Related Terms

• 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).

Background

• 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.

Scientific Evidence

Tradition/Theory

The below uses are based on tradition or scientific theories. 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 health care professional.

• Allergic rhinitis, allergies, anti-aging, antihistamine, anti-inflammatory, antioxidant, antispasmodic, anti-thrombotic, arthritis, antiviral, asthma, atherosclerosis (hardening of the arteries), autoimmune diseases, brain tumors, breast cancer prevention, cancer treatment, capillary fragility, cataract prevention, cerebrovascular disease, chronic venous insufficiency, coronary heart disease, cutaneous blood flux, diabetes mellitus, diarrhea (acute), depression, DNA damage, draft healing, eczema, edema, fibromyalgia, gout, hepatitis, herpes, high cholesterol, hives, ileostomy use, inflammation, interstitial cystitis, kidney toxicity, local anesthesia, lung problems, macular degeneration, mucosal mast cell inhibition, osteoporosis, peptic ulcers, platelet aggregation, prostate cancer prevention, protection against chemotherapy side effects, retinopathy, rheumatoid arthritis, schizophrenia, systemic lupus erythematosus (SLE), ulcers, vasoprotective, viral infections, vision problems (age-related).

Dosing

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.

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.

Safety

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.

Allergies

• 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.

Interactions

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.

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.

Author Information

• This information is based on a systematic review of scientific literature edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

References

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 www.naturalstandard.com. Selected references are listed below.

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2. 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. View Abstract

3. 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. View Abstract

4. 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.

5. 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. View Abstract

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13. 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.

14. 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. View Abstract

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