Quercetin Supplements Review
Find the Best Quercetin
Supplements. ConsumerLab Tests Reveal Not All
Quercetin Supplements Contain What They Claim.
Medically reviewed and
edited by Tod Cooperman, M.D. 
Last Updated
: 12/06/2021 | Initially Posted:
03/23/2019Latest Update: Quercetin for Severe COVID?
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Table of Contents
Summary
·
What is quercetin? 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 quercetin 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's tests of quercetin 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 for quercetin: ConsumerLab
selected two Top Picks among
quercetin supplements based on quality and cost.
·
How much quercetin should I 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.
·
Quercetin safety and side effects: 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 ConsumerTips™.
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.
Prostatitis
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 µIU/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).
Arthritis
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
Laboratory and animal studies have shown that 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).
According to preliminary research, quercetin appears to work by preventing
viruses from entering cells, thereby reducing "viral load" (Agrawal, Nat Prod Commun 2020). However, research in people has
been limited and inconsistent.
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).
Two small, company-funded
studies in Pakistan suggest a possible benefit from quercetin in people
with early-stage COVID-19; however, because the studies
were open-label (i.e. participants and investigators knew who was taking
quercetin and who was not) and not placebo-controlled, it's
not possible to draw conclusions about quercetin's effectiveness. Quercetin
does not appear to be beneficial among people with severe COVID-19.
One of the studies, among 152 people with early-stage COVID-19,
showed that taking a quercetin tablet (Quevir,
Pharmextracta S.p.A.) containing 500 mg of Quercetin
Phytosome (by Indena
S.p.A.) corresponding to 200 mg of quercetin twice daily (total daily dose: 400
mg) along with standard care for 30 days reduced the percentage of patients
requiring hospitalization to 9.2% vs. 28.9% for those only receiving standard
care, requiring oxygen (1.3% vs. 19.7%), or admitted to the ICU (0% vs. 10.5%),
and reduced deaths (0 vs. 3) and shortened the duration of hospitalization by
about 5 days. However, a significantly greater percentage of patients not given
quercetin had comorbid conditions than the treatment group. When only
patients without comorbidities were considered, there was no
between-group difference in rate of hospitalization, ICU admission, or deaths,
although the duration of hospitalization and need for oxygen was still reduced.
Standard of care consisted of pain-relievers/fever-reducers, oral steroids, and
antibiotics. Both Indena S.p.A. and Pharmextracta S.p.A. funded the study (Di Pierro, Int J Gen Med 2021).
The second ongoing, open-label study conducted by the same researchers (and
also manufacturer-funded) has shown that, among people with early-stage
COVID-19, taking the same quercetin supplement three times daily for one week
followed by twice daily for a second week along with standard of care increased
the percentage of patients who were RT-PCR-negative for COVID-19 after the
first week compared to standard of care alone (76% vs. 9.5%). However, all
patients in both groups were negative by week three (Di Pierro, In J Gen Med 2021).
An
open-label study in Iran among 60 people diagnosed between December 2020 and
January 2021 with severe COVID-19, all of whom were treated
with remdesivir or favipiravir and half of whom also received 500 mg of
quercetin (Jarrow Formulas, USA) twice daily for the
first week of COVID-19 treatment, showed that quercetin treatment did not reduce
the number of patients admitted to the ICU, the duration of ICU admission, or
the number of deaths, nor did it increase the number of patients discharged,
although the duration of hospitalization following the first week of treatment
was about 1.5 days shorter for those who received quercetin compared to those
who did not. The rate of clinical symptoms, including cough, shortness of
breath, chest pain, headache, fever, runny nose, diarrhea, nausea and vomiting
were similar between the two groups, although fewer people in the quercetin
group experienced weakness and lethargy (Shohan, Eur J Pharmacol 2021).
Exercise
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).
Sleep
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).
Cancer
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.
Quality Concerns and
Tests Performed:
Neither
the Food and Drug Administration (FDA) nor any other federal or state agency
routinely tests supplements for quality prior to sale. ConsumerLab.com 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.
·
PipingRock.com Ultra Quercetin 650 mg - contained 557.3
mg of quercetin dihydrate per capsule instead of 650 mg.
Cost
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.
Top Picks:
Among
the Approved products, the following are our Top Picks for
quercetin.
All-around:
Top Pick: BulkSupplements.com 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.
Capsules:
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 ConsumerLab.com 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 ConsumerLab.com'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.
|
RESULTS OF CONSUMERLAB.COM TESTING OF QUERCETIN SUPPLEMENTS |
|||||
|
Product Name, Amount
Listed of Quercetin per Unit, Serving Size, and Suggested Daily Serving on
Label |
Claimed Amount and
Form of Quercetin |
--TEST RESULTS-- |
Cost For Daily
Suggested Serving On Label |
||
|
OVERALL RESULTS: |
Contained Labeled
Amount of Quercetin |
Did Not Exceed
Contamination Limits for Lead, Cadmium, Arsenic and Mercury |
|||
|
|
250 mg to 1,000 mg quercetin dihydrate |
APPROVED |
✔ |
✔ |
$0.05-$0.19 |
|
|
1,500 mg quercetin |
APPROVED |
✔ |
✔ |
$0.60 |
|
|
500 mg quercetin |
NOT |
Found only 438.6 mg
quercetin per daily serving |
✔ |
$0.30 |
|
|
650 mg quercetin dihydrate (equivalent to
580.8 mg quercetin) |
NOT |
Found only 557.3 mg
quercetin dihydrate per daily serving |
✔ |
$0.25 |
|
|
500 mg quercetin |
APPROVED |
✔ |
✔ |
$0.17 |
|
|
1,000 mg quercetin |
APPROVED |
✔ |
✔ |
$0.79 |
|
|
67 mg to 134 mg quercetin |
APPROVED |
✔ |
✔ |
$0.16-$0.31 |
|
|
|||||
|
Unless otherwise noted, information about the products
listed above is based on the samples purchased by ConsumerLab.com (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. |
|||||
|
Copyright ConsumerLab.com, LLC, 2019. All rights reserved.
Not to be reproduced, excerpted, or cited in any fashion without the express
written permission of ConsumerLab.com LLC. |
|||||
ConsumerTips™:
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, ConsumerLab.com 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. ConsumerLab.com will
periodically re-evaluate these products to ensure their compliance with ConsumerLab.com's standards.
Information on this site
is provided for informational purposes only. It is not an endorsement of any
product nor 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. Consumers should
inform their healthcare providers of the dietary supplements they take.
Latest Clinical Research Updates for Quercetin Supplements
12/07/2021
Does quercetin help
people with severe COVID-19? See what a recent study suggests in the Viral Infections section of our Quercetin
Supplements Review. Also see our Top Picks for quercetin.
11/05/2021
Find out if quercetin
helped people with early-stage COVID-19 in a recent study. Details are in
the Viral Infections section of our Quercetin
Supplements Review, which includes our test results and Top Picks among quercetin supplements.
3/31/2020
Quercetin has been
proposed as a treatment for a range of medical conditions, including COVID-19,
but is it safe? Find out what studies have found, including results of a
recent, small clinical trial, in the Concerns and Cautions section
of the Quercetin Supplements Review. Also see our Top Picks for
quercetin.
Related CL Answers (4)