Quercetin

botanical

What is it

Quercetin is a plant flavonoid pigment found in many fruits, vegetables, and grains, including onions, apples, capers, and berries. It is widely used as a supplement for its antioxidant and anti-inflammatory properties.

How it works

Quercetin's biological effects derive from its flavonol structure, which contains multiple hydroxyl groups that enable it to scavenge free radicals and chelate certain metal ions. It modulates several inflammatory and signaling pathways, including inhibition of nuclear factor kappa B (NF-kB), lipoxygenases, and cyclooxygenases involved in producing inflammatory mediators. Quercetin is best known for stabilizing mast cells, which release histamine and other mediators during allergic reactions. By inhibiting mast cell degranulation, it can attenuate symptoms of allergies and certain inflammatory conditions. It also modulates the activity of immune cells and may influence the bioavailability and effect of other compounds, including vitamin C. Oral bioavailability of quercetin aglycone (the free form) is poor. Glycoside forms found in food, such as quercetin-3-O-rutinoside (rutin) and quercetin-4'-O-glucoside, are more efficiently absorbed. After absorption, quercetin is rapidly metabolized to glucuronide and sulfate conjugates that retain some biological activity. Bromelain, vitamin C, and certain phospholipid formulations are sometimes paired with quercetin to improve absorption.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Blood pressure

Grade B

Good evidence

Meta-analyses of randomized trials suggest quercetin doses of 500 mg or more per day can modestly lower blood pressure, with greater effects in people with hypertension. Reductions are typically a few mmHg in systolic and diastolic pressure.

Seasonal allergies

Grade C

Moderate evidence

Quercetin's mast cell stabilizing and antihistamine effects support its traditional use for allergies. Small clinical trials and mechanistic studies suggest it may reduce allergy symptoms, but high-quality RCTs are limited. Effects are typically modest and may take weeks to develop.

Inflammation

Grade C

Moderate evidence

Some studies report reductions in markers like CRP and TNF-alpha with quercetin supplementation, particularly in people with elevated baseline inflammation. Clinical significance is unclear.

Exercise-induced oxidative stress

Grade C

Moderate evidence

Research suggests quercetin may modestly reduce markers of oxidative stress and inflammation after intense exercise. Effects on actual performance or recovery are smaller and less consistent.

Antiviral activity (including respiratory infections)

Grade D

Mixed evidence

Mechanistic and preclinical studies suggest quercetin has antiviral properties, but human clinical evidence is limited. Some interest exists for upper respiratory infection prevention, but conclusions cannot yet be drawn.

4 commercial forms

Quercetin aglycone (free quercetin)

Poor oral bioavailability without enhancers.

Most common and inexpensive form. Often combined with bromelain or vitamin C in supplements.

Quercetin phytosome

Bound to phospholipids; substantially improved absorption.

Higher cost but better bioavailability documented in clinical studies.

Rutin (quercetin-3-rutinoside)

Glycoside form found in foods; broken down in gut to release quercetin.

Traditional form used for vascular health; gentler effect than free quercetin.

Isoquercetin / EMIQ (enzymatically modified)

Glucoside form with markedly improved absorption versus aglycone.

Better-absorbed alternative, often used in research.

Dosage

There is no RDA for quercetin. Typical supplement doses range from 250 to 1,000 mg per day, often divided into two or three doses. For allergy support, 500 to 1,000 mg per day is commonly used, sometimes combined with bromelain or vitamin C. Higher doses (up to 2,000 mg) have been used in research but are not generally recommended without supervision.

When and how to take it

WHEN: Quercetin can be taken at any time of day. For allergy support, dosing 1 to 2 weeks before peak allergen exposure (e.g., pollen season) may be most effective. HOW: Take with food, ideally a fat-containing meal, to improve absorption since quercetin is poorly water-soluble. Combining with bromelain or vitamin C may further enhance bioavailability. Split doses (morning and evening) help maintain steady blood levels given quercetin's short half-life.

Food sources

Food	Amount	%DV
Capers	1 tbsp	—
Red onion	1/2 cup	—
Apples (with skin)	1 medium	—
Kale	1 cup raw	—
Broccoli	1 cup	—
Blueberries	1 cup	—
Black/green tea	1 cup	—
Cherries	1 cup	—

Safety

Quercetin is generally well tolerated at typical supplement doses. Reported side effects include headache, tingling in the extremities, and gastrointestinal symptoms such as nausea or stomach upset. High doses may rarely cause kidney issues, particularly in people with pre-existing kidney disease. There is no established Tolerable Upper Intake Level for supplements. Intravenous quercetin has been associated with kidney damage in some cases and is not recommended outside clinical research.

Who should be cautious

Pregnant and breastfeeding women should avoid quercetin supplementation due to limited safety data; food sources are considered safe. People with kidney disease should consult a clinician before use. Those on prescription medications, especially blood thinners, immunosuppressants, or drugs metabolized by CYP3A4, should check for interactions. People with hypothyroidism should be cautious at high doses.

Interactions

Quercetin inhibits several cytochrome P450 enzymes (CYP3A4, CYP2C9) and the drug efflux transporter P-glycoprotein, which can affect the metabolism of many medications. It may increase blood levels of cyclosporine, certain statins, and other drugs metabolized by these pathways. It has mild antiplatelet effects and may interact with anticoagulants. Quercetin can interfere with certain antibiotics (such as fluoroquinolones) and may affect thyroid hormone activity at high doses.

Frequently asked questions

Does quercetin help with allergies?⌄

Quercetin stabilizes mast cells and may reduce histamine release. Clinical evidence is modest but supports use for seasonal allergies, often started 1 to 2 weeks before allergen exposure begins.

Is quercetin worth taking with bromelain?⌄

Bromelain may improve quercetin absorption and adds its own anti-inflammatory effects. Many allergy-focused supplements combine the two, though direct head-to-head evidence is limited.

Can I get enough quercetin from food?⌄

Diets rich in onions, apples, berries, capers, and tea can deliver meaningful quercetin (typically 10 to 100 mg per day). Supplemental doses are 5 to 10 times higher than typical dietary intake.

Are there side effects?⌄

Quercetin is generally well tolerated. High doses can cause headache, GI upset, or, rarely, kidney problems. Avoid intravenous use outside clinical settings.

Does quercetin interact with medications?⌄

Yes. Quercetin can affect liver enzymes that metabolize many drugs. Check with your pharmacist if you take blood thinners, immunosuppressants, statins, or certain antibiotics.

References

Wikidata: Quercetin — Wikidata link
PubChem: Quercetin (CID 5280343) — PubChem link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.