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What Is Quercetin?

Quercetin is a naturally occurring flavonoid found in many fruits, vegetables, and other plant foods. Common dietary sources include onions, apples, berries, broccoli, and tea. Quercetin is also sold as a dietary supplement, sometimes combined with ingredients such as bromelain or vitamin C. Researchers have investigated its antioxidant, anti-inflammatory, and other biological properties. Although laboratory studies suggest several possible activities, human evidence remains insufficient to establish quercetin as a treatment for most medical conditions. Quercetin supplements are not FDA-approved medications for preventing or treating diseases.

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What Is Quercetin?

Quercetin is a naturally occurring flavonoid found in many fruits, vegetables, and other plant foods. Common dietary sources include onions, apples, berries, broccoli, and tea. Quercetin is also sold as a dietary supplement, sometimes combined with ingredients such as bromelain or vitamin C. Researchers have investigated its antioxidant, anti-inflammatory, and other biological properties. Although laboratory studies suggest several possible activities, human evidence remains insufficient to establish quercetin as a treatment for most medical conditions. Quercetin supplements are not FDA-approved medications for preventing or treating diseases.

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How Does Quercetin Work?

Quercetin belongs to the flavonol subclass of flavonoids and can interact with biological pathways involved in oxidation, inflammation, and cellular signaling. Laboratory research has identified antioxidant activity and effects on certain enzymes and inflammatory mediators. Quercetin is metabolized after ingestion, and the forms circulating in the body differ from the compounds frequently tested in laboratory experiments. Its absorption and metabolism also vary according to the food or supplement formulation. These factors limit how directly laboratory findings can be applied to human health outcomes, and a clinically proven mechanism for treating a specific disease has not been established.

What Health Benefits Have Been Studied for Quercetin?

Researchers have investigated quercetin for potential effects on inflammatory markers, blood pressure, immune responses, and respiratory infections. Some small human studies have reported changes in selected measurements, but findings are inconsistent and do not establish reliable treatment benefits. Quercetin has also been studied for possible antiviral activity in laboratory settings, which should not be interpreted as proof that supplements prevent or treat viral illnesses. The National Institutes of Health notes that evidence for preventing respiratory infections remains uncertain. Eating quercetin-containing foods can contribute to a varied diet without relying on concentrated supplement doses.

How Is Quercetin Taken?

Quercetin is consumed naturally through foods or taken as an oral dietary supplement, typically in capsules or tablets. Commercial products contain different amounts and combinations of ingredients, and there is no FDA-approved therapeutic dosing schedule for quercetin. Clinical studies have used varying doses, including amounts up to 1,000 mg daily for limited periods, but these research doses are not established recommendations for routine use. Long-term supplement safety remains less certain. People taking prescription medications should discuss quercetin supplements with a healthcare professional because interactions may alter drug exposure or blood pressure.

What Are the Side Effects and Warnings of Quercetin?

Quercetin is generally consumed safely in ordinary food amounts. Short-term clinical studies of supplements have not identified frequent serious adverse effects, although mild headache or gastrointestinal discomfort may occur. Information about long-term use of concentrated supplements, pregnancy, and breastfeeding is limited. Quercetin may affect enzymes involved in medication metabolism and could interact with certain drugs, including cyclosporine, pravastatin, and fexofenadine. It may also increase the blood pressure-lowering effects of antihypertensive medications. People with kidney disease or complex medication regimens should seek medical advice before taking concentrated quercetin products.

Common Questions About Quercetin

Is quercetin a vitamin?

No. Quercetin is a flavonoid, a type of plant compound, rather than an essential vitamin.

What foods naturally contain quercetin?

Quercetin is found in onions, apples, berries, broccoli, tea, and various other plant foods.

Can quercetin supplements prevent viral infections?

There is insufficient reliable clinical evidence to conclude that quercetin supplements prevent or treat viral infections. Laboratory findings do not establish clinical benefit.

Can quercetin interact with prescription medications?

Yes. Quercetin may affect the metabolism or action of certain medications, including some cholesterol, allergy, immune-suppressing, and blood pressure drugs.

References

Dietary Supplements for Exercise and Athletic Performance - Health Professional Fact Sheet. National Institutes of Health Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/ExerciseAndAthleticPerformance-HealthProfessional/. Date Accessed November 3, 2026.

Dietary Supplements in the Time of COVID-19 - Health Professional Fact Sheet. National Institutes of Health Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/COVID19-HealthProfessional/. Date Accessed November 3, 2026.

Quercetin. LiverTox. https://www.ncbi.nlm.nih.gov/books/NBK556474/. Date Accessed November 3, 2026.

Quercetin. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/5280343. Date Accessed November 3, 2026.

Quercetin. National Institutes of Health Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/DietarySupplementsInTheTimeOfCOVID19-Consumer/. Date Accessed November 3, 2026.