Product Reviews
Quercetin Supplements Review

Reviewed and edited by Tod Cooperman, M.D. Tod Cooperman, M.D.
Initial Posting: 3/23/19 Last Update: 3/31/2020
Quercetin Supplement Reviewed By
Sections: Jump to a section by clicking on its name.
  • What is it? Quercetin is a yellow-colored flavonoid found in plant-based foods such as capers, onions, and kale. In its natural form it is attached to sugar molecules, but supplements typically contain just the quercetin molecule or quercetin that has been attached to water molecules (quercetin dihydrate, which is about 90% quercetin). Quercetin from plant-based foods has been shown to be better absorbed than quercetin from supplements. See What It Is.
  • How is it used? Many short-term and relatively small clinical studies have been conducted with quercetin, suggesting modest benefit in conditions such as prostatitis, insulin resistance, and rheumatoid arthritis. Quercetin may also modestly reduce blood pressure. See What It Does.
  • What did CL find? Tests by ConsumerLab revealed that two products contained 12% to 14% less quercetin than claimed. Five other products were Approved, including one tested through CL's Quality Certification Program. The cost to obtain 500 mg of quercetin was found to range from just 11 cents to $1.17, depending on the product. See What CL Found, the Cost graph, and the Results table to compare amounts and costs of quercetin in products.)
  • Top Picks — ConsumerLab selected two Top Picks among quercetin supplements based on quality and cost.
  • How much to take and when? Typical dosage is 500 mg taken once or twice daily. To enhance absorption, it is best to take quercetin with a meal containing fats or oils. See the ConsumerTips section for the dosage for specific uses.
  • Cautions: Side effects of quercetin are uncommon and generally mild. However, quercetin may interact with a variety of prescription medications. For details, see Concerns and Cautions.

What It Is:
Quercetin is a yellow flavonoid that occurs in many plant-based foods. It is found at very high concentration in capers (200 mg, based on a 100 gram serving) and in moderate concentration in onions (primarily in the outer rings) (20-30 mg), radicchio (10-30 mg), kale (20 mg), certain berries (chokeberry, cranberry, and lingonberry) (about 15 mg), black plums (12 mg), and buckwheat (roasted, 7 mg), and in smaller concentrations (about 1 to 4 mg) in many other foods such as apples (primarily in the skin) and broccoli (USDA 2011). Average daily intake of quercetin from food sources in the U.S. has been estimated at about 25 to 50 mg (Formica, Food Chem Tox 1995). It also naturally occurs in botanical ingredients used in supplements such as Ginkgo biloba, St. John's wort, and sea buckthorn.

The quercetin in supplements is typically derived from plant extracts chemically treated to remove sugar molecules naturally attached to quercetin, creating free-form (aglycone) quercetin. Most supplements contain either this free form or a dihydrate form in which two water molecules are chemically attached to each quercetin molecule. For more information about forms of quercetin and how they compare, see What to Consider When Buying and Using.

What It Does:
In-vitro and animal studies have shown quercetin to have anti-carcinogenic, anti-inflammatory and antiviral effects (Li, Nutrients 2016; Somerville, Adv Nutr 2016). In people, short-term and relatively small clinical studies suggest modest benefits in certain conditions, as discussed below.

Preliminary evidence suggests that quercetin may be helpful to men with prostatitis. A one-month, double-blind study among 28 men (average age 44) with inflammatory or non-inflammatory chronic prostatitis found that 67% of those who took quercetin (500 mg twice daily) had significant overall improvement in symptoms in contrast to only 20% of men who took a placebo. Specific symptoms that improved were pain and quality of life, but urinary symptoms, such as urgency or difficulty voiding, did not improve significantly (Shoskes, Urology 1999). Quercetin is sometimes added as an ingredient in prostate supplement formulas, although these combinations generally have not been clinically tested.

Blood sugar and insulin resistance
Several clinical studies suggest that quercetin supplementation may slightly decrease fasting blood sugar levels and insulin levels. A placebo-controlled study among 82 women in Iran with polycystic ovary disease (PCOS), high-normal fasting blood sugar levels and moderate insulin resistance found that those who took 500 mg of quercetin (Jarrow Formulas) after breakfast and lunch (a total daily dose of 1,000 mg quercetin) for three months experienced small decreases in fasting blood sugar (1.99 mg/dL), blood insulin levels (1.74 ┬ÁIU/mL) and HOMA-IR (0.44), a measure of insulin resistance (Resvan, Cell J 2018). However, studies in adults with metabolic syndrome using lower daily doses of quercetin (100 mg to 730 mg) have failed to demonstrate improvement in glycemic control (Ostadmohammadi, Phytother Res 2019).

Cardiovascular health
Some, but not all, studies suggest quercetin may modestly lower blood pressure. A review of seven clinical trials lasting four to ten weeks found that taking 500 mg to 1,000 mg of quercetin daily reduced systolic and diastolic blood pressure by approximately 3.04 mm Hg and 2.63 mm Hg, respectively, in people with and without high blood pressure (Serban, J Am Heart Assoc 2016).

A study among 40 women in Iran (average age 47) with rheumatoid arthritis found that 500 mg of quercetin taken once daily for two months decreased morning stiffness and pain, and after-activity pain, but did not reduce the number of tender or swollen joints, or disease activity scores, compared to placebo. The quercetin supplement (Solaray) was taken after lunch with a glass of water (Solaray did not fund the study) (Javadi, J Am Coll Nutr 2017). There do not appear to be studies on the effects of quercetin supplementation in people with osteoarthritis.

Memory and Cognition
A combination of quercetin and resveratrol (160 mg quercetin plus 100 mg resveratrol twice daily with meals) has shown mixed results for improving memory. One study among overweight but otherwise healthy adults found the combination improved memory performance and activity in the hippocampus, a region in the brain involved with memory function (Witte, J Neurosci 2014). However, a second study using the same combination found no benefit (Huhn, Neuroimage 2018). There do not appear to be any studies on the effects of quercetin alone on memory or cognition.

Seasonal allergy
Laboratory studies suggest that quercetin inhibits the release of histamine and release of antigen-specific antibodies (IgE) from mast cells, both of which are involved in allergic and inflammatory responses to seasonal allergens (Mlcek, Molecules 2016). Given orally, quercetin has been shown to reduce sneezing and nasal rubbing in rats with chemically-induced nasal allergy-like symptoms (Kashiwabara, BMC Complement Altern Med 2016). However, research with people appears to be limited to a study among 24 men and women allergic to cedar pollen. The study found that 100 mg of a specially formulated quercetin supplement taken during pollen season reduced self-reported eye symptoms such as itching and tearing and decreased allergy medication use, but it did not reduce nasal symptoms, compared to placebo. There was no reduction in blood levels of total or Japanese cedar pollen-specific IgE antibodies in those who took quercetin. The quercetin used in the study (EMIQ, San-Ei-Gen F.F.I. Inc., Japan) is described as an enzymatically modified iso-quercitrin that may be better-absorbed than regular quercetin (see Forms of quercetin below). One 100 mg capsule of EMIQ (providing 50 mg of quercetin) was taken once daily one month before and one month during cedar pollen season in Japan (Hirano, Allergol Int 2009).

Viral infections
As briefly noted above, laboratory and animal studies show quercetin has anti-viral properties (Li, Nutrients 2016; Somerville, Adv Nutr 2016) but research in people has been limited.

A small study in 40 trained male cyclists showed that taking 1,000 mg of quercetin daily for three weeks before and for two weeks after intense exercise (cycling for three hours per day for three consecutive days) significantly reduced the incidence of upper respiratory tract infections during the two weeks after exercise compared to placebo (1 infection among those who took quercetin vs 9 infections among those that took a placebo). However, it did not alter certain blood markers of inflammation and immune system function (IL-6, IL-8, IL-10, IL-1ra, and TNF-a) compared to placebo (Niemen, Med Sci Sports Exerc 2007).

Laboratory and animal studies have shown the quercetin and its major metabolites, such as quercetin 3-beta-O-d-glucoside (Q3G, also called isoquercetin), inhibit a wide variety of viruses, including influenza virus, Chikungunya virus, Epstein-Barr virus, hepatitis C virus, Ebola virus, the Zika virus (ZIKV) and severe acute respiratory syndrome coronavirus (SARS-CoV) (Qiu, Antimicrob Agents Chemother 2016; Wong, Virol Sin 2017, Chen, Bioorg Med Chem 2006). For example, one of these studies showed that when mice were injected with high doses (50 mg/kg of body weight) of Q3G every other day for three weeks before they were infected with a lethal dose of the Ebola virus all survived Ebola infection, while all of the mice that did not receive QG3 died (Qiu, Antimicrob Agents Chemother 2016). One of the researchers involved in the study, Michel Chrétien, has partnered with researchers in China to begin a clinical trial that will investigate the effects of high oral doses of quercetin in people with COVID-19, the disease caused by the novel coronavirus (SARS-CoV-2). Details about exact form (quercetin or 3-beta-O-d-glucoside) and dose of the formula (produced by Swiss drug manufacturer, Quercegen Pharmaceuticals) do not appear to have been made public, but in a February 2020 interview posted online by the Canadian Broadcasting Company (CBC News), Chrétien stated that they will give "high doses" of quercetin to patients, four times daily. He noted that similar high doses have not been shown to cause toxicity in previous human clinical trials (and that amounts of quercetin found naturally in foods are likely too small to have an effect against the virus). He also stated that he hopes to have preliminary results of the trial in the upcoming months, but cautioned that he does not want to give "false hope" about the potential benefits of quercetin until more research is conducted. Isoquercetin (3-beta-O-d-glucoside) is sold as a supplement, although it appears to be less common than quercetin. Animal studies suggest that taking quercetin increases blood and tissue levels of 3-beta-O-d-glucoside in the body (Yang, Sci Rep 2016). Until more is known, it's not clear if taking isoquercetin or quercetin supplements can help to prevent or treat COVID-19, and if so, what dosage would be effective.

Quercetin does not seem to improve exercise performance. A review of seven randomized, placebo-controlled studies lasting between one week and 1 ½ months concluded that quercetin supplementation (600 mg to 1,000 mg per day, typically taken in divided doses with meals) is unlikely to have beneficial effects for aerobic exercise in trained or untrained individuals (Pelletier, Int J Sport Nutr Exerc Metab 2013).

A study in rats found that quercetin altered the sleep-wake cycle and decreased time spend in REM (rapid eye movement) sleep, but there do not appear to any studies on the effects of quercetin supplementation on sleep or insomnia in people (Kambe, Brain Res 2010).

Studies have not been conducted with quercetin supplementation to determine effects on cancer in people. A study in which quercetin was given intravenously to 51 people with a variety of cancers (of the colon, pancreas, liver, stomach, kidney, lung, skin or ovaries) found no reduction in cancers based on based on radiologic imaging, although one patient with ovarian cancer and one with liver cancer each had an improvement in a single marker of disease activity (Clin Cancer Res 1996).

A study in Iran among 20 people undergoing chemotherapy for blood cancer found that quercetin (250 mg quercetin hydrate taken twice daily for one month) did not reduce the occurance of a chemotherapy side effect (oral mucositis -- painful swelling in the mouth) compared to placebo. The study did not report cancer progression or outcomes (Kooshyar, J Clin Diagn Res 2017).

Although quercetin is commonly promoted for a wide range of other uses - including allergies and asthma - the evidence supporting these uses is limited. For more information see the Encyclopedia article about Quercetin.

Quality Concerns and What CL Tested For:
Neither the Food and Drug Administration (FDA) nor any other federal or state agency routinely tests supplements for quality prior to sale. tested quercetin supplements determine the amounts of quercetin or quercetin dihydrate contained and whether these matched the amounts and forms listed on product labels. Products were also tested for contamination with the heavy metals lead, cadmium, and arsenic.

See How Products Were Evaluated for more information on testing.

What CL Found:
Four of the six quercetin supplements selected for review passed testing, containing their expected amounts of quercetin without contamination with heavy metals. One product tested through ConsumerLab's voluntary Quality Certification Program also passed testing.

As shown in the Results table further below, the following two products failed testing for providing about 12% to 14% less quercetin than claimed on their labels:
  • Natrol Quercetin Complex Immune Health — contained 219.3 mg of quercetin per capsule instead of 250 mg.
  • Ultra Quercetin 650 mg - contained 557.3 mg of quercetin dihydrate per capsule instead of 650 mg.
Shown in the graph below, the cost to get 500 milligrams of quercetin (about the amount in a large capsule or two small capsules) ranged from just 11 cents (from a powder) to $1.17 (from a liquid). The cost among capsules (the most common form) ranged from 17 cents to 40 cents.

Cost per 500 mg of quercetin

Top Picks
Among the Approved products, the following are our Top Picks for quercetin.

Top Pick: Quercetin Dihydrate
In addition to being the least expensive source of high-quality quercetin (11 cents per 500 mg), the quercetin in BulkSupplements is a finer powder than in many of the capsules. As a result, it may be more easily absorbed and digested. It can be mixed into liquids, although it won't dissolve well in water (like any quercetin). Keep in mind that this product contains the dihydrate form of quercetin, so around 10% of its listed amount is actually water molecules and not quercetin.

Top Pick: MRM Quercetin 500 mg
The least-expensive quercetin sold as a capsule was Solaray Quercetin 500 mg (17 cents per capsule) but our Top Pick among capsules is MRM Quercetin 500 mg, the second least-expensive capsule (20 cents). The reason for choosing MRM is that, if you open the capsule, you'll find that the quercetin is a finer powder than Solaray's. As quercetin has poor solubility (as noted earlier), a finer powder may be more easily digested and, therefore, more easily absorbed and/or used in the gut. It also turns out that the form of quercetin in MRM, QU995, is a patented form that is the only quercetin ingredient to have GRAS (Generally Regarded as Safe) status with the FDA.

Test Results by Product:
Listed below are the test results for seven quercetin supplements. Six were selected by and one (denoted with a CL flask) is included for having passed the same evaluation through the Quality Certification Program.

Also shown are the labeled amounts and suggested serving sizes for each product. Products listed as "Approved" contained their expected amounts of quercetin and met's additional quality criteria (see Passing Score). The last column shows price and cost comparisons as well as notable ingredient features. A full list of ingredients is available for each product by clicking on the word "Ingredients" in the first column.

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Product Name, Amount Listed of Quercetin per Unit, Serving Size, and Suggested Daily Serving on Label

Click on "Ingredients" for Full Listing
Claimed Amount and Form of Quercetin --TEST RESULTS--

(See How Products Were Evaluated)
Cost For Daily Suggested Serving On Label

[Cost per 500 mg Quercetin1]

Other Notable Features2

Price Paid

Contained Labeled Amount of Quercetin Did Not Exceed Contamination Limits for Lead, Cadmium, Arsenic and Mercury Quercetin Dihydrate (500 mg quercetin dihydrate per ½ tsp [500 mg]; ¼ to ½ tsp, once to twice daily3)

Dist. by
250 mg to 1,000 mg quercetin dihydrate
(equivalent to 223.4 mg to 893.5 mg quercetin)

Powder in bag

Lowest cost for quercetin

Free of gluten and yeast

$18.96/3.53 oz [100 g] bag (approx. 200 servings4)

MRM® Quercetin 500 mg (500 mg quercetin per vegan capsule; 1 vegan capsule, three times daily)

Dist. by MRM
1,500 mg quercetin

Large vegan capsule


Suitable for vegans, contains no wheat, gluten and yeast

$11.99/60 vegan capsules
Natrol® Quercetin Complex Immune Health (250 mg quercetin per capsule; 2 capsules, once daily)

Mfd. by Natrol LLC
500 mg quercetin

Large capsule
Found only 438.6 mg quercetin per daily serving
(87.7% of listed amount)

[$0.30 based on amount claimed]
[$0.34 based on amount found]

Vitamin C (1,000 mg), citrus bioflavonoid complex (66 mg)

No wheat and yeast

$7.56/50 capsules® Ultra Quercetin 650 mg (650 mg quercetin dihydrate per capsule; 1 capsule, once daily)

Mfd. by Piping Rock Health Products
650 mg quercetin dihydrate (equivalent to 580.8 mg quercetin)

Large capsule
Found only 557.3 mg quercetin dihydrate per daily serving
(85.7% of listed amount)

[$0.22 based on amount claimed]
[$0.25 based on amount found]

$14.99/60 capsules
Solaray® Quercetin 500 mg (500 mg quercetin per VegCap; 1 VegCap, once daily)

Mfd. by Nutraceutical Corp.
500 mg quercetin

Large vegetable capsule

Lowest cost for quercetin from pill

$15.12/90 VegCaps
Solgar® Quercetin Complex (250 mg quercetin per vegetable capsule; 2 vegetable capsules, twice daily)

Mfd. by Solgar, Inc.
1,000 mg quercetin

Large vegetable capsule


Vitamin C (1,000 mg), calcium (100 mg), bromelain (6,500 GDU), citrus bioflavonoid complex (100 mg), rose hips (100 mg), acerola (100 mg), rutin (20 mg)

Kosher, free of wheat and yeast, gluten free

$19.80/100 vegetable capsules
Source Naturals® NutraDrops™ Quercetin (134 mg quercetin per 2 droppers [2 ml]; 1 to 2 droppers, once daily5)

Dist. by Source Naturals, Inc.
67 mg to 134 mg quercetin

Liquid from bottle


$18.55/4 fl oz [118.28 ml] bottle (approx. 59 servings)
Tested through CL's Quality Certification Program prior to, or after initial posting of this Product Review.

* Based on measured density.

1 Cost analysis is based on amount of quercetin. Figures for products in the quercetin dihydrate form are calculated on the basis that 89.35% of quercetin dihydrate is quercetin.
2 Not tested but claimed on label.
3 Label states "As a dietary supplement, take 250 mg (1/4 tsp) to 500 mg (1/2 tsp) once or twice daily with food, or as directed by a physician.
4 Based on a 500 mg serving.
Unless otherwise noted, information about the products listed above is based on the samples purchased by (CL) for this Product Review. Manufacturers may change ingredients and label information at any time, so be sure to check labels carefully when evaluating the products you use or buy. If a product's ingredients differ from what is listed above, it may not necessarily be of the same quality as what was tested.

The information contained in this report is based on the compilation and review of information from product labeling and analytic testing. CL applies what it believes to be the most appropriate testing methods and standards. The information in this report does not reflect the opinion or recommendation of CL, its officers or employees. CL cannot assure the accuracy of information.
Copyright, LLC, 2019. All rights reserved. Not to be reproduced, excerpted, or cited in any fashion without the express written permission of LLC.


What to Consider When Buying and Using:

Forms of quercetin
Quercetin is generally sold either in its free form (i.e., just the quercetin molecule and labeled simply as "quercetin") or as quercetin dihydrate, in which the quercetin molecule is bound to two molecules of water, with quercetin representing 89.35% of the weight of the compound (i.e., 500 mg of quercetin dihydrate provides 446.75 mg of quercetin). However, both forms are poorly soluble in water at body temperature and little of either (about 2%) will be absorbed if taken with water. Taking quercetin with a high-fat meal (15.4 grams) was shown to increase bioavailability by one-third -- although still representing just a small fraction of quercetin ingested (Guo, Nol Nutr Food Res 2013).

A branded form of quercetin described as enzymatically modified iso-quercitrin (a glycoside of quercetin), called EMIQ (San-Ei-Gen F.F.I. Inc., Japan) has been shown to be better-absorbed than other quercetin glycosides as purified powders in an animal study and one small study in people (Makino, Biol Pharm Bull 2009; Murota, Arch Biochem Biophys 2010). None of the products in this review contain EMIQ, but it is sold as an ingredient in Natural Factors Bioactive Quercetin EMIQ and Bioclinic Naturals EMIQ Activated Quercetin, both of which appear to be available in the U.S.

When occurring naturally in foods, quercetin is attached to sugar molecules, and these forms of quercetin appear to be absorbed better than the quercetin or quercetin dihydrate in supplements, with 3% to 17% of the quercetin naturally in foods being absorbed (Li, Nutrients 2016). In fact, a study in healthy men found that consuming a red onion soup naturally containing 47 mg of quercetin was roughly equivalent to taking 544 mg of quercetin (from quercetin dihydrate) as a tablet with water (Shi, Food Funct 2015). Quercetin that is not absorbed (which is most of the quercetin that you consume) gets transformed by microbes and intestinal enzymes into smaller phenolic compounds.

Delivery Forms
Quercetin is generally sold in capsules. Labels typically suggest taking with "a meal." However, to maximize absorption, it would seem best to take with a meal that contains a good amount of fats or oils. Quercetin won't dissolve well in water but can still be mixed into drinks. It has no flavor and, as noted earlier, is yellow in color.

Dosage: (See What It Does for details)

Chronic prostatitis: 500 mg of quercetin twice daily (Shoskes, Urology 1999).

Blood sugar and insulin: 500 mg of quercetin twice daily may slightly decrease fasting blood sugar levels and decrease insulin resistance (Resvan, Cell J 2018).

Blood pressure: 500 mg to 1,000 mg of quercetin daily may modestly lower blood pressure (Serban, J Am Heart Assoc 2016).

Rheumatoid arthritis: 500 mg of quercetin taken daily may reduce morning pain and stiffness (Javadi, J Am Coll Nutr 2017).

Concerns and Cautions:
Side-effects with quercetin have been uncommon in short-term (three months or less) clinical studies at doses between 500 mg and 1,000 mg per day. Reported side effects have generally been mild nausea, stomach upset and /or headache. Taking quercetin with food may reduce stomach upset (Andres, Mol Nutr Food Res 2018), as well as increase absorption if the foods include fats or oils. In a small one-week study among older adults with chronic obstructive pulmonary disease (COPD), a daily dose of 500 mg, 1,000 mg, or 2,000 mg of quercetin (from 500 mg soft chews taken with food) was generally well-tolerated except for the development or worsening of gastro-esophageal reflux disease (GERD) in one participant -- although one of three participants who received placebo also reported GERD (Han, BMJ Open Respir Res 2020). One out of 15 participants in another study reported "mild tingling in the extremities after each dose" (Shoskes, Urology 1999). There is not enough research to determine the safety of taking more than 1,000 mg of quercetin daily for longer than 3 months or of taking more than 2,000 mg daily for longer than a week.

Quercetin may modestly lower blood pressure, as noted earlier. People with low blood pressure, or those taking blood-pressure lowering medications, should use with caution.

Laboratory studies show that quercetin may inhibit peptides that are involved in the uptake of certain drugs, including the organic anion-transporting peptide OATP1B1. This means that quercetin may increase the absorption, effects, and side effects of drugs that are substrates of OATP1B1. For example, in a study among ten healthy men who took 500 mg of quercetin daily for two weeks, followed by a single, 40 mg dose of pravastatin (Pravachol -- a substrate of OATP1B1) absorption of the drug was increased by 24%, and the elimination half-life was prolonged by 14% (Br J Clin Pharmacol. 2012). Drugs that are OATP1B1 substrates include certain other statin drugs, such as rosuvastatin (Crestor), fluvastatin (Lescol), pitavastatin (Livalo) and atorvastatin (Lipitor), as well as conjugated estrogens, valsartan (Diovan), enalpril (Vasotec), methotrexate (Otrexup (PF), fexofenadine (Aller-ease, Allegra) and others (Kalliokoski, Br J Pharmacol 2009; Niemi, Br J Clin Pharmacol 2005; Izumi, Drug Metab Dispos 2015).

Quercetin may inhibit the enzyme CYP2C9 (Rastogi, Phytother Res 2014), potentially increasing the effects of drugs that are metabolized by this enzyme. For example, a study among healthy adults showed that 500 mg of quercetin taken twice daily for 10 days significantly increased the absorption of the nonsteroidal anti-inflammatory drug diclofenac (Voltaren, Flector), which is metabolized by CYP2C9 (Bedada, Phytother Res 2018). Other drugs that are metabolized by this enzyme include celecoxib (Celebrex), fluvastatin (Lescol), glipizide (Glucotrol), ibuprofen (Advil, Motrin), losartan (Cozaar), phenytoin (Dilantin), piroxicam (Feldene), tamoxifen (Nolvadex), and warfarin (Coumadin).

Quercetin may increase the activity of the enzyme CYP3A, and therefore may decrease the effects of drugs metabolized by this enzyme. For example, in a small study among healthy adults, 500 mg of quercetin taken daily for 13 days significantly reduced the activity of the sedative midazolam (a single, 7.5 mg dose) taken on day 14) (Duan, J Clin Pharmacol 2012).

Laboratory and animal studies suggest quercetin may be a thyroid disruptor, interfering with iodide uptake and inhibiting an enzyme (thyroid type 1 5'-deiodinase) involved in the conversion of thyroid hormone T4 to its active form, T3 (Giuliani, Food Chem Toxicol 2014; de Souza Dos Santos,Food Chem Toxicol 2011). However, there do not appear to be studies investigating the effects of quercetin supplementation in people with thyroid disease. Some researchers advise that, out of caution, people with hypothyroidism, those who may undergo radioiodine treatment, and pregnant and lactating women avoid using quercetin (Giuliani, Food Chem Toxicol 2014).

Animal studies suggest that quercetin may potentially exacerbate pre-existing kidney disease or damage (Andres, Mol Nutr Food Res 2018). In people, giving quercetin intravenously has caused kidney toxicity in some people (Ferry, Clin Cancer Res 1996).

Some evidence from laboratory and animal studies suggest that quercetin may promote estrogen-receptor positive breast cancer cell growth, but not estrogen-negative breast cancer cells; however, there do not appear to be any studies on the effects quercetin supplementation on breast cancer in people (Andres, Mol Nutr Food Res 2018).

To further assist consumers, licenses its flask-shaped CL Seal of Approved Quality (see The CL Seal) to manufacturers for use on labels of products that have passed its testing. will periodically re-evaluate these products to ensure their compliance with's standards.

Information on this site is provided for informational purposes only. It is not an endorsement of any product nor it is it meant to substitute for the advice provided by physicians or other healthcare professionals. The information contained herein should not be used for diagnosing or treating a health problem or disease.
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