Quercetin is a flavonoid—a substance found in fruits, flowers, and vegetables—that, among other functions, gives them their color. Flavonoids have antioxidant properties, which help protect against disease, as well as anti-inflammatory and antihistamine effects.

Quercetin offers a variety of potential therapeutic uses, primarily in the prevention and treatment of the following conditions:

• Allergies, asthma, and hives: Quercetin may inhibit histamine release from basophils (a type of white blood cell) and mast cells (large cells in connective tissue).
• Canker sores: Quercetin may reduce the frequency of mouth sores and produce mild symptomatic relief.
• Heart disease: Individuals with very low intakes of flavonoids are at higher risk for heart disease.
• Infection: Quercetin may control the spread of certain viruses within the body.
• Rheumatoid arthritis: Quercetin may help reduce tissue destruction.

Quercetin may also be beneficial in the treatment of dysentery (an intestinal infection causing severe diarrhea), gout (a disease where crystals of uric acid, a component of urine, are deposited in the joints and cause swelling), and psoriasis (a chronic skin disease).

Fruits and vegetables—particularly citrus fruits, apples, onions, parsley, tea, and red wine—are the primary dietary sources of quercetin.

Quercetin is available as a supplement in several strengths in powder or capsule form. It is often packaged with bromelain as an anti-inflammatory agent. Flavonoid-rich extracts include those from grape seed, bilberry, Ginkgo biloba, and green tea.

Pediatric

• For the treatment of allergies, adjust the recommended adult dose to account for the child's weight. Most adult dosages are calculated on the basis of a 150 lb (70 kg) adult. Therefore, if the child weighs 50 lb (20 to 25 kg), the appropriate dose of quercetin for this child would be 1/3 of the adult dosage. 

Adult

Recommended adult dosages of quercetin vary depending on the health condition being treated. The following are guidelines for some of its common uses:

• General supplementation: 100 to 250 mg three times per day.
• Allergy symptoms: 250 to 600 mg per day divided in several doses.
• Gout: 200 to 400 mg of quercetin taken with bromelain between meals three times per day
• Chronic hives: 200 to 400 mg of quercetin taken approximately 20 minutes before each meal

There are no known reported problems with the use of quercetin.

Quercetin may increase the effects of adriamycin and cisplatin, two chemotherapy medications. More research is needed to determine if quercetin should be taken in conjunction with chemotherapy.

Duthie SJ, Collins AR, Duthie GG, Dobson VL. Quercetin and myricetin protect against hydrogen peroxide-induced DNA damage (strand breaks and oxidised pyrimidines) in human lymphocytes. Mutat Res. 1997;393(3):223-231.

Ferry DR, Smith A, Malkhandi J, et al. Phase I clinical trial of the flavonoid quercetin pharmacokinetics and evidence for in vivo tyrosine kinase inhibition. Clin Cancer Res. 1996;2(4):659-668.

Frolov VM, Peresadin NA, Khomutianskaia NI, Pshenichnyi I. The efficacy of quercetin and tocopherol acetate in treating patients with Flexner's dysentery [in Ukrainian]. Lik Sprava. 1993;4:84-86.

Gross M, Pfeiffer M, Martini M, Campbell D, Slavin J, Potter J. The quantitation of metabolites of quercetin flavonols in human urine. Cancer EpidemiolBiomarkers Prevent. 1996;5(9):711-720.

Haas EM. Staying Healthy with Nutrition. Berkley, Calif: Celestial Arts Publishing; 1992:272, 882-884.

Hofmann J, Fiebig HH, Winterhalter BR, Berger DP, Grunicke H. Enhancement of the antiproliferative activity of cis-diamminedichloroplatinum (II) by quercetin. Int J Cancer. 1990;45(3):536-539.

Hollman PC, Van Trijp JM, Mengelers MJ, De Vries JH, Katan, MB. Bioavailability of the dietary antioxidant flavonol quercetin in man. Cancer Lett. 1997;114(1-2):139-140.

Knekt P, Jarvinen R, Reunanen A, Maatela J. Flavonoid intake and coronary mortality in Finland: a cohort study. BMJ (Clinical Research Ed.). 1996;312(7029):478-481.

Murray MT. Encyclopedia of Nutritional Supplements. Rocklin, Calif: Prima Publishing; 1996: 320-321.

Murray MT, Pizzorno JE. Encyclopedia of Natural Medicine. 2nd ed. Rocklin, Calif: Prima Publishing; 1996:268, 314, 422, 494, 546-47, 766.

Scambia G, Ranelletti FO, Benedetti Panici P, et al. Quercetin potentiates the effect of adriamycin in a multi-drug-resistant MCF-7 human breast-cancer cell line: P-glycoprotein as a possible target. Cancer Chemother Pharmacol. 1994;34:459-464.

Shils ME, Olson JA, Shike M, Ross AC. Modern Nutrition in Health and Disease. 9th ed. Baltimore, Md: Williams & Wilkins; 1999:1274-1277.

Werbach MR. Nutritional Influences on Illness. 2nd ed. Tarzana, Calif: Third Line Press; 1993:179, 259, 267, 389.

White L, Mavor S. Kids, Herbs, Health. Loveland, Colo: Interweave Press; 1998:22, 84.

Young JF, Nielsen SE, Haraldsdottir J, et al. Effect of fruit juice intake on urinary quercetin excretion and biomarkers of antioxidative status. Am J Clin Nutr. 1999; 69(1):87-94.