- Allium cepa, American elder, apples, Artemisia abrotanum L, AS195 Folia vitis viniferae (red vine leaf extract), biflavonoids, bilberries, black currants, black tea, brassica vegetables (cabbage, broccoli, cauliflower, turnips, kale), buckwheat tea, citrus bioflavonoid, endive, flavones, flavonoids, flavonols, flavon(ol)-glycosides, Ginkgo biloba extract, ginkgo flavone glycosides, grapefruit, green tea, Hypericum perforatum L., isoflavones, isorhamnetin, isoquercitrin, kaemferol, meletin, Myrtaceae, naphthodianthrones, onion, parsley (Petroselinum crispum (Mill.) Nym.), phytodrug (QG-5), phytoestrogens, pine bark extract, polyphenol, Psidium guajava L. (Fam.), quercetin aglycone, quercetin chalcone, quercetin dimethyl-ethers, quercetin glucoside, quercetin glucuronides, quercetin rutin, quercetin rutinoside, red vine leaf extract, red wine, red wine phenolics, rhamnose molecule, rutin, Sambucas nigra L., sophretin, St. John’s wort, STW-3, Tycho, Venenkapseln, Venoruton (O-(beta-hydroxyethyl) rutosides (HR).
- Quercetin is a major flavonol, one of the almost 4,000 flavonoids (antioxidants) that occur in foods of plant origin, such as red wine, onions, green tea, apples, berries, and Brassica vegetables (cabbage, broccoli, cauliflower, turnips). Quercetin is also found in Gingko biloba, St. John’s wort, and American elder.
- Quercetin and rutin (another flavonol) are used in many countries as vasoprotectants (protects blood vessels) and are ingredients of numerous multivitamin preparations and herbal remedies. They occur mainly as glycosides, which means they are linked with various sugars. However, the ability of the body to absorb these compounds is questionable.
- Quercetin and other flavonols have a wide variety of biological effects, but the scientific evidence for use in the prevention or treatment of disease is weak. Quercetin has been considered as a therapy for cardiovascular diseases, high cholesterol, diabetic cataracts, inflammation, ischemic injury, chronic prostatitis, chronic venous insufficiency, gastrointestinal ulceration, hepatitis, allergies, asthma, viral infections, and hay fever.
- Review of the literature shows that there have been several studies on the association of quercetin and with the risk reduction for coronary heart disease and stroke, high blood pressure, cancers, and a few studies on other medical conditions. However, there is a lack of strong evidence to support any of these conditions.
These uses have been tested in humans or animals. Safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider.
Several of the effects of flavonoids that have been observed in laboratory and animal studies suggest that they might be effective in reducing cardiovascular disease risk. Studies in humans using polyphenolic compounds from red grapes showed improvement in endothelial function in patients with coronary heart disease. Antioxidant and cholesterol-lowering effects are proposed.
Population-based studies report that quercetin is associated with a reduced risk of coronary heart disease and stroke. Quercetin supplementation reduced blood pressure in hypertensive rodents and hypertensive humans.
Quercetin does not appear to affect changes in the immune system caused by intense exercise. However, it reduced the number of respiratory tract infections in people who participated in intense cycling. More research is needed.
Some research suggests that quercetin may help prevent pancreatic cancer in smokers. However, quercetin did not have this effect in non-smokers or former smokers. More research is needed to determine if quercetin is beneficial.
There is some evidence that quercetin may be useful for the treatment of chronic prostatitis and chronic pelvic pain syndrome. Further research is needed to confirm these results.
*Key to grades:
The below uses are based on tradition, scientific theories, or limited research. They often have not been thoroughly tested in humans, and safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider. There may be other proposed uses that are not listed below.
Adults (over 18 years old)
- It has been suggested that quercetin should be taken 20 minutes before meals. Quercetin has been ingested from onions, juice, black tea, and red wine. Doses of 100-500 milligrams of quercetin have been taken 2-3 times daily. It is available in tablet, capsule, or softgel form. Intravenous and intramuscular forms have been injected by a healthcare provider.
Children (under 18 years old)
- Insufficient available evidence to recommend.
The below doses are based on scientific research, publications, traditional use, or expert opinion. Many herbs and supplements have not been thoroughly tested, and safety and effectiveness may not be proven. Brands may be made differently, with variable ingredients, even within the same brand. The below doses may not apply to all products. You should read product labels, and discuss doses with a qualified healthcare provider before starting therapy.
- Avoid in individuals with a known allergy/hypersensitivity to quercetin by ingestion or skin contact. Eye, skin, gastrointestinal and/or respiratory tract irritation is possible.
Side Effects and Warnings
- Being a common food component, quercetin is generally safe and well tolerated at usual dietary intake. However it has been associated with headache, gastrointestinal effects, hematoma, and kidney toxicity.
- Intravenous injection has been associated with pain at the injection site that is dose related and can be controlled by reducing the rate of infusion. Intravenous administration of quercetin has also resulted in flushing, sweating, difficulty breathing (dyspnea), nausea, mild tingling of the extremities, lethargy, and vomiting.
- Concern had been expressed about the possible tumor-promoting effect of quercetin.
- Mild constipation and hair thinning were reported by two of 260 patients taking AS195, which contains red vine leaf extract that is rich in quercetin.
Pregnancy and Breastfeeding
- Insufficient available evidence.
The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.
Interactions with Drugs
- Based on laboratory study, platelet aggregation may be inhibited, which may increase bleeding risk.
- Based on laboratory study, quercetin may enhance the effects of the cancer chemotherapy drug busulphan. Quercetin may decrease liver and kidney damage caused by the cancer chemotherapy drug cisplatin. Quercetin may also affect the levels of cyclosporine.
- Quercetin may increase the effects of steroids or interact with estradiol and nifedipine by interfering with the way that the liver breaks them down.
- Laboratory studies also suggest that quercetin may affect certain hormone levels.
- Quercetin may interact with quinolone antibiotics like ciprofloxacin or levofloxacin.
- Based on laboratory studies, short-term use of quercetin may result in higher uptake or influx of ritonavir and erythromycin. Quercetin may also inhibit cortisol.
- Based on studies performed in animals and humans, quercetin supplementation may reduce blood pressure in people with hypertension.
Interactions with Herbs and Dietary Supplements
- Quercetin may interact with herbs or supplements that are broken down by the liver.
- Based on laboratory study, platelet aggregation may be inhibited that may increase bleeding risk.
- Quercetin is found in elder, St. John’s wort, parsley, green tea, and ginkgo, and thus there may be additive effects if taken together. Consumption of black currants, lingonberries, and bilberries increases quercetin levels in the blood.
- Papain or bromelain may increase the absorption of quercetin. Vitamin C may enhance the antioxidant activity of quercetin. Consumption of rutin-containing herbs may cause additive effects due to quercetin content.
Most herbs and supplements have not been thoroughly tested for interactions with other herbs, supplements, drugs, or foods. The interactions listed below are based on reports in scientific publications, laboratory experiments, or traditional use. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy.
- This information is based on a systematic review of scientific literature edited and peer-reviewed by contributors to the Natural Standard Research Collaboration ().
- Hibatallah J, Carduner C, Poelman MC. In-vivo and in-vitro assessment of the free-radical-scavenger activity of Ginkgo flavone glycosides at high concentration. J Pharm Pharmacol 1999;51(12):1435-1440.
- Hubbard GP, Wolffram S, Lovegrove JA, et al. Ingestion of quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in humans. J Thromb Haemost 2004;2(12):2138-2145.
- Janssen K, Mensink RP, Cox FJ, et al. Effects of the flavonoids quercetin and apigenin on hemostasis in healthy volunteers: results from an in vitro and a dietary supplement study. Am J Clin Nutr 1998;67(2):255-262.
- Kiesewetter H, Koscielny J, Kalus U, et al. Efficacy of orally administered extract of red vine leaf AS 195 (folia vitis viniferae) in chronic venous insufficiency (stages I-II). A randomized, double-blind, placebo-controlled trial. Arzineimittelforschung 2000;50(2):109-117.
- Knekt P, Isotupa S, Rissanen H, et al. Quercetin intake and the incidence of cerebrovascular disease. Eur J Clin Nutr 2000;54(5):415-417.
- Lekakis J, Rallidis LS, Andreadou I, et al. Polyphenolic compounds from red grapes acutely improve endothelial function in patients with coronary heart disease. Eur J Cardiovasc Prev Rehabil 2005 Dec;12(6):596-600.
- Mulholland PJ, Ferry DR, Anderson D, et al. Pre-clinical and clinical study of QC12, a water-soluble, pro-drug of quercetin. Ann Oncol 2001;12(2):245-248.
- Nieman DC, Henson DA, Davis JM, et al. Quercetin’s influence on exercise-induced changes in plasma cytokines and muscle and leukocyte cytokine mRNA. J Appl Physiol 2007 Nov;103(5):1728-35.
- Nieman DC, Henson DA, Gross SJ, et al. Quercetin reduces illness but not immune perturbations after intensive exercise. Med Sci Sports Exerc 2007 Sep;39(9):1561-9.
- Nothlings U, Murphy SP, Wilkens LR, et al. Flavonols and pancreatic cancer risk: the multiethnic cohort study. Am J Epidemiol 2007 Oct 15;166(8):924-31.
- Pal D, Mitra AK. MDR- and CYP3A4-mediated drug-herbal interactions. Life Sci 2006 Mar 27;78(18):2131-45.
- Schulz HU, Schurer M, Bassler D, et al. Investigation of pharmacokinetic data of hypericin, pseudohypericin, hyperforin and the flavonoids quercetin and isorhamnetin revealed from single and multiple oral dose studies with a hypericum extract containing tablet in healthy male volunteers. Arzneimittelforschung 2005;55(10):561-8.
- Schaefer E, Peil H, Ambrosetti L, et al. Oedema protective properties of the red vine leaf extract AS 195 (Folia vitis viniferae) in the treatment of chronic venous insufficiency. A 6-weel observational trial. Arzineimittelforschung 2003;53(4):243-246.
- Shoskes DA, Zeitlin SI, Shahed A, et al. Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial. Urology 1999;54(6):960-963.
- Theoharides TC, Sant GR. A pilot open label study of Cystoprotek in interstitial cystitis. Int J Immunopathol Pharmacol 2005;18(1):183-188.
Natural Standard developed the above evidence-based information based on a thorough systematic review of the available scientific articles. For comprehensive information about alternative and complementary therapies on the professional level, go to . Selected references are listed below.