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What does quercetin do?

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

Quercetin acts as an antihistamine and has anti-inflammatory properties. As an antioxidant, it protects LDL cholesterol ("bad" cholesterol) from becoming damaged. A variety of evidence indicates that quercetin possesses potent antioxidant properties. Cardiologists believe that damage to LDL cholesterol is an underlying cause of heart disease. Quercetin blocks an enzyme that leads to accumulation of sorbitol, which has been linked to nerve, eye, and kidney damage in those with diabetes. However, no human research has demonstrated these actions of quercetin in people with diabetes patients.

Quercetin is considered a phytoestrogen (i.e., a plant substance with similar functions as that of estrogen). Some phytoestrogens are believed also to have antiestrogenic effects that might lead to reduced risks of certain cancers. Quercetin was found to have this antiestrogenic activity by inhibiting breast cancer cells in a test tube.1

In a double-blind trial, 67% of people taking quercetin had an improvement of prostatitis symptoms, compared with a 20% response rate in the placebo group.2

Where is quercetin 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)

Capillary fragility
Childhood diseases
Edema (water retention)
Hay fever
Peptic ulcer

Reliable and relatively consistent scientific data showing a substantial health benefit.
Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
For an herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support and/or minimal health benefit.


Who is likely to be deficient of quercetin?

No clear deficiency of quercetin has been established.

How much quercetin 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 with quercetin?

No clear toxicity has been identified. Early quercetin research suggested that large amounts of quercetin could cause cancer in animals.3 Most,4 5 6 but not all,7 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.8 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.