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.
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.
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.