Avoid if known allergy/hypersensitivity to quercetin by ingestion or skin contact.
Possible eye, skin, gastrointestinal and/or respiratory tract irritation.
Possible hypersensitization to quercetin.

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 administration of quercetin has resulted in flushing, sweating, difficulty breathing (dyspnea), nausea, and vomiting.
One out of 260 patients taking AS195, which contains red vine leaf extract, rich in quercetin, reported hematoma at a site of mild trauma.
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, rich in quercetin. Vomiting has occurred with high intravenous dosing as well as flushing, sweating, difficulty breathing (dyspnea), nausea, and vomiting.
In a phase one study of bolus intravenous doses, there was reversible kidney damage in three out of 14 patients.
Intravenous injection has been associated with pain at the injection site which is dose related and can be controlled by reducing the rate of infusion.
Headaches and mild tingling of the extremities have been reported. Intravenous QC12 was associated with lethargy in two patients who also had cancer progression.
High doses have been associated with severe dyspnea (difficulty breathing) lasting as long as five minutes.

Pregnancy & Breastfeeding

Insufficient available evidence.


Adults (over 18 years old)

Oral (by mouth):

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. 100-500 milligrams of quercetin has been taken two or three 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 in human leukemia cell lines.
Based on laboratory study, quercetin may decrease liver and kidney damage caused by the cancer chemotherapy drug cisplatin.
Quercetin may increase the effects of steroids by interfering with the way that the liver breaks them down.
Quercetin may affect the levels of cyclosporine.
Quercetin may interact with drugs that are broken down by the liver like estradiol and nifedipine.
Laboratory studies suggest that quercetin may affect certain hormone levels.
Food rich in flavonoids, including quercetin, have been found to delay the metabolism of nifedipine in humans; however this effect cannot be specifically related to quercetin.
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.

Interactions with Herbs & 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 which may increase bleeding risk.
Consumption of black currants, lingonberries and bilberries increases quercetin levels in the blood.
Quercetin is found in elder, St. John's wort, parsley, green tea, and ginkgo, and thus there may be additive effects if taken together.
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.