Quercetin supplement benefit and side effects, a flavonoid used for prostate, allergy, blood vessel health, athletic performance
Drug interactions, influence on allergy and how to best use this product without adverse reactions
November 9 2017 by Ray Sahelian, M.D. 

Content in food, dietary intake
Quercetin is a plant pigment found in many foods such as onions, apples, berries, tea, grapes and red wine. It's not a nutrient, but is classified as a flavonoid. Once thought to be vitamins, flavonoids were given such names as vitamin P and vitamin C2. Like many other plant chemicals, it is sold as a supplement. Oral quercetin is relatively well absorbed, and it is metabolized mainly to isorhamnetin, tamarixetin and kaempferol.   

This nutrient often occurs in plants as glycosides, such as rutin (quercetin rutinoside) in tea. Quercetin and rutin are used in many countries for blood vessel health and are ingredients of numerous multivitamin preparations and herbal remedies. The average U.S. adult eating a normal, healthy diet consumes about 25 to 50 milligrams a day.

Health benefit, why take a pill?
Quercetin, a member of the bioflavonoids family, has anti-inflammatory, anti-atherogenic, and anti-hypertensive properties leading to the beneficial effects against cardiovascular diseases. Epidemiological studies report that this antioxidant flavonol found in apples, berries, and onions, is associated with reduced risk of coronary heart disease and stroke.

Allergy prevention or treatment
I have not come across any good human studies evaluating the influence of allergy by quercetin supplements. However, as a flavonoid, it has been shown in lab studies to blunt the effects of IgE mediated reactions and perhaps have anti histamine effects. There are several nutrients and herbs that have had some research in terms of their influence on allergies. These include urtica dioica, bromelain, acetylcysteine, and vitamin C.

Antioxidant benefit
Anticancer Research. 2009. Antioxidant effects of quercetin and coenzyme Q10 in mini organ cultures of human nasal mucosa cells.
Oxidative DNA damage is a known risk factor of head and neck cancer. Antioxidants, such as coenzyme Q10  and quercetin, a member of flavonoids present in red wine and tea, are thought to play a significant role in protecting cells from oxidative stress induced by reactive oxygen species. Our study provides considerable evidence that quercetin and CoQ10 have strong antioxidant effects on mucosal cells of the nasal turbinates.

Am J Physiol Lung Cell Mol Physiol. 2013. Quercetin acutely relaxes airway smooth muscle and potentiates β-agonist-induced relaxation via dual phosphodiesterase inhibition of PLCβ and PDE4. Sata show that the natural PDE4-selective inhibitor quercetin may provide therapeutic relief of asthma symptoms and decrease reliance on short-acting β-agonists.

Blood pressure, influence on hypertension
Quercetin exerts antihypertensive effects and reduces left ventricular hypertrophy, endothelial dysfunction, and the plasma and hepatic oxidative status in spontaneously hypertensive rats.
Those with hypertension may consider adding quercetin supplements to their regimen.

Br J Nutr. 2015. Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial. The polyphenol quercetin may prevent heart disease due to its antihypertensive and vasorelaxant properties. We investigated the effects of quercetin after regular intake on blood pressure (BP) in overweight-to-obese patients with pre-hypertension and stage I hypertension. In addition, the potential mechanisms responsible for the hypothesised effect of quercetin on BP were explored. Subjects (n 70) were randomised to receive 162 mg/d quercetin from onion skin extract powder or placebo in a double-blinded, placebo-controlled cross-over trial with 6-week treatment periods separated by a 6-week washout period. Supplementation with 162 mg/d quercetin from onion skin extract lowers ABP in patients with hypertension, suggesting a cardioprotective effect of quercetin. The mechanisms responsible for the BP-lowering effect remain unclear.

Quercetin reduces blood pressure in hypertensive subjects.
J Nutr. 2007. Edwards RL, Lyon T, Litwin SE, Rabovsky A. Division of Nutrition, University of Utah, Salt Lake City, UT, USA.
Quercetin supplementation reduces blood pressure in hypertensive rodents. The efficacy of quercetin supplementation to lower blood pressure in hypertensive humans has never been evaluated. We tested the hypothesis that quercetin supplementation reduces blood pressure in hypertensive patients. We then determined whether the antihypertensive effect of quercetin is associated with reductions in systemic oxidant stress. Men and women with prehypertension and stage 1 hypertension were enrolled in a randomized, double-blind, placebo-controlled, crossover study to test the efficacy of 730 mg quercetin/d for 28 d vs. placebo. Blood pressure at enrollment was 137/86 in prehypertensives and 148/96 in stage 1 hypertensive subjects. Blood pressure was not altered in prehypertensive patients after quercetin supplementation. In contrast, reductions in systolic, diastolic, and mean arterial pressures were observed in stage 1 hypertensive patients after quercetin treatment. These data are the first to our knowledge to show that quercetin supplementation reduces blood pressure in hypertensive subjects. Contrary to animal-based studies, there was no quercetin-evoked reduction in systemic markers of oxidative stress.

Q. I have hypothyroidism from to much radioactive treatment from the year of 1974, also I have hypoglycemia and hypertension and a little​ gout. Does quercetin effect any of these conditions?
   A. It is of benefit but does not solve the problems alone.

Blood vessel health
Pure dietary flavonoids quercetin and epicatechin augment nitric oxide products and reduce endothelin-1 acutely in healthy men.
Am J Clin Nutr. 2008. Loke WM, Hodgson JM, Proudfoot JM. School of Medicine and Pharmacology and the School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia, Perth, Western Australia.
Dietary flavonoids may improve endothelial function and ultimately lead to beneficial cardiovascular effects. The objective was to assess whether pure dietary flavonoids can modulate nitric oxide and endothelin-1 production and thereby improve endothelial function. A randomized, placebo-controlled, crossover trial in 12 healthy men was conducted to compare the acute effects of the oral administration of 200 mg quercetin, (epicatechin, or epigallocatechin gallate on nitric oxide, endothelin-1, and oxidative stress. Relative to water (control), quercetin and epicatechin resulted in a significant increase in plasma S-nitrosothiols, plasma nitrite, and urinary nitrate concentrations, but not in plasma nitrate or urinary nitrite. Epigallocatechin gallate did not alter any of the measures of nitric oxide production. Quercetin and epicatechin resulted in a significant reduction in plasma endothelin-1 concentration.

Quercetin has anti-tumor potential in laboratory studies, and has potential to be useful in various cancers including pancreatic cancer, however human trials are difficult to find.
Food-derived flavonoid quercetin, widely distributed in onions, apples, and tea, is able to inhibit growth of various cancer cells indicating that this compound may be considered as a good candidate for anticancer therapy.

Indian J Exp Biol. 2005. Antimutagenic and antioxidant / prooxidant activity of quercetin.
The present study has been performed to evaluate the antimutagenic activity of quercetin, ascorbic acid and their combination against an oxidative mutagen. Quercetin and ascorbic acid showed significant effect. Quercetin when combined with ascorbic acid showed an increase in the antimutagenic activity. In vitro antioxidant activity of quercetin was better than ascorbic acid in all the test systems used. The study indicated that the antimutagenic activity of quercetin was not solely accountable by its antioxidant nature. However, in vitro free radical scavenging activity of quercetin correlated well with the antimutagenic activity.

Common cold, and immune health
Quercetin, a naturally occurring, powerful antioxidant found in red grapes, red wine, red apples, green tea and broccoli, appears to be one of the first plant compound proven in a double-blind, randomized, placebo-controlled clinical trial to reduce susceptibility to viral illnesses. Dr. David Nieman, a professor at Appalachian State University was the researcher and presented his findings at the southeastern regional meeting of the American College of Sports Medicine in Charlotte, N.C. Participants in the study -- long-distance cyclists -- ingested 1,000 milligrams of pure quercetin, combined with vitamin C and niacin.

Quercetin reduces illness but not immune perturbations after intensive exercise.
Med Sci Sports Exerc. 2007. Nieman DC, Henson DA, Gross SJ, Jenkins DP, Davis JM, Murphy EA, Carmichael MD, Dumke CL. Department of Health, Leisure, and Exercise Science, Appalachian State University, Boone, NC.
To investigate the effects of quercetin supplementation on incidence of upper respiratory tract infections and exercise-induced changes in immune function. Trained male cyclists were randomized to quercetin or placebo  groups and, under double-blind procedures, received 3 weeks quercetin (1000 mg a day) or placebo before, during, and for 2 weeks after a 3-d period in which subjects cycled for 3 hours a day at approximately 57% Wmax. Quercetin versus placebo ingestion did not alter exercise-induced changes in several measures of immune function, but it significantly reduced upper respiratory tract infection incidence in cyclists during the 2-wk period after intensified exercise.

Endurance, exercise
A study included twelve volunteers who were randomly assigned to one of two treatments. The first treatment was 500 mg of quercetin twice daily dissolved in Tang and the second treatment group received only Tang. Supplementation resulted in a modest 4 percent increase in VO2max and a substantial 13 percent increase in ride time endurance. These findings suggest that it improves endurance in as little as seven days, which may have important implications for enhancement of athletic performance. Davis JM, Carlstedt CJ The dietary flavonoid quercetin increases VO(2max) and endurance capacity. Int J Sport Nutr Exerc Metabolism. 2010.

Int J Prev Med. 2013. The effects of quercetin supplementation on body composition, exercise performance and muscle damage indices in athletes. Flavonoids comprise a large group of plant metabolites, 6,000 of which have been identified to date. Some studies have shown the increased aerobic performance and maximal oxygen consumption (VO(2max)) and therefore fitness following quercetin intake as a result of elevated number of intracellular mitochondria caused by the flavonoid. This double-blind clinical trial comprised 60 male students having an athletic history of at least 3 years. The individuals were assigned into four groups of equal number. The first to fourth groups received a 500 mg supplemental quercetin capsule plus a 250 mg vitamin C pill, a 500 mg supplemental quercetin capsule plus a 250 mg placebo vitamin C pill, a 500 mg placebo quercetin capsule plus a 250 mg vitamin C pill, and a 500 mg placebo quercetin capsule plus a 250 mg placebo vitamin C pill, respectively, daily for 8 weeks. The participants were asked to continue their routine diet and physical activity during the study and they were monitored through phone calls or text messages. Lean body mass, total body water, basal metabolic rate, and total energy expenditure increased significantly in the quercetin group after intervention.

Erectile dysfunction, impotence?
Phytomedicine. 2006. FRS 1000, an extract of red onion peel, strongly inhibits phosphodiesterase 5A (PDE 5A). As part of our ongoing search for flavonoids that are bioactive in humans, it was determined that FRS 1000, a beverage containing flavonoids extracted from onion peel, showed unexpected improvement of male sexual function. An in vitro enzyme assay clearly showed that FRS 1000 has a strong phosphodiesterase 5A (PDE 5A) inhibitory activity, which is considered to be important for treatment of erectile dysfunction. Detailed assays of each major ingredient indicated that the antioxidative flavonoid quercetin was responsible for the activity. Results also suggested that PDE 5A inhibition is not directly related to the free radical scavenging activity of flavonoids.

J Biosci Bioeng. 2011. Antioxidant treatment with quercetin ameliorates erectile dysfunction in streptozotocin-induced diabetic rats.

Gout, uric acid level lowering
Br J Nutr. 2016. Quercetin lowers plasma uric acid in pre-hyperuricaemic males: a randomised, double-blinded, placebo-controlled, cross-over trial. Quercetin, a flavonoid found in high levels in onions, tea and apples, inhibits xanthine oxidoreductase in vitro, the final step in intracellular uric acid production, indicating that quercetin might be able to lower blood uric acid in humans. We determined the effects of 4 weeks of oral supplementation of quercetin on plasma uric acid, blood pressure and fasting glucose. This randomised, double-blinded, placebo-controlled, cross-over trial recruited twenty-two healthy males (19-60 years) with baseline plasma uric acid concentration in the higher, but still considered healthy. The intervention included one tablet containing 500 mg quercetin daily for 4 weeks, compared with placebo, with a 4-week washout period between treatments. After quercetin treatment, plasma uric acid concentrations were significantly lowered without affecting fasting glucose, urinary excretion of uric acid or blood pressure. Daily supplementation of 500 mg quercetin, containing the bioavailable amount of quercetin as present in approximately 100 g red onions, for 4 weeks, significantly reduces elevated plasma uric acid concentrations in healthy males.

Heart disease, cardiovascular health
Quercetin may benefit those with heart disease. it inhibits the proliferation and migration of aortic smooth muscle cells, inhibits platelet aggregation, improves the health of the endothelium and protects vascular endothelial cells against oxidative and proinflammatory insults.

Ingestion of onion soup high in quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in man: a pilot study.
Br J Nutr. 2006. School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, Berkshire, RG6 6AL, UK.
Those who consume a diet rich in quercetin containing foods may have a reduced risk of cardiovascular disease. In vitro and ex vivo studies have observed inhibition of collagen-induced platelet activation. The aim of the present study was to investigate the possible inhibitory effects of quercetin ingestion from a dietary source on collagen-stimulated platelet aggregation. Subjects ingested a soup containing either a high or a low amount of quercetin. The high- quercetin soup contained 69 mg total quercetin compared with the low- quercetin soup containing 5 mg. Plasma quercetin concentrations were significantly higher after high- quercetin soup ingestion than after low. The ingestion of quercetin from a dietary source of onion soup could inhibit some aspects of collagen-stimulated platelet aggregation and signalling. This further substantiates the epidemiological data suggesting that those who preferentially consume high amounts of quercetin containing foods have a reduced risk of thrombosis and potential cardiovascular disease risk.

Long-Term Prognosis in Patients With Acute ST-Segment Elevation Myocardial Infarction and Acute Heart Failure: Prospective Randomised Study. Poster Session 2. This study was presented at the Heart Failure 2008 Congress.
   Intravenous quercetin reduces infarct size, improves left ventricular function, and significantly lowers cardiac mortality in patients with ST-segment elevation myocardial infarction (STEMI).  In patients with acute STEMI, reduced survival has been linked to infarct size and left ventricular enlargement. Alexander Parkhomenko, MD, Institute of Cardiology, National Scientific Centre, Kiev, Ukraine, demonstrations that this IV, carrier form of the nitric-oxide remodelling, lipoxygenase inhibitor quercetin has anti-ischemic actions and limits left ventricular remodeling provided promise for its addition to standard therapy for these patients. Prof. Alexander Parkhomenko concludes, "In these patients with systolic heart failure, we have good data concerning myocardial protection, even while using all of the modern approaches. Reducing the final infarct size, we see an improvement in function; the injection fraction; and, extremely important, we have a very good long-term survival rate improvement. Furthermore, quercetin is a safe inhibitor; it is not toxic."

Dietary flavonoids improve endothelial function and ultimately lead to beneficial cardiovascular effects. Together with its blood thinning and anti-inflammatory effects, the latter mainly mediated through the inhibition of cytokines and nitric oxide, quercetin is a good supplement choice for those who wish to improve their blood vessel and heart health. Arjuna herb is also used by Ayurvedic doctors for heart health.

Liver health, prevention or treatment of acetaminophen toxicity
Pathophysiology. 2014. Quercetin protects against acetaminophen-induced hepatorenal toxicity by reducing reactive oxygen and nitrogen species. Our results suggest that the protective role of quercetin in the prevention of acetaminophen induced hepatorenal toxicity in rats was associated with the decrease of oxidative and nitrosative stress in hepatic and renal tissues as well as its capacity to improve the mitochondrial energy production. However, clinical studies are warranted to investigate such an effect in human subjects.

Niacin flush reduction
A quercetin containing supplement reduces niacin-induced flush in humans.
Int J Immunopathol Pharmacol. 2008; Kalogeromitros D, Makris M, Aggelides X, Kempuraj D, Theoharides TC. Allergy Clinical Research Center, Allergy Section, Attikon Hospital, University of Athens Medical School, Athens, Greece.
Coronary artery disease is associated with increased serum levels of cholesterol, triglycerides and LDL, but low levels of HDL. The most potent agent capable of reversing this trend is the vitamin nicotinic acid (niacin). However, compliance even with extended-release preparations and addition of acetylsalicylic acid (ASA) is hampered by the development of a feeling of erythema and burning ("flush"), especially on the face. We recently showed that the natural flavonoids quercetin and luteolin can eliminate "flush", as well as inhibit both niacin-induced plasma prostaglandin D2 (PGD2) and serotonin increase in an animal model. We conducted a pilot clinical study in humans. Four normal male subjects received (a) 1 g immediate release niacin either alone or after (b) the dietary formulation (Algonot-plus) containing 150 mg quercetin per capsule. Subjects completed a visual scale symptom assessment. Erythema and burning sensation scores were were reduced. Quercetin also inhibited methylnicotinate-induced human mast cell PGD2 release. These preliminary results suggest that quercetin could reduce niacin-induced flush in humans.

Prostate health
Preliminary  studies indicate that quercetin my inhibit the proliferation of androgen-independent human prostatic tumor cells. In rats, in combination with finasteride, it reduces prostate weight. Whether therapy alone benefits those with benign prostate hyperplasia is not clearly known at this time. Many flavonoids act as aromatase inhibitors.

Prostate Power Rx has several key Ingredients for support of normal prostate size and is carefully formulated with important herbs and nutrients to provide optimal prostate health.
Saw Palmetto extract standardized to contain 45% fatty acids - serenoa repens fruit
Stinging-Nettle 4:1 extract (urtica diocia root)
Rosemary 4:1 extract
Beta Sitosterol
Pygeum afrcianum extract
Genistein, and Lycopene.

Q. I am a researcher in Australia. I note that quercetin interacts with finasteride, which may affect that product's effect in reducing hair loss: "Finasteride alone caused a significant decrease in serum DHT level and prostate weight. Co-administration of quercetin with finasteride prevented the finasteride-induced decrease in serum DHT levels." My question is; in your opinion, will this impact on the benefit of finasteride's reduction in hair loss by reducing serum DHT?
  A.  I have a summary of the study below. The amount of quercetin given the rats per body weight is much higher than what a person would normally ingest. I think rodent studies are helpful but I do not rely on them much since over the years I have seen very different or even completely opposite results in humans. The 100 mg per kg of body weight given to a rat would be about 7000 mg in humans. Even if 3 capsules of quercetin are taken, this would amount to 1000 mg, seven times less than in the rodent. This could make a significant difference. Add to this the different hormonal status in humans compared to rodents, different diets, sleep patterns, liver metabolism, etc. All this gives too many variables to make heads or tails (excuse the pun) from the results of this study. If I interpret the results of this study further, you can see that the different dosage of quercetin led to different levels of DHT, one dose let to a 125% increase, while another dosage led to a decrease to 73 percent of the control.

Reduction of rat prostate weight by combined quercetin - finasteride treatment is associated with cell cycle deregulation.
J Endocrinol. 2004.
Benign prostate hyperplasia and prostate cancer are major public health problems. We report herein that daily treatment of male rats with 50, 100 or 150 mg quercetin per kg body weight. Concomitantly, serum testosterone levels were increased by 1.79-, 1.83- and 3.48-fold, while serum dihydrotestosterone (DHT) levels were 125%, 92% and 73% of the control. A slight increase in prostate weight coupled with dilated prostate lumens full of secretory materials were observed. Finasteride alone caused a significant decrease in serum DHT level and prostate weight. Co-administration of quercetin with finasteride prevented the finasteride-induced decrease in serum DHT levels but significantly enhanced the reduction in wet prostate weight, which was reduced by 26.9% in finasteride-treated animals to 31.8%, 40.0% and 48.2% after finasteride given together with the three doses of quercetin. The combined treatment altered cell cycle-regulated proteins in a wide spectrum. In conclusion, quercetin - finasteride treatments caused wide cell cycle deregulation in rat prostates, which, in turn, decreased the proliferation rate, changed the secretion activities of epithelial cells and resulted in a marked reduction in wet prostate weight. The results suggest that quercetin synergizes with finasteride to reduce the wet prostate weight through a cell cycle-related pathway, which may be androgen independent.

Kidney disease
Quercetin reduces damage to kidneys in rats given a toxin or a chemotherapy drug such as cisplatin.

Protective Effect of Quercetin on the Evolution of Cisplatin-Induced Acute Tubular Necrosis.
Kidney Blood Press Res. 2004.
The bioflavonoid quercetin may be a potential alternative to reduce cisplatin-induced nephrotoxicity.

Drug Chem Toxicol. 2004. Quercetin, a bioflavonoid, attenuates ferric nitrilotriacetate-induced oxidative renal injury in rats.

Liver health
The liver of rats given toxins was much better protected when they were pre-treated with quercetin.

Phytomedicine. 2004. Studies on the protective effects of caffeic acid and quercetin on chemical-induced hepatotoxicity in rodents. Our results indicate that caffeic acid and quercetin exhibited hepatoprotective activity possibly through multiple mechanisms.


I was made aware of this study by Dr. David C. Leopold, MD  who is at Scripps Center for Integrative Medicine in La Jolla, California.

Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial.
Urology. 1999. Shoskes DA, Zeitlin SI, Shahed A. Institute for Male Urology, Encino, California, USA.
The National Institutes of Health (NIH) category III chronic prostatitis syndromes (nonbacterial chronic prostatitis and prostatodynia) are common disorders with few effective therapies. Thirty men with category IIIa and IIIb chronic pelvic pain syndrome were randomized in a double-blind fashion to receive either placebo or the bioflavonoid quercetin 500 mg twice daily for 1 month. The NIH chronic prostatitis symptom score was used to grade symptoms and the quality-of-life impact at the start and conclusion of the study. In a follow-up unblind, open-label study, 17 additional men received 1 month of a supplement containing quercetin, as well as bromelain and papain (Prosta-O), which enhance bioflavonoid absorption. Patients taking placebo had a mean improvement in NIH symptom score from 20 to 18 (not significant), while those taking the bioflavonoid had a mean improvement from 21 to 13. Twenty percent of patients taking placebo and 67% of patients taking the bioflavonoid had an improvement of symptoms of at least 25%. In the 17 patients who received Prosta-Q in the open-label study, 82% had at least a 25% improvement in symptom score. Therapy with the bioflavonoid quercetin is well tolerated and provides significant symptomatic improvement in most men with chronic pelvic pain syndrome.

Is quercetin effective for prostatitis?
   One study shows it helps prostatitis, a prostate infection or inflammation. Quercetin has potential as one nutrient out of many to be helpful in prostate enlargement. The dose of quercetin used in the prostatitis study was 500 mg twice daily. We will need a few more studies regarding the role of quercetin in the treatment of prostatitis to have a clearer idea of the benefits of this supplement for this condition.

Schizophrenia and brain tissue
Quercetin has potential for the treatment of neuroleptic-induced extrapyramidal side effects, such as from the drug haloperidol. Quercetin is a powerful antioxidant that may protect brain cells from damage.

Quercetin, a bioflavonoid, reverses haloperidol-induced catalepsy.
Methods Find Exp Clin Pharmacol. 2004.
Neuroleptics are extensively used in the treatment of schizophrenia and other affective disorders. Unfortunately their use is often associated with distressing side effects involving the extrapyramidal tract, such as Parkinsonism and tardive dyskinesia. Neuroleptic-induced catalepsy has long been used as a model for extrapyramidal side effects such as Parkinsonian-like bradykinesia associated with antipsychotic use in humans. In the present study, haloperidol was administered to mice to induce catalepsy. Pretreatment with quercetin dose-dependently reduced the catalepsy score in haloperidol-treated animals. The findings of the present study strongly suggest that quercetin can be screened as a potential drug candidate or as an adjuvant for the treatment of neuroleptic-induced extrapyramidal side effects.

Skin health and radiation damage
Radiat Res. 2013. Quercetin inhibits radiation-induced skin fibrosis in mice.

Statin drug use
In an article from 2011, 'Quercetin is a potential rationale therapeutic choice in order to treat statin myotoxicity' Joffrey Zoll, PhD, Jamal Bouitbir, Anne-Laure Charles, Bernard geny say, "We fully believe that quercetin or more largely some efficient natural antioxidant molecules targeting mitochondria should be beneficial for patients with statin-induced myalgias or myopathy."

Chronic ingestion of high doses of quercetin will decrease the bioavailability of simvastatin to a significant extent. Eur J Pharm Sci. 2009. Effects of the flavonol quercetin on the bioavailability of simvastatin in pigs.

Tylenol toxicity
Pathophysiology. 2014. Quercetin protects against acetaminophen-induced hepatorenal toxicity by reducing reactive oxygen and nitrogen species. Results suggest that the protective role of quercetin in the prevention of APAP-induced hepatorenal toxicity in rats was associated with the decrease of oxidative and nitrosative stress in hepatic and renal tissues as well as its capacity to improve the mitochondrial energy production. However, clinical studies are warranted to investigate such an effect in human subjects.

Quercetin side effects, is the supplement safe?
At this time quercetin side effects have been reported in the medical literature. I have personally taken 3 capsules of quercetin and bromelain for 3 days in a row without side effects. Adverse reacations with long term supplementation over several years is not known. Thyroid function could be influenced.

Food Chem Toxicol. 2014. The flavonoid quercetin inhibits thyroid-restricted genes expression and thyroid function. Quercetin is the most abundant flavonoid present in a broad range of fruit and vegetables. Furthermore, quercetin is available as dietary supplements that are based on its antioxidant, antiproliferative and anti-inflammatory properties. However, concerns have been raised about the potential toxic effects of excessive intake, and several studies have demonstrated that flavonoids can interfere with thyroid function. In a previous report, we showed that quercetin inhibits thyroid-cell growth and iodide uptake. The latter effect was associated with down-regulation of sodium/iodide symporter gene expression. In the present study, we have evaluated the effects of quercetin on the expression of other thyroid-restricted genes, and we show that quercetin decreases the expression of the thyrotropin receptor, thyroid peroxidase and thyroglobulin genes. We further investigated the inhibitory effects of quercetin on thyroid function in vivo through evaluation of radioiodine uptake in the Sprague-Dawley rat, which was significantly decreased after 14days of quercetin treatment. These data confirm that quercetin can act as a thyroid disruptor, and they suggest that caution is needed in its supplemental and therapeutic use.

Quercetin is readily absorbed from the intestinal tract and blood levels rise as the dosage of the quercetin supplement increases. Quercetin is commonly present as a glycoside and is converted to glucuronide / sulfate conjugates during intestinal absorption and conjugated metabolites are found in circulating blood.

Quercetin and drug, medication interactions
Quercetin influences activities of several drug metabolizing enzymes including CYP1A2, CYP2A6, NAT2 and XO.

Short-term effect of quercetin on the pharmacokinetics of fexofenadine, a substrate of P-glycoprotein, in healthy volunteers.
Eur J Clin Pharmacol. 2009. Department of Clinical Pharmacology and Toxicology, Anam Hospital, Korea University College of Medicine, Sungbuk-gu, Seoul, Korea.
The aim of the present study was to assess whether quercetin exhibited any inhibitory effect on P-glycoprotein (P-gp)-mediated drug disposition in humans using fexofenadine as a P-gp substrate. Twelve healthy subjects were enrolled in the study and treated daily for 7 days with 500 mg quercetin or placebo 3 times a day. On day 7, a single dose of 60 mg fexofenadine was administered orally. Plasma and urinary fexofenadine concentrations were measured, and pharmacokinetic differences between placebo and quercetin phases were assessed. The mean plasma concentrations of fexofenadine were significantly increased after quercetin treatment compared to those of the placebo phase. The results of the present study showed that short-term use of quercetin elevated the plasma concentrations of fexofenadine, probably by the inhibition of P-gp-mediated efflux in humans.

Q. Can you comment on the interaction of quercetin and clopidogrel? Am I correct in understanding that it significantly increases the platelet inhibition of clopidogrel.

Arch Pharm Res. 2012. Pharmacokinetic interactions of clopidogrel with quercetin, telmisartan, and cyclosporine A in rats and dogs. Following the oral administration of clopidogrel with or without the P-gp inhibitors, quercetin (250 mg/kg), telmisartan (8 mg/kg), and cyclosporine A (10 mg/kg), in rats and dogs, the plasma concentration-time profiles of clopidogrel carboxylic acid, a surrogate marker for the bioavailability of clopidogrel, were determined. Co-administration of the quercetin, telmisartan and cyclosporine A significantly increased the area under the curve and peak plasma concentration of clopidogrel carboxylic acid in rats. However, in dogs, the plasma concentrations of clopidogrel carboxylic acid were not considerably changed by the coadministration of three different kinds of P-gp inhibitors. These findings suggest potential interaction of clopidogrel with quercetin, telmisartan, and cyclosporine A, although there are differences between animal models.

Food content of flavonoids
The average U.S. adult eating a normal, healthy diet consumes about 25 to 50 milligrams of quercetin a day. Studies were conducted on the flavonoids (myricetin, quercetin, kaempferol, luteolin, and apigenin) contents of 62 edible tropical plants. The highest total flavonoids content was in onion leaves (1497 mg/kg quercetin, 391 mg/kg luteolin, and 832 mg/kg kaempferol), followed by Semambu leaves (2041 mg/kg), bird chili (1663 mg/kg), black tea (1491 mg/kg), papaya shoots (1264 mg/kg), and guava (1128 mg/kg). The major flavonoid in these plant extracts is quercetin, followed by myricetin and kaempferol. Luteolin could be detected only in broccoli (74 mg/kg dry weight), green chili (33 mg/kg), bird chili (1035 mg/kg), onion leaves (391 mg/kg), carrot (37 mg/kg), white radish (9 mg/kg), local celery (80 mg/kg) and dried asam gelugur (107 mg/kg). Apigenin was found only in Chinese cabbage (187 mg/kg), bell pepper (272 mg/kg), garlic (217 mg/kg), French peas (176 mg/kg), snake gourd (42 mg/kg), guava (579 mg/kg), wolfberry leaves (547 mg/kg), local celery (338 mg/kg), and kadok (34 mg/kg). In vegetables, quercetin glycosides predominate, but glycosides of kaempferol, luteolin, and apigenin are also present. Fruits contain almost exclusively quercetin glycosides, whereas kaempferol and myricetin glycosides are found only in trace quantities.

Consumption of black currants, lingonberries and bilberries increases serum quercetin concentrations.
Eur J Clin Nutr. 2003.

Content of the flavonols quercetin, myricetin, and kaempferol in 25 edible berries.
J Agric Food Chem. 1999.
Sixteen species of cultivated berries and nine species of wild berries were collected in Finland. Quercetin was found in all berries, the contents being highest in bog whortleberry (158 mg/kg, fresh weight), lingonberry (74 and 146 mg/kg), cranberry (83 and 121 mg/kg), chokeberry (89 mg/kg), sweet rowan (85 mg/kg), rowanberry (63 mg/kg), sea buckthorn berry (62 mg/kg), and crowberry (53 and 56 mg/kg). Amounts between 14 and 142 mg/kg of myricetin were detected in cranberry, black currant, crowberry, bog whortleberry, blueberries, and bilberry. Kaempferol was detected only in gooseberries (16 and 19 mg/kg) and strawberries (5 and 8 mg/kg). Total contents of these flavonols (100-263 mg/kg) in cranberry, bog whortleberry, lingonberry, black currant, and crowberry were higher than those in the commonly consumed fruits or vegetables, except for onion, kale, and broccoli.

I came across your website after doing a search dot 'quercertin sleepy'. I couldn't see anything further though in the side effects heading but I just wanted to raise the point that after using this supplement with food, I always had a drowsy effect which would conk me out for a couple hours at least. I do have a questionable thyroid function though.

Quercetin research studies
An apple a day really does keep the doctor away, thanks to strong antioxidants that fight cell damage. Rat brain cells exposed to the antioxidant quercetin resisted damage much better than those not treated, a team at Cornell University in New York found. The researchers say their study adds strength to the theory that the risk of developing Alzheimer's and similar brain diseases might be reduced by eating plenty of fresh fruits and vegetables. Writing in the December 1st issue of the Journal of Agricultural and Food Chemistry, the Cornell team said they soaked rat brain cells in either quercetin or vitamin C -- another potent antioxidant. The cells were then exposed to hydrogen peroxide to mimic the type of oxidative cell damage that is believed to occur with Alzheimer's disease. Brain cells that were treated with quercetin had significantly less damage than the cells treated with vitamin C and cells that were not treated with antioxidants.

Plant compounds may curb cancer growth
Several studies have shown that a group of antioxidant compounds found in grapes, green tea, soybeans and wine may lower the risk of a range of cancers, but exactly how these powerful compounds work has remained unclear. Now, researchers report that a plant-derived polyphenol can slow the growth of pancreatic cancer cells in mice and curb the spread of cells by triggering a series or reactions that causes the cells to self-destruct, a process known as apoptosis. In the study, the researchers investigated the effects of quercetin -- a type of antioxidant polyphenol commonly found in apples -- in mice given injections of human pancreatic cells. The mice used in the study are a specially bred strain that lacks an immune system, so that they quickly grow tumors when cancer cells are injected. Quercetin caused apoptosis, decreased the growth of the main tumor and inhibited the spread of malignant cells, the researchers report. Mice treated with quercetin survived for about 75 days, compared with 67 days in mice not given the compound. Genistein, another type of polyphenol compound found in soy, also prevented the spread of cells and inhibited the growth of the primary tumor in mice while resveratrol, found in wine and grapes, caused apoptosis in laboratory-grown cells, according to the report. International Journal of Cancer 2002:98;761-769.

Q. What is the difference between quercetin and quercetin chalcone?
   A. I don't know for sure. Chalcones are also intermediates in the biosynthesis of flavonoids, and quercetin chalcone may just be a form slightly different biochemically.

Q. As one professional to another, I would appreciate your clinical advice. Some studies indicate quercetin can exert a net pro-inflammatory response (albeit at high doses in combination with other bioflavinoids). Some researchers have claimed quercetin might cause inflammatory responses involving the PIP, MIP and Dip joints of the hands and feet; some claim an altered uric acid metabolism. Have you any experience in this regard? I have started quercetin at 250 mg bid (along with saw palmetto, zinc/copper and nettle leaf) to balance my DHT-T-E ratio and have experienced a new onset of said joint effects.
   A. I have not come across any studies and do not have any personal feedback from quercetin users regarding the association between quercetin and joint pain.

Q. I have been taking a formula which contains 250 MG of quercetin. I have been taking 4 pills a day. It works great for allergies but I recently found out I am pregnant. I came across some warnings of birth defects. Of course I am very upset and hoping you might have some access to studies or info regarding the use of quercetin during conception. I am only 5 weeks pregnant.
   A. I have not seen any human studies regarding the use of quercetin supplements during pregnancy. I found this rodent study which appears to be reassuring regarding implantation. The human dosage often taken is about 10 to 20 mg quercetin per kilo. Your doctor needs to make the final decision regarding continuing or stopping the quercetin.

Q. Let me again thank you for the great work you do. Whenever I encounter new research or information on a supplement, my first stop is your website, which I think is balanced and trustworthy. I was especially interested in the research on quercetin and upper respiratory tract infections. This past year I started to get serious about fitness and have a routine that is quite intense. Although it's helped me look and feel great (and took my blood pressure from borderline high to perfect), I had a winter filled with colds and other sinus problems, which I attribute in hindsight to overdoing it on exercise. Although I've slowed down a little, I don't want to stop being at a top fitness level. Thus, I'm quite interested in the study about quercetin and colds. My question is this: do you believe there would be an ideal time (e.g. before, during, or after workouts) to take the quercetin supplement to get the most antiviral benefit on days of intense training?
   A. I have not seen studies evaluating the timing of quercetin supplement intake and relationship to viruses and colds. I don't think timing is a major influence but I could be wrong. The proper use of vitamin C and zinc lozenges could reduce the frequency or severity of a cold. Deep sleep is utmost importance in maintaining a healthy immune system. 

I just read on your website about quercetin products. About 3 weeks ago I broke out in hives all over my body. I have been on 24hr antihistamines every since. There were only two things that I can think of that changed in my routine / diet. The first was that I took a Greens plus Tropical flavoured smoothie one morning and the other being I started to take quercetin capsules 500mg. I had taken 1 tablet each day for two days before I first noticed the histamine reaction. Since I had taken the Greens+ regular flavoured smoothie regularly for months with no issues, I figured the reaction would more likely be to the quercetin. I have not take either since the reaction began. I have read a little about the possibility of being allergic to quercetin. Are you familiar with this? The product is AOR Clasic Series, Quercetin 3,3',4',5,7-Pentahydroxyflavone, contains 500mg of quercetin capsule - hypromellose, sorbitol, silicon dioxide, water.
    This is the first time we have had an email regarding a possible allergic reaction to quercetin products.

I read that quercetin can be helpful for people with interstitial cystitis. I want to try it but read about interactions between quercetin and Estrace. I take 1 mg. estrace daily. Is there any problem with taking the estrace with 1500 mg. of quercetin daily?
    I have not seen studies regarding the interaction of this nutrient and estradiol.

I am currently taking one tablet of 500mg of quercetin supplement daily for about 9 months per year for the last two years. I would like to know about possible side effects of such an intake for a prolonged period of time. Since I am a marathon runner, I would also be interested in possible benefits in the athletic sector of this dosage or any other dosage that would be safe. Moreover, is it meaningful to search for the pros and cons of synthetic quercetin capsules versus natural, or is everything relatively safe?
   No long term studies have been done with daily use, but it seems to be safe. Nevertheless it is a good idea to take breaks once in a while since we don't know everything yet about its long term use. If the actual chemical is quercetin in the capsule, it does not matter what the source is.

Dietary supplements over the counter
There are a number of nutritional supplement companies that sell
Quercetin, some by itself, others combined with bromelain, vitamin C, or other nutrients.

Quercetin 500 mg per 2 capsules (Dimorphandra mollis) (fruit). It is not clear from the supplement fact panel of this product whether it has 500 mg of quercetin per two capsules or whether it has 500 mg of the fruit with some of it being quercetin.
500 mg, 120 capsules
500 mg 60 tablets.

Activated Quercetin and Bromelain by Source Naturals
Activated quercetin with bromelain is a unique bioflavonoid derived product from plant sources. In human cell culture studies, it has been shown to inhibit histamine release. Additional research needs to be conducted to increase our awareness of the benefits in humans. Bromelain is a pineapple enzyme.

Supplement facts:
Vitamin C (as magnesium ascorbate)
Magnesium (as magnesium ascorbate)
Quercetin 1 gram per 3 capsules, is a potent antioxidant

MRM Quercetin 500 mg each pill
Buy Quercetin 500 mg

Supplement Facts
Serving Size: 1 Vegetarian Capsules
Servings Per Container: 60
  Amount per Serving % Daily Value*
Quercetin 500 mg
L-lysophosphatidyl Choline 40 mg
*Percent Daily Values based in a 2,000 calorie diet.
†Daily Value not established.

Other products to buy online
Now Foods, Quercetin with Bromelain, 240 Veg Capsules
Solaray, Quercetin, 500 mg, 90 VegCaps

Buy Quercetin
Activated quercetin with bromelain is a unique bioflavonoid derived product from plant sources. In human cell culture studies, it has been shown to inhibit histamine release. Additional research needs to be conducted to increase our awareness of the benefits in humans. Bromelain is a pineapple enzyme.