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):
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.
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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
Are there any drug
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
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
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
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.
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