronic infections of the prostate
• Skin disorders, including dermatitis and hives

iii. Quercetin Benefits for Hives

Hives, also known as urticaria, are itchy, generalised, raised red discolouration that are found on the skin. They are usually red, pink, or flesh-colored, and sometimes they sting or hurt. In most cases, hives are caused by an allergic reaction to a medication or food or a reaction to an irritant in the environment.

In many cases, hives are an acute (temporary) problem that may be alleviated with allergy medications. Most rashes go away on their own. As with any allergic reaction, if hives are accompanied by difficulty breathing or swallowing, this can be a sign of a much more severe and serious anaphylactic reaction. This requires immediate emergency medical attention to treat. Please see your doctor and go to the nearest 24 hr emergency clinic or hospital immediately.

However, chronic hives can occur over and over. In some cases, the red lesions can result from an allergic reaction to the food you eat.

Dr Deborah Gordon recommends trying a food elimination diet to pinpoint the source of the problem. Thus, if you get rid of the source, you won’t have to worry about treating the hives.

Antihistamine drugs also stop the release of the histamines that cause the hives in an allergic reaction. However, people who suffer from chronic hives don’t usually want to take medication for months or years if they don’t have to.

For this reason, some allergy sufferers turn to natural alternatives such as quercetin; that works by stabilizing the membranes of cells that release histamine. This often helps brings inflammatory and allergic reactions under control. You can buy quercetin products in natural foods stores or Amazon. The best form is a coated 400 mg tablet, taken twice a day between meals; as recommended by Dr Andrew Weil. It can take six to eight weeks of continuous use for quercetin to build up to therapeutic levels in the body.

iv. Does Quercetin Help Allergies? : Research Studies

Quercetin has been shown to reduce inflammation in multiple studies. In this 8 week study involving 50 women with rheumatoid arthritis, participants took 500mg of quercetin per day or a placebo. The quercetin group reported less early morning stiffness, morning pain, and after-activity pain. 

Early studies on quercetin and inflammation are promising, although more large scale human studies need to be performed to verify these benefits.

There’s evidence that quercetin reduces allergy symptoms. Researchers believe quercetin’s anti-inflammatory effects may relieve allergy symptoms.

This study published in Molecules in 2016, for example, found that quercetin could be effective for treating the anaphylactic (allergic) reaction in someone with peanut allergies. A similar study from 2006 concluded that quercetin was a “safe, natural therapy that may be used as primary therapy or in conjunction with conventional methods” for blocking allergies. 

It’s possible that quercetin has the same anti-allergy effect in humans, although more research needs to be done.

According to the research, quercetin has been shown to help fight obesity, Type 2 diabetes, circulatory dysfunction, chronic inflammation and mood disorders. It has even been found to help lower blood pressure. Researchers have found that quercetin can trigger tumor regression and begin the process of apoptosis. This is programmed cell death, without which cells can grow uninterrupted and develop into cancerous growths.

b. Quercetin Benefits for Cancer

There’s also some evidence that quercetin has cancer-fighting properties. Researchers believe quercetin can fight cancer cells with its powerful antioxidant properties. This includes helping to prevent cancer cell growth from the breasts, colon, prostate, endometrial and lungs.

i. Does Quercetin Fight Cancer?: Research Studies

A Boston University School of Medicine study published in the Journal of Biological Regulators and Homeostatic Agents shows a link between a nutrient-dense diet rich in quercetin and a lowered risk of cancer. Quercetin seems to have potential chemo-preventive activity and might have a unique antiproliferative effect on cancer cells, making it an effective addition to any natural treatment approach. Research shows that this may result from the modulation of either EGFR or estrogen-receptor pathways. Recent studies have also found that quercetin can help stop the processes involved in cell proliferation and mutation, the growth of tumors, and symptoms related to typical cancer treatments, such as radiation or chemotherapy.

Animal studies suggest that quercetin effectively suppresses breast tumor growth, and further studies demonstrated that quercetin enhanced the therapeutic effects of Adriamycin® while suppressing its toxic effects on normal cells.

An animal study with rats conducted in 2009 demonstrated that quercetin’s antioxidant effects may act to prevent hepatocellular carcinoma (HCC). 

Another animal study showed that a combination of pterostilbene (derivative of resveratrol) and quercetin significantly inhibited the growth of liver metastases from malignant melanoma and also increased the lifespan of the subject mice.

In 2015, researchers reviewed available test tube and animal studies on quercetin and prostate cancer. After reviewing available evidence, researchers found that quercetin suppressed cell growth and induced cell death in prostate cancer cells.

This study published in 2017 in Oncology Reports took things a step further, finding that quercetin induced cancer cell death in nine types of cancer, including prostate cancer, colon cancer, and breast caner.

In another 2017 study, researchers gave quercetin to mice with tumors. Researchers found that mice in the quercetin-treated group showed delayed tumor growth, no significant changes in daily behavior, significantly better survival ratings, and increased rates of cell death.

Quercetin may also target bladder cancer. In 2016, researchers published a landmark study in the American Journal of Cancer Research. Researchers analyzed quercetin’s effect on cancer cells in a test tube. They concluded, “We are the first to show that quercetin displays potent inhibition on bladder cancer cells via activation of AMPK pathway.”

Early research on the cancer-fighting benefits of quercetin is promising, although more research needs to be performed to verify these effects in humans.

c. Quercetin Benefits for Brain

There’s mounting evidence that quercetin offers neuroprotective benefits due to its ability to defend the brain against oxidation and inflammation, leading to potentially less risk for cognitive conditions such as Alzheimer’s disease and dementia. Other evidence suggests quercetin lowers your risk of chronic brain disorders, including Alzheimer’s and dementia.

i. Does Quercetin Reduce the Risk of Alzheimer’s? : Research Studies

In this study published in Neuropharmacology in 2015, researchers gave quercetin to mice with Alzheimer’s, injecting them with quercetin every two days for three months. By the end of the study, the injections had reversed several markers of Alzheimer’s, and the mice performed much better on learning tests.

In a separate study published in 2018, researchers gave mice with Alzheimer’s a quercetin-rich diet. Researchers found the diet improved brain function in mice with early-middle stage Alzheimer’s, although it had no significant effect on middle-late stage Alzheimer’s.

A 2018 study concluded that “findings suggest a possible new protective role for dietary flavonoids on Alzheimer’s disease (AD).” The study found that quercetin helps ameliorate cognitive dysfunction and may help reduce destruction of neurons.

You may have heard that coffee is linked to a reduced risk of Alzheimer’s. There’s certainly some research to back that claim up. However, recent research has suggested that quercetin (not caffeine) is the primary compound in coffee responsible for protective effects against Alzheimer’s.

d. Quercetin Benefits for Blood Pressure

Some people use quercetin to reduce blood pressure and improve other measurements of cardiovascular health. High blood pressure raises your risk of heart disease, which is the leading cause of death in the United States. Approximately 1 in 3 American adults have high blood pressure.

Research suggests that quercetin reduces blood pressure levels. In this study published in 2002, researchers found that quercetin exhibited vasodilator effects, widening blood vessels and reducing blood pressure. 

Multiple studies on humans have suggested similar benefits. Researchers reviewed nine human studies involving 580 people. After reviewing available evidence, researchers found that taking more than 500 mg of quercetin supplement per day reduced systolic and diastolic blood pressure by 5.8mm Hg and 2.6 mm Hg, respectively. In other words, quercetin was shown to significantly improve blood pressure readings.

i. Quercetin Heart Benefits

While it may not directly improve your love life, quercetin might be just what the doctor ordered in terms of a healthy heart and vascular system. One of the most prevalent flavonoids in the food we eat, this polyphenol is also one of the most studied, including on our cardiovascular system.

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Eating lots of deeply colored fruits and veggies that contain flavonoids is linked to a lower risk of cardiovascular disease, and even death, in older adults. Quercetin has also been connected to reduced risk for type 2 diabetes and obesity, which have many of the same risk factors as heart disease.

In those already experiencing some level of cardiovascular disease with high blood pressure and cholesterol levels, studies show quercetin can significantly reduce these two markers, demonstrating its therapeutic uses. (Reference)

Another clinical trial conducted in 2005 at the University of Utah with 41 pre-hypertensive or stage 1 hypertensive subjects suggest that quercetin supplementation may help control high blood pressure. Pre-hypertension was defined as having a blood pressure of 120–139/80–89, while those with 140–159/90–99 were categorized with stage 1 hypertension (as per clinically accepted guidelines at the time). Potential study candidates were carefully screened for chronic disease or other possible interfering factors (i.e., blood pressure medication or other supplementation), and only non-smokers who were not pregnant or nursing were allowed to complete the study. Participants were randomly divided into two groups, and for four weeks, took 365 mg of quercetin twice a day (based on in vivo animal studies) or a placebo. Results demonstrated that quercetin supplements reduced both systolic and diastolic pressures by five to seven points for those suffering from hypertension, regardless of age, gender, or weight, with no changes in those with borderline normal blood pressures. Given the statistical data, these reductions are considered significant and are associated with decreased risks of mortality from stroke and heart disease. (Reference)

ii. Quercetin for Cardiovascular Support

Results of a clinical study involving 12 healthy men of normal weights and blood pressures suggest that oral ingestion of 200 mg of quercetin could improve the functioning of arteries by significantly increasing production of vasodilating nitric oxide molecules while reducing levels of endothelin-1, a vasoconstricting peptide. (Reference, Loke et al)

Another clinical study demonstrated that quercetin’s antioxidant properties may also reduce the cardiovascular risks of atherosclerosis that patients in end-stage kidney failure typically suffer from. Thirty-eight patients undergoing regular hemodialysis for renal failure were divided into two groups: 26 given 100 ml of concentrated red grape juice (RGJ) a day over two weeks, and the remaining 12 as a control group not taking the RGJ supplement. A group of 15 healthy volunteers were also included in the study. (Reference)

Quercetin is the main polyphenol constituent of RGJ (red grape juice), and results of the study were substantial increases in HDL (the “good” cholesterol) as well as clinically significant decreases in LDL (the “bad” cholesterol), both of which lower the risks of atherosclerosis. Interestingly, even though not show in most previous studies, this study demonstrated the same effects on cholesterol levels in the healthy test subjects as in the unhealthy patients. One explanation may be the substantially higher amount of RGJ supplement given in this study (644 mg/day vs. 432 mg/day). (Reference)

e. Quercetin for Blood Sugar

Some diabetics take quercetin to help manage blood sugar. 

i. Does Quercetin Help with Diabetes? : Research Studies

There’s some evidence that quercetin helps with diabetes, helping your body manage blood sugar levels during fasting. However, more research needs to be done to verify these benefits.

This study from 2019 found that taking 500mg of quercetin per day significantly reduced fasting plasma glucose (FPG) levels, suggesting that quercetin helped control fasting blood sugar. This study in 2019 found similar results, finding that quercetin lowered serum glucose levels at doses of 10, 25, and 50mg per kg.

f. Quercetin May Be Anti Aging

Some people take quercetin for its anti-aging effects. Antioxidants and anti-inflammatories – including quercetin – seem to have powerful anti-aging benefits. One study on young mice found that quercetin reduced mortality and extended lifespan. Another study from 2017 on human cells in test tubes found that quercetin reduced aging markers, with researchers concluding there was “anti-aging potential” for quercetin.

i. Does Quercetin Work for Anti-aging? : Research Studies

A: Several studies have examined the effect of quercetin on aging. Quercetin is rich with anti-inflammatories and antioxidants, which could make it a powerful anti-aging compound. That said, quercetin reduces glutathione and inhibits sirtuin-6 and NRF-2. All of these might hurt your longevity (see this clip, 2:30).

g. Benefits of Quercetin for Weight Loss

Some people believe that consuming quercetin, whether in foods or as a natural supplement, can help promote weight loss.

However, there have been mixed reviews and conclusions on the effects of quercetin for weight loss and its effect on the expression of certain genes. 

These inconsistent findings from the studies maybe are due to the difference in the range of fitness of the subjects, differences in research design, different methods in the preparation of quercetin extract, etc. In order to assist you in doing your own research, let's dive into some relevant studies and reviews.

Matthew Solan, author of Doctor’s Detox Diet reports that quercetin can mimic the effect of calorie restriction. However, it shows to have a greater effect on weight loss when combined with other fat-fighting phytochemicals such as resveratrol and genistein.

Another interesting side effect of quercetin, as reported by MuscularDevelopment.com, is that quercetin may offer some of the same psychostimulant properties of caffeine.

Therefore, consuming it may help people increase their activity throughout the day and delay fatigue. Plus, this can lead to expending more energy during workouts, thus burning more calories.

Dr Josh Axe also adds that some athletic supplements contain quercetin because some people believe that is increases endurance and athletic performance.

In a biochemical study, quercetin exerts anti-fat cell formations at the molecular level.

In another study, a combination of Quercetin along with Resveratrol inhibited the formation of mature fat cells, which subsequently resulted in weight loss. Quercetin increases the secretion of the enzyme, triglyceride lipase that breaks down fat, thus aiding in weight loss.

It was also discovered that consumption of 500 mg of quercetin twice a day increased the VO2 max (the maximum amount of oxygen that is utilised by an individual) in untrained individuals as a result of an increase in mitochondrial biogenesis.

This enhancement in fitness without exercise training can be used in disease prevention such as obesity.

In two animal studies, supplementation with quercetin brought about a significant reduction in obesity related gene expression.

i. Can Quercetin Help You Recover After Exercise? : Research Studies

Quercetin is added to some athletic supplements because it’s believed to help increase athletic performance and endurance, likely because of its positive effects on blood flow. Researchers from the School of Applied Physiology at the Georgia Institute of Technology found that, on average, “quercetin provides a statistically significant benefit in human endurance exercise capacity (VO2 max) and endurance exercise performance.”

Other studies show that quercetin helps increase immune function and prevents susceptibility to illnesses that can occur when someone trains intensely and experiences exhaustion. One study found evidence that taking a dosage of 500 mg of quercetin twice daily helped protect cyclers from exercise-induced respiratory infections.

Because it can boost energy, quercetin could affect sleep patterns. One study found evidence that it may alter the sleep-wake cycle partly through activation of GABA receptors. However, insomnia is generally not believed to be a common side effect of taking quercetin.

Some studies show that quercetin boosts recovery after exercise. In some small studies, athletes taking quercetin after exercise had faster recovery than a control group. Researchers believe quercetin reduces oxidative stress and post-exercise inflammation, helping your body recover.

This 2011 study reviewed 11 quercetin studies involving 254 human subjects and found that quercetin provided a statistically significant benefit in human endurance exercise capacity (VO2 max) and endurance exercise performance, although the effect “is between trivial and small”.

h. Quercetin and COVID-19

In treating COVID-19, quercetin may also lower inflammation, help clear mucus, prevent ventilator-induced damage and support immunity.

Some suggest that quercetin could treat the COVID-19 coronavirus. Quercetin was shown to be effective against SARS, the Ebola virus, and the Zika virus. In 2003, Quercetin was initially found to provide broad-spectrum protection against SARS coronavirus in the aftermath of the SARS epidemic that broke out across 26 countries. Now, some doctors are advocating its use against SARS-CoV-2, in combination with vitamin C, noting that the two have synergistic effects.

An animal study published in the Nature (Aug 2012), concluded that quercetin has anti-oxidant, anti-inflammatory and anti-fibrotic (anti-scarring) properties. Another study (JCI May 2012) on rutin (quercetin molecule bound to a sugar molecule called rutinose), commonly found in fruits and vegetables and sold over the counter as a dietary supplement, has been shown to inhibit the formation of blood clots in an animal model of thrombosis.

As of October 2023 there have been more than 10 published studies of quercetin against COVID-19. For the list of studies, check out c19quercetin.com.

i. Quercetin Benefits for Asthma

Quercetin may help asthma by relaxing smooth muscles lining the airway, improving airway flow.

j. Quercetin Benefits for Bones

Quercetin may help to prevent osteoporosis.

One study found that 500 mg of quercetin a day could improve symptoms of rheumatoid arthritis.

k. Quercetin Benefits for Immune System

Quercetin’s antiviral properties have been attributed to three main mechanisms of action:

- Inhibiting the virus’ ability to infect cells

- Inhibiting replication of already infected cells

- Reducing infected cells’ resistance to treatment with antiviral medication

For example, research funded by the U.S. Defense Advanced Research Projects Agency (DARPA), published in 2008, found it lowers your risk of viral illness such as influenza and boosts mental performance following extreme physical stress, which might otherwise undermine your immune function and render you more susceptible to infections.

Here, cyclists who received a daily dose of 1,000 mg of quercetin in combination with vitamin C (which enhances plasma quercetin levels) and niacin (to improve absorption) for five weeks were significantly less likely to contract a viral illness after bicycling three hours a day for three consecutive days, compared to untreated controls. While 45% of the placebo group got sick, only 5% of the treatment group did.

i. Is Quercetin Good For Herpes?

In the lab, studies have demonstrated quercetin’s antiviral effect against both herpes simplex virus type 1 (HSV-1) and 2 (HSV-2). Quercetin extracted from apples inhibited both HSV-1 and HSV-2 replication by greater than 50%.

The in vitro antiviral effects of quercetin on herpesviruses (HSV-1, 2) and adenoviruses (ADV-3,−8,−11) suggest inhibition of early stage viral replication in a dose dependent manner (for HSV-1 100% inhibition at 60 mg/L) (PubMed, PubMed) as well as inhibition of viral DNA and RNA polymerase (PubMed, PubMed, PubMed).

ii. Quercetin for Canker Sores

Canker sores, also called aphthous ulcers, are small, shallow lesions that develop on the soft tissues in your mouth or at the base of your gums. Unlike cold sores, canker sores don't occur on the surface of your lips and they aren't contagious. They can be painful, however, and can make eating and talking difficult.

A clinical trial was conducted to assess quercetin’s effects on canker sores, and the results were promising.

In the forty male patients randomly assigned to apply quercetin topical cream 2-3 times a day to canker sores, 90% had completely healed ulcers in 4-7 days. By 10 days, all 20 test subjects had resolved sores. Compared to the control group treated with a medicated mouthwash, the quercetin cream group healed faster and found the cream easy to use.

iii. Quercetin Works Against Many Common Viruses

Quercetin lowers your risk of secondary bacterial infections by attenuating oxidative damage, which is actually the primary cause of influenza-related deaths.

Importantly, quercetin increases mitochondrial biogenesis in skeletal muscle, which suggests part of its antiviral effects are due to enhanced mitochondrial antiviral signaling. Quercetin also works against other viruses, as demonstrated in the following studies:

- A 1985 study found quercetin inhibits infectivity and replication of herpes simplex virus type 1, polio-virus type 1, parainfluenza virus type 3 and respiratory syncytial virus (RSV).

- A 2016 animal study found quercetin inhibited mouse dengue virus and hepatitis virus.

- Other studies have confirmed quercetin’s power to inhibit both hepatitis B23 and C24 infection.

- A March 2020 study found quercetin provides “comprehensive protection” against Streptococcus pneumoniae infection, both in vitro and in vivo, primarily by neutralizing pneumolysin (PLY), one of the toxins released from pneumococci that encourages S. pneumoniae infection to blossom in the first place. Streptococcus pneumoniae is responsible not only for pneumonia, but can also be involved in some ear and sinus infections, meningitis and certain blood infections.

As reported by the authors of this study:

“The results indicated that quercetin significantly reduced PLY-induced hemolytic activity and cytotoxicity via repressing the formation of oligomers.

In addition, treatment with quercetin can reduce PLY-mediated cell injury, improve the survival rate of mice infected with a lethal dose of S. pneumoniae, alleviate the pathological damage of lung tissue and inhibit the release of cytokines (IL-1β and TNF-α) in bronchoalveolar lavage fluid.

Considering the importance of these events in antimicrobial resistant S. pneumoniae pathogenesis, our results indicated that quercetin may be a novel potential drug candidate for the treatment of clinical pneumococcal infections.”

l. Quercetin Benefits for Skin

Capable of blocking “mast cells,” which are immune cells critical in triggering allergic reactions, inflammatory disease, and autoimmune disease, quercetin can help protect skin from the effects of disorders like dermatitis and photosensitivity. Flavonoids like quercetin block the release of many proinflammatory cytokines, such as IL-8 and TNF, which helps stop symptoms related to skin inflammation, even in people who don’t find relief from other conventional treatments or prescriptions.

Studies have found that quercetin has antioxidant and anti-inflammatory effects that help fight allergic and inflammatory diseases as well as some prescriptions when taken in oral supplement form. For example, some people take quercetin for eczema since it can inhibit the secretion of histamine and pro-inflammatory markers.

m. Quercetin Benefits for Liver

Recent research has shown that quercetin has protective effects when administered to rats with ethanol-induced acute liver injury. Researchers concluded that “quercetin, by multiple mechanisms interplay, demonstrates hepatoprotective effect on liver-injury induced by alcohol by increasing ethanol metabolizing enzyme activities, increasing antioxidant system activities against oxidative stress, and lowering the expressions of proinflammatory cytokines.”

A 2017 study found evidence indicating that quercetin attenuates liver inflammation and fibrosis in mice through inhibiting macrophages infiltration. Researchers believe it “holds promise as a potential therapeutic agent for human fibrotic liver disease,” a condition triggered by liver injury and inflammation.

Related: NOW Testing of Quercetin in Brands Sold on Amazon Reveals “Deceptive Labeling Tricks”

n. Quercetin Helps to Keep Gray Hair Away

Research suggests the use of onion juice as a natural, topical treatment to support hair regrowth, and one reason could be due to quercetin. In addition to helping reduce inflammation, quercetin enhances the expression of the antioxidant enzyme catalase in your scalp, which helps break down hydrogen peroxide that contributes to cell damage and thinning hair. (R)

By breaking down hydrogen peroxide, catalase — and by extension, quercetin — may also be beneficial for slowing down hair graying. Hydrogen peroxide is a well-known tool for bleaching your hair, but many people aren't aware that your hair cells make hydrogen peroxide, too. 

As you age, the amount produced increases, which researchers believe ultimately bleaches out your hair pigment, turning your hair gray and then white. Quercetin is well known for maintaining oxidative balance in the body, both by inhibiting and alleviating oxidative stress. This is another way that it may help keep gray hair away. Oxidative stress is the state in which your body’s free radicals (from pollution, poor diet, stress) outnumber your antioxidant defenses (from healthy diet). Graying hair may be an indicator of oxidative stress-induced damage. 

Research has also shown that people with premature graying had a higher level of pro-oxidants and lower levels of antioxidants than those with normal hair. As researchers noted in the FASEB Journal, compounds that slow oxidative stress, such as quercetin, may be useful for slowing down or stopping hair graying. 

3. Quercetin Dosage

Most quercetin studies use a dosage of around 500mg per day, although some studies use a dosage of 500mg taken twice per day.

Most supplements have a similar dosage, offering 500mg to 1,200mg of quercetin per serving.

In some studies, researchers have given participants up to 5,000mg of quercetin per day with no reported side effects.

Quercetin has poor bioavailability. You might take a 1,200mg quercetin supplement, but your body only absorbs a small percentage of it. That’s why many quercetin supplements contain vitamin C or bromelain, as some evidence suggests they boost absorption.

a. Quercetin Dosage for Hay Fever

Experts recommend taking 400mg of quercetin twice a day between meals for hay fever. Also consult your doctor to help provide a more comprehensive and effective treatment for hay fever.


b. Quercetin Dosage for Covid 19

The FLCCC I-MASK+ protocol recommends 250 mg/day for COVID-19 prevention and 250 mg twice a day for treatment. 

c. How Much Quercetin Should You Take Per Day?

A: Researchers typically use a dose of 500 mg of quercetin per day in supplement form. Most quercetin supplements contain between 500 and 1200 mg of quercetin, although this can vary. Check out the nutritional label for your quercetin supplement to be sure.

4. Food Sources of Quercetin

You may already get a small amount of quercetin in your normal diet: quercetin is found in onions, kale, tomatoes, broccoli, asparagus, berries, red wine, citrus fruits, cherries, and tea. Many people also take quercetin in supplement form.

In fact, it’s estimated that the average person consumes about 10 to 100 mg of quercetin per day through a normal diet. Quercetin is one of the most abundant flavonoids in the human diet.

a. What are the best foods for quercetin?

Arguably, red onions are some of the most potent sources of quercetin; one study found that absorption of onion-derived quercetin was 52% compared to only 24% in supplement form. In fact, onions can give you three times the amount of quercetin than you’d get from apples. For an especially concentrated source, consider onion skins (R). They may have 77 times more quercetin than the flesh.

Other food sources that are high in quercetin include:

• Organically grown apples, grapes and tomatoes
• Dark cherries and dark berries like blueberries, blackberries and bilberries (eaten in moderation)
• Cocoa, unsweetened
• Hot green chili peppers
• Lingonberries

Other excellent foods for Quercetin include capers, peppers (yellow and green), onions (red and white), shallots, asparagus, cherries, broccoli, kale, red leaf lettuce, cranberries, raspberries, green tea, black tea, and coffee.

b. Do you get enough quercetin in your daily diet?

A: The average person gets 10 to 100 mg of quercetin per day through normal dietary sources. This can vary greatly. Carefully track your diet over an extended period to figure out if you have a dietary deficiency of quercetin.

5. Quercetin Supplement Types

While the best way to get quercetin is through your diet, it also is available in supplement form, as pills or capsules. 

a. What’s better – quercetin tablets, capsules, powders, or liquid?

Quercetin liquid claims to be more bioavailable than other sources of quercetin. You take the liquid sublingually (underneath your tongue). However, limited research shows quercetin capsules or powders are more effective, and all forms of quercetin are absorbed poorly by the body.

Related: Best Organic Quercetin Supplements

6. Quercetin Comparison

a. Quercetin Dihydrate vs Quercetin

The key difference between quercetin and quercetin dihydrate is that quercetin is a plant flavonoid, whereas quercetin dihydrate is a synthetic chemical compound. Moreover, quercetin is a yellow crystalline powder, whereas quercetin dihydrate is a bright yellow colour powder.

Quercetin Dihydrate has the highest bioavailability when it comes to quercetin supplements.

According to Examine.com, this form is preferable to glycosides, aglycone and rutinoside versions.

b. Quercetin phytosome vs Quercetin dihydrate

On its own, quercetin has a low bioavailability, which means your body absorbs it poorly. That’s why the supplements may include other compounds, such as vitamin C or digestive enzymes like bromelain, as they may increase absorption.

Additionally, some research indicates that quercetin has a synergistic effect when combined with other flavonoid supplements, such as resveratrol, genistein, and catechins.

A new food-grade lecithin-based form of quercetin, Quercetin Phytosome, was developed and validated in healthy volunteers. Quercetin Phytosome overcomes the low bioavailability hurdle of quercetin and should help to fulfill the great health benefit potential of this flavonoid in the diet and as food supplements.

Most in vitro studies used a form of quercetin called 'quercetin aglycone'. However, this particular form of quercetin is never found in the blood, even after ingested, as it it gets changed in the liver.

According to Examine, the quercetin dihydrate form has the apparent best bioavailability followed by glycosides, aglycone, and finally rutinoside.

Quercetin Phytosome Supplement > MoxyVites Whole Food Quercetin 500mg with Bromelain Supplement

c. Rutin vs Quercetin

Often referred to as “Vitamin P”, Rutin is comprised of a quercetin molecule bound to a sugar molecule called rutinose. It is chemically very similar to Quercetin and the two compounds are invariably found together in fruits and vegetables.

An animal study published in the Nature (Aug 2012), concluded that rutin exerts stronger protection against liver damage but has weaker antioxidant and anti-inflammatory activities and antifibrotic (anti-scarring) potential than quercetin

Another study (JCI May 2012) on rutin, commonly found in fruits and vegetables and sold over the counter as a dietary supplement, has been shown to inhibit the formation of blood clots in an animal model of thrombosis.

d. Rutin vs Glycosidic Quercetin

Quercetin is found in two forms, including in rutin or glycosidic form. Quercetin glucosides, such as isoquercitrin and isoquercetin, seem to be much more bioavailable and more quickly absorbed than quercetin aglycone or quercetin glycosides, such as rutin (quercetin rutinoside).

e. Quercetin Anhydrous vs Dihydrate

Quercetin supplements come in different forms. Most of the quercetin ingredients on the market are in the quercetin dihydrate form. Quercetin anhydrous and dihydrate differ in the amount of water they contain. Quercetin anhydrous contains only 1% to 4% moisture and the sugar molecules that are attached to quercetin in its natural form have been extracted. This translates into 13% more quercetin per gram for quercetin anhydrous vs quercetin dihydrate. For formula manufacturers, this means there is substantially less bulk per capsule or tablet.

f. Quercetin vs Hydroxychloroquine

Some physicians are recommending quercetin to reduce viral illnesses because quercetin acts as a zinc ionophore to improve zinc uptake into cells. 

It is much less potent than HCQ (hydroxychloroquine) as a zinc transporter, and it does not reach high concentrations in lung cells that HCQ does. Quercetin may help reduce risk of viral illness if you are basically healthy. But it is not potent enough to replace HCQ for treatment of COVID once you have symptoms, and it does not adequately get into lung tissue unless you take massive doses (3-5 grams a day), which cause significant GI (gastrointestinal) side effects such as diarrhea.

i. What is Hydroxychloroquine?

 

Hydroxychloroquine, a less toxic derivative of Chloroquine is a widely used medication by people with lupus or arthritis. It was developed in the 1950s from chloroquine, an old anti-malarial drug, is registered in around 60 countries under trade names such as Plaquenil, Quensyl and Plaquinol.

Hydroxychloroquine is among the handful of COVID-19 treatments that are being studied as potential candidates that might influence the outcome in the management of COVID-19. According to a real time meta-analysis of more than 200 studies, early treatment is most successful.

Hydroxychloroquine, Quercetin and EGCG (EpiGalloCatechin Gallate) are all zinc ionophores. Meaning they all transport zinc into the cells. 

f. Quercetin vs Hydroxychloroquine vs Ivermectin

Clinical evidence to date has reported promising results (see above) for Ivermectin in prevention, early treatment as well as late treatment for COVID-19.

While both Ivermectin and Hydroxychloroquine might be useful for early treatment, Ivermectin has a broader potential benefit i.e. prevention, early treatment as well as late treatment / hospital treatment. An important to note is that both ivermectin and hydroxychloroquine perform better when given early (within first 3 - 5 days of symptoms) as opposed to late treatment.

That said, if you simply cannot get hydroxychloroquine or ivermectin, quercetin is a viable stand-in. Quercetin works best when taken with vitamin C and Bromelain, as vitamin C helps activate it and bromelain helps with the absorption.

Although ivermectin and hydroxychloroquine are relatively safe drugs, they are still synthetic chemicals that can have side effects. Quercetin and Vitamin D, C, Zinc are nutrients that your body require for optimal health. Nutrients are safer alternatives especially if your risk is low e.g. age below 50 and no other chronic illness.

 

h. Quercetin vs Quinine

Is quercetin Same as Quinine? No. Quercetin is a phytonutrient whereas quinine is a naturally occurring compound found in cinchona bark and was used as an antimalarial agent. However, both quercetin and quinine are known to have zinc ionophore properties i.e. they transport zinc into the cells.


i. Isoquercetin vs Quercetin

Quercetin and isoquercetin are both natural plant flavonoids. Quercetin is an aglycone, meaning that it lacks a glycoside side chain. In contrast, naturally occurring quercetin compounds are primarily glycosides, with only very small quantity occurring as an aglycone. Once absorbed from the gut, most quercetin compounds are metabolized to quercetin glucuronides, the primary metabolic form detected in plasma.

As mentioned, quercetin does not principally occur in the form in which it is available as a dietary supplement (an aglycone). Instead, it occurs mostly as quercetin glycosides. Isoquercetin is one of the naturally occurring glucosides of quercetin. Isoquercetin is also sometimes called isoquercitrin, a nearly identical quercetin-3-monoglucoside. Technically the two are different (isoquercetin has a pyranose ring whereas isoquercitrin has a furanose ring), but functionally the two molecules are indistinguishable. The literature often considers them as one and uses the names interchangeably.

7. Quercetin and Other Supplements

There’s also some evidence that quercetin has a synergistic effect when combined with other flavonoid supplements. That’s why some quercetin supplements contain resveratrol, catechins, genistein, and other flavonoids.

a. Zinc and Quercetin

i. Should Quercetin Be Taken With Zinc?

Yes. Quercetin can likely be used with zinc supplement to act as antiviral against many RNA viruses including influenza and COVID-19. 

ii. How Zinc and Quercetin Work Together

Quercetin acts as a zinc ionophore (PubMed 2014), the same mechanism of action that hydroxychloroquine has via helping zinc pass the cell wall where it might halt viral replication.

Suggested benefits of zinc supplementation along with zinc ionophores to prevent and treat COVID-19 and other respiratory tract infections are supported by countless studies (source). For example, it is known that zinc will slow down the replication of coronavirus through inhibition of enzyme RNA polymerase (PubMed 2010). 

iii. Covid-19

The Zelenko Covid-19 Protocols was developed by Dr Vladimir Zelenko. The protocol has as its centerpiece, but not exclusive piece, the combined use of Hydroxychloroquine (HCQ) and Zinc as a means by which a person can both help to prevent or mitigate the contraction of COVID-19, or can actually treat the disease once it has been diagnosed. 

The COVID-19 is an RNA (RiboNucleicAcid) virus and requires the RNA polymerase to replicate. Do take note that the study publication was a 2010 publication and is referring to a different coronavirus as compared to the latest coronavirus (COVID-19); though both are from the same family of coronaviruses.

A word about quercetin: Some physicians are recommending this supplement to reduce viral illnesses because quercetin acts as a zinc ionophore to improve zinc uptake into cells. It is much less potent than HCQ (hydroxychloroquine) as a zinc transporter, and it does not reach high concentrations in lung cells that HCQ does. Quercetin may help reduce risk of viral illness if you are basically healthy. But it is not potent enough to replace HCQ for treatment of COVID once you have symptoms, and it does not adequately get into lung tissue.

That said, if you simply cannot get hydroxychloroquine or ivermectin, quercetin is a viable stand-in. Quercetin works best when taken with vitamin C and Bromelain, as vitamin C helps activate it and bromelain helps with the absorption. In this study (PubMed), Bromelain also has anti-viral property against COVID-19 virus and anti-clotting property, and therefore may be useful against COVID-19.

Update: Recently, Ivermectin has come to the fore as a more effective (also long standing, cheap, generic drug), in all three stages of COVID-19. Dr Zelenko has also updated his protocols to incorporate ivermectin as an option for both the prophylactic and treatment protocols as shown below.

Best Quercetin and Zinc Supplement

• Supplements Studio Quercetin with Bromelain-Zinc-Vitamin C-Ginger > Buy on Amazon
• Kappa Nutrition Immune C Plus > Buy on Amazon

b. Quercetin and Bromelain

Bromelain is a naturally occurring, protein-digesting enzyme found in the stem of the pineapple. Bromelain seems to boost the absorption of quercetin by inhibiting inflammatory chemicals called prostaglandins. Bromelain may also decrease inflammation on its own.

Your body doesn’t absorb quercetin very effectively on its own. Some studies have shown that bromelain boosts the absorption of quercetin, which is why you find bromelain (or vitamin C) in many quercetin supplements.

i. Quercetin with Bromelain for Sinusitis

About 35 million Americans develop sinusitis each year. This is inflammation and swelling of the tissue lining the sinuses in your nasal cavity or the areas surrounding your nose (figure below). It can be caused by infections but also nose related problems like a polyps (benign growth), blockages or allergies.

Types of Sinusitis

Acute sinusitis - an infection that is often triggered by the flu or cold. The flu or cold virus attacks your sinuses causing them to swell and become narrow. Your body responds by producing mucus. When the sinuses become blocked and are filled with mucus, viruses, fungi or bacteria can grow and cause infections that cause pressure and headache.

Chronic sinusitis - an infection that lasts for more than 3 weeks and can continue indefinitely if not treated. It is often caused by allergies, fungal infections or weakened immune system.

For those with chronic or recurring sinusitis, you could consider rinsing your sinuses once or twice a day with a nasal saline solution. 

Bromelain helps support the sinuses because of its powerful anti-inflammatory and mucus thinning properties. A 2016 review of studies suggests that bromelain may reduce the duration of sinusitis symptoms in children, improve breathing, and reduce nasal inflammation.

Quercetin has amazing anti-inflammatory and anti-oxidant effects. Since bromelain and quercetin are recommended to be taken together for maximum anti-inflammatory benefit, formulas that contain both ingredients are ideal.

ii. Quercetin with Bromelain for Inflammation

Studies have shown that systemic enzymes for inflammation (like bromelain) combined with quercetin, produce greater anti-inflammatory benefits than either one used alone. Thus, the combination of bromelain and quercetin is ideal for effectively inhibiting the inflammatory response in both acute and chronic inflammatory conditions.

Since bromelain and quercetin are recommended to be taken together for maximum anti-inflammatory benefit, formulas that contain both ingredients are ideal. This also simplifies the dosing schedule and reduces the total number of capsules needed to be taken throughout the day.

iii. Quercetin with Bromelain for Allergies and Hives

In general, there are two types of allergies; generalized and local. Hives are generalized allergies, could be triggered by food (e.g. peanuts) or antibiotics or certain medications. Local allergies instead affect a certain area of the body e.g. scalp (allergic to hair dye) or face (allergic to certain make-up).

According to this review article, quercetin is known for its antioxidant activity and anti-allergic properties. 

A study (2013) that was done on people with chronic rhinosinusitis (inflammation in the sinuses) indicated good tolerability, symptom control, and improvement in QoL (quality of life) for the treatment of CRS (chronic rhinosinusitis) using bromelain tablets.

On top of improving absorption, bromelain together with quercetin, can help alleviate sinus inflammation, with quercetin functioning as an anti-inflammatory. 

Can you take a quercetin supplement with a pineapple allergy? If you’re allergic to pineapples, you should avoid quercetin supplements with bromelain. Bromelain, an enzyme in pineapples, is added to quercetin supplements to boost bioavailability. Overuse of bromelain can create problems for consumers.

iv. Quercetin with Bromelain with Prostatitis

As an antioxidant that helps fight inflammation, Quercetin may help reduce pain and symptoms of chronic non-bacterial prostatitis. In one study, combining quercetin with bromelain and papain produced even greater effects. Bromelain and papain help the body better absorb quercetin and they also have anti-inflammatory effects. Quercetin may interact with several common medications, so ask your doctor before taking it. Both bromelain and quercetin may increase the risk of bleeding, especially if you also take blood thinners, such as warfarin (Coumadin), clopidogrel (Plavix), or aspirin. Very high doses of quercetin have been associated with kidney damage. DO NOT take quercetin for more than 2 to 3 months without taking a break.

Best Quercetin with Bromelain Supplement > Vitaraw Quercetin, Zinc, Bromelain, Vitamin C, Vitamin D3

c. Quercetin and Vitamin C

There is evidence that quercetin and vitamin C co-administration exerts a synergistic antiviral action due to overlapping antiviral and immuno-modulatory properties and the capacity of ascorbate to recycle quercetin, increasing its efficacy.

i. COVID-19 (FLCCC I-MASK+ Protocol)

Ascorbic acid (vitamin C) and the bioflavonoid quercetin (originally labeled vitamin P) were both discovered by the same scientist — Nobel prize winner Albert Szent-Györgyi. Quercetin and vitamin C also act as an antiviral drug, effectively inactivating viruses. 

There is evidence that vitamin C and quercetin co-administration exerts a synergistic antiviral action due to overlapping antiviral and immunomodulatory properties and the capacity of ascorbate to recycle quercetin, increasing its efficacy.

June 19, 2020, Dr Marik published the paper “Quercetin and Vitamin C: An Experimental, Synergistic Therapy for the Prevention and Treatment of SARS-CoV-2 Related Disease (COVID-19)” in the journal Frontiers in Immunology. The paper presents evidence for the use of vitamin C and quercetin — based on their biological actions and pharmacokinetics profiles — both as prophylaxis in high-risk populations, and as an adjunct to drugs such as Remdesivir or convalescent plasma in the treatment of hospitalized COVID-19 patients.

For updated prevention and early outpatient protocol  for COVID-19 positive, please check out FLCCC I-MASK+ protocol.

d. Quercetin, Bromelain and Vitamin C

Quercetin was initially found to provide broad-spectrum protection against SARS coronavirus in the aftermath of the SARS epidemic that broke out across 26 countries in 2003. Now, some doctors are advocating its use against SARS-CoV-2, in combination with Bromelain and vitamin C.

Quercetin works best when taken with Vitamin C and Bromelain, as vitamin C helps activate it and bromelain helps with the absorption. 

Disclaimer

Do not take supplements without consulting your doctor. The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. It is intended as a sharing of knowledge and information from the research and experience of third party sites. If you are pregnant, nursing, taking medication, or have a medical condition, consult your health care professional before using products based on this content.

Quercetin with Bromelain and Vitamin C Supplements > Forest Leaf Quercetin 500mg with Bromelain, Vitamin C and Stinging Nettle

e. Quercetin and EGCG (Green Tea Extract)

EGCG is a powerful plant compound and immune nutrient primarily found in green tea. The beauty of green tea is that it also contains quercetin, another zinc ionophore. Both are known for its many health benefits. EGCG is rich in polyphenols and antioxidants. It may help to lower oxidative stress, decrease inflammation, improve heart health, support brain health, aid weight loss and help you live longer.

EGCG also acts as a zinc ionophore (PubMed 2014), the same mechanism of action that hydroxychloroquine has via helping zinc pass the cell wall and get into cells where it might halt viral replication. Conversely, zinc helps to make EGCG more bioavailable, meaning the two nutrients work hand in hand to enhance each other’s therapeutic potential.

An Indian study found that EGCG has the ability to target numerous key structures inside the virus, disabling its functional capacity.

“EGCG showed a very high binding affinity and a low inhibition constant among all the phytoconstituents screened, especially in the case of 6vw1, which is a potential target of SARS-CoV-2,” the paper found.

“EGCG exhibited better binding with the viral proteins and hence, is expected to show better antiviral activity than the reference drugs, remdesivir and chloroquine,” it added.

It is important to remember, though, that by itself, EGCG is both unstable and bio-unavailable unless coupled with nutrients like zinc that studies show help to maximize its transport into cells.

Quercetin Supplement with Green Tea Extract > Codeage Antioxidant Polyphenols Broad Spectrum

f. Quercetin and Curcumin

Both Quercetin and Curcumin have similar benefits. Thay are both antioxidant molecules with anti-proliferative, anti-inflammatory and immunosuppressive activities.

A study shown that combining low concentrations of Quercetin and Curcumin yield similar or better anti-inflammatory effectiveness when compared to treatment with each agent alone. Moreover, they co-operate synergistically in the context of antimicrobial activity, with an increased effectiveness when compared to Quercetin or Curcumin alone at high concentrations.

Another study suggests that curcumin and to a lesser extent quercetin may offer therapeutic potential for the treatment of crystal-induced arthritis or rheumatoid arthritis.

Both curcumin and quercetin had protective effects, reduced the oxidative stress and stimulated the antioxidant defense. Curcumin had stronger antioxidant effects compared to quercetin, probably because of its higher bioavailability and absorption. (Source)

Quercetin Supplement with Curcumin > Organic Cadane Quercetin Capsule

g. Quercetin, Resveratrol and Curcumin

Quercetin, Resveratrol and Curcumin may provide substantial therapeutic benefit especially in aging and chronic inflammatory diseases. This study suggests that delivering these compounds in combination may improve the acute bioavailability of Curcumin and Resveratrol compared to supplementation with single compounds, allowing for lower overall doses and simpler treatment protocols using combination therapies.

Combination of Quercetin, Resveratrol and Curcumin may improve intestinal absorption of Resveratrol and Curcumin without affecting Quercetin absorption. 

Quercetin, Resveratrol and Curcumin Supplement > Ortho Nutrition Antioxidant Boost

8. What Not To Take With Quercetin?

Quercetin may interact with a number of medications:

- Antibiotics: May prevent the action of antibiotics

- Blood thinners: Could increase the effects of blood-thinning medication

- Chemotherapy: May interact with chemotherapeutic medication

- Corticosteroids: Could cause these medications to stay in your body for longer

- Cyclosporine: May interact with the absorption of this medicine

- Digoxin: Could increase the risk of side-effects associated with this chemical agent

- Fluoroquinolones: May decrease the effectiveness of these medicines

- Medications altered/activated by the liver: Quercetin may change how your body metabolizes medicine that is activated in the liver 

Be cautious with the following combination below! An adverse outcome could occur. Talk with your health provider before taking this combination.

a. Quercetin Interactions with Antibiotics (Quinolone antibiotics) 

Taking quercetin along with some antibiotics might decrease the effectiveness of some antibiotics. Some scientists think that quercetin might prevent some antibiotics from killing bacteria. But it's too soon to know if this is a big concern.

Some of these antibiotics that might interact with quercetin include ciprofloxacin (Cipro), enoxacin (Penetrex), norfloxacin (Chibroxin, Noroxin), sparfloxacin (Zagam), trovafloxacin (Trovan), and grepafloxacin (Raxar).


b. Quercetin Interactions with Cyclosporin (Neoral, Sandimmune)

Cyclosporin (Neoral, Sandimmune) is changed and broken down by the liver. Quercetin might decrease how quickly the liver breaks down cyclosporin (Neoral, Sandimmune). Taking quercetin might increase the effects and side effects of this medication. Before taking quercetin talk to your healthcare provider if you take cyclosporin (Neoral, Sandimmune).

c. Quercetin Interactions with Drugs

i. Medications changed by the liver

Some medications are changed and broken down by the liver. Quercetin might decrease how quickly the liver breaks down some medications. Taking quercetin along with these medications that are changed by the liver might increase the effects and side effects of your medication. Before taking quercetin talk to your healthcare provider if you take any medications that are changed by the liver.

Some medications that are changed by the liver include paclitaxel (Taxol), rosiglitazone (Avandia), amiodarone (Cordarone), docetaxel (Taxotere), repaglinide (Prandin), verapamil (Calan, Isoptin, Verelan), celecoxib (Celebrex), diclofenac (Voltaren), fluvastatin (Lescol), glipizide (Glucotrol), ibuprofen (Advil, Motrin), irbesartan (Avapro), losartan (Cozaar), phenytoin (Dilantin), piroxicam (Feldene), tamoxifen (Nolvadex), tolbutamide (Tolinase), torsemide (Demadex), warfarin (Coumadin), amitriptyline (Elavil), codeine, flecainide (Tambocor), haloperidol (Haldol), imipramine (Tofranil), metoprolol (Lopressor, Toprol XL), ondansetron (Zofran), paroxetine (Paxil), risperidone (Risperdal), tramadol (Ultram), venlafaxine (Effexor), lovastatin (Mevacor), clarithromycin (Biaxin), cyclosporine (Neoral, Sandimmune), diltiazem (Cardizem), estrogens, indinavir (Crixivan), triazolam (Halcion), verapamil (Calan, Isoptin, Verelan), alfentanil (Alfenta), fentanyl (Sublimaze), losartan (Cozaar), fluoxetine (Prozac), midazolam (Versed), omeprazole (Prilosec), lansoprazole (Prevacid), propranolol (Inderal), fexofenadine (Allegra), amitriptyline (Elavil), amiodarone (Cordarone), citalopram (Celexa), sertraline (Zoloft), ketoconazole (Nizoral), itraconazole (Sporanox) and others.

ii. Medications moved by pumps in cells (P-glycoprotein Substrates) 

Some medications are moved by pumps in cells. Quercetin might make these pumps less active and increase how much of some medications get absorbed by the body. This might cause more side effects from some medications.

Some medications that are moved by these pumps include diltiazem (Cardizem), verapamil (Calan, Isoptin, Verelan), digoxin (Lanoxin) cyclosporine (Neoral, Sandimmune), saquinavir (Invirase), amprenavir (Agenerase), nelfinavir (Viracept), loperamide (Imodium), quinidine, paclitaxel (Taxol), vincristine, etoposide (VP16, VePesid), cimetidine (Tagamet), ranitidine (Zantac), fexofenadine (Allegra), ketoconazole (Nizoral), itraconazole (Sporanox), and others.

9. Quercetin Side Effects

Quercetin is generally recognized as safe. It’s found in many fruits and vegetables, and most people will experience no side effects when taking normal dosages of quercetin.

Any side effects tend to be mild. Quercetin side effects include tingling and numbness, headache, and nausea. Your chance of experiencing side effects increases at higher dosages (over 1,000mg).

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

• Blurred vision
• dizziness
• dull ache or feeling of pressure or heaviness in legs
• fluid accumulation in the knee
• headache
• itching skin near damaged veins
• nervousness
• pounding in the ears
• red or scaling skin
• slow or fast heartbeat
• swollen feet and ankles
• tingling of the arms and legs
• stomach upset

a. Who should not take a quercetin supplement?

Quercetin appears safe for anyone to take when used in normal dosages. However, there’s limited research on how quercetin supplements affect women who are pregnant or breastfeeding. 

People with high blood pressure, or anyone taking blood pressure medication, may also want to take to their doctor before taking a quercetin supplement. Although there are 30 CYP450 (Cytochrome P450) liver enzymes that metabolize drugs, only 6 do most of the work. Quercetin inhibits 5 of them (Source).

Therefore, you should talk to your doctor before taking Quercetin.

b. Can you overdose on quercetin?

In one study, researchers gave participants 2,000mg to 5,000mg of quercetin per day with no adverse effects or signs of toxicity reported. Generally, quercetin is safe to take even in high doses, although you may experience mild side effects like nausea, digestive issues, and headaches at high doses. Excessively high doses of quercetin could lead to kidney problems.

10. Online Shopping Guide

The cost to obtain 500 mg of quercetin was found to range from just 11 cents to $1.17, depending on the product and brand.

Quercetin Phytosome Supplement 

1. THORNE Quercetin Complex (Formerly Quercenase)

2. MoxyVites Whole Food Quercetin 500mg with Bromelain Supplement

Quercetin, Zinc, Vitamin D and C

3. Z-Stack Quercetin 500 mg with Zinc, Vitamin C and Vitamin D3 (Dr Zelenko's Z-Stack)

Quercetin and Zinc Supplement

4. Kappa Nutrition Immune C Plus 

Quercetin with Bromelain and Vitamin C Supplements

5. Forest Leaf Quercetin 500mg with Bromelain, Vitamin C and Stinging Nettle

6. Vitaraw Quercetin, Zinc, Bromelain, Vitamin C, Vitamin D3 

Quercetin with Bromelain, Vitamin C and Zinc Supplements

7. Whole Food Quercetin Plus with Bromelain, Zinc, Vitamin C and Ginger

8. Quercetin Supplement with Green Tea Extract > Codeage Antioxidant Polyphenols Broad Spectrum

9. Quercetin with Curcumin > Organic Cadane Quercetin Capsule

10. Quercetin, Resveratrol and Curcumin Supplement > Ortho Nutrition Antioxidant Boost