Quercetin

See also:

• Drug interactions

Quercetin belongs to a class of water-soluble plant pigments called flavonoids.

Where is it found?

Quercetin can be found in onions, apples, green tea, and black tea. Smaller amounts are found in leafy green vegetables and beans.

Quercetin has been used in connection with the following conditions (refer to the individual health concern for complete information):

Rating	Health Concerns
	Prostatitis (nonbacterial prostatitis, prostadynia)
	Allergies Asthma Atherosclerosis Capillary fragility Cataracts Childhood diseases Edema (water retention) Gout Hay fever Peptic ulcer Retinopathy Type 1 diabetes Type 2 diabetes
Reliable and relatively consistent scientific data showing a substantial health benefit. Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit. An herb is primarily supported by traditional use, or the herb or supplement has little scientific support and/or minimal health benefit.

Who is likely to be deficient?

No clear deficiency of quercetin has been established.

How much is usually taken?

Some doctors recommend 200-500 mg of quercetin taken two to three times per day. Optimal intake remains unknown.

Are there any side effects or interactions?

No clear toxicity has been identified. Early quercetin research suggested that large amounts of quercetin could cause cancer in animals.¹ Most,^{2, 3, 4} but not all,⁵ current research finds quercetin to be safe or actually linked to protection from cancer.

Quercetin has been shown to cause chromosomal mutations in certain bacteria in test tube studies.⁶ Although the significance of this finding for humans is not clear, some doctors are concerned about the possibility that birth defects could occur in the offspring of people supplementing with quercetin at the time of conception or during pregnancy.

Since flavonoids help protect and enhance vitamin C, quercetin is often taken with vitamin C.

Are there any drug interactions?

Certain medicines may interact with quercetin. Refer to drug interactions for a list of those medicines.

1. Ishikawa M, Oikawa T, Hosokawa M, et al. Enhancing effect of quercetin on 3-methylcholanthrene carcinogenesis in C57B1/6 mice. Neoplasma 1985;43:435-41.
2. Hertog MGL, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and cancer risk in the Zutphen elderly study. Nutr Cancer 1994;22:175-84.
3. Castillo MH, Perkins E, Campbell JH, et al. The effects of the bioflavonoid quercetin on squamous cell carcinoma of head and neck origin. Am J Surg 1989;351-5.
4. Stavric B. Quercetin in our diet: from potent mutagen to probably anticarcinogen. Clin Biochem 1994;27:245-8.
5. Barotto NN, López CB, Eyard AR, et al. Quercetin enhances pretumourous lesions in the NMU model of rat pancreatic carcinogenesis. Cancer Lett 1998;129:1-6.
6. Stoewsand GS, Anderson JL, Boyd JN, Hrazdina G. Quercetin: a mutagen, not a carcinogen in Fischer rats. J Toxicol Environ Health 1984;14:105-14.

Last Review: 09-07-2006

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The information presented in Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires August 2007.

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