Supplement analysis backed by scientific evidence · PT

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Moderate evidenceHeart & metabolic

Quercetin

One of the better-studied flavonoids, with a small but real blood-pressure-lowering effect at doses above 500 mg/day. The immune and "zinc ionophore" claims that made it popular in 2020 are far weaker than the marketing suggests.

What the science says

  • Blood pressure Moderate evidence

    A meta-analysis of 7 RCTs (587 participants) in JAHA found quercetin reduced systolic BP by 3.0 mmHg and diastolic by 2.6 mmHg, with the effect concentrated in trials using at least 500 mg/day for 8 weeks or more. A later 2020 meta-analysis in Nutrition Reviews confirmed the systolic reduction but found no meaningful effect on glucose or lipids. The magnitude is modest but consistent.

  • Exercise performance Limited evidence

    Meta-analyses of quercetin and endurance performance find a very small average benefit (roughly 1-3% in VO2max or time-to-exhaustion) that is largely restricted to untrained individuals and disappears in trained athletes. Doses of 1,000 mg/day for 1-2 weeks are typical. Not worth taking for this purpose if you already train.

  • Antiviral and immune effects Limited evidence

    Despite the 2020 boom, the human data are thin. A 2023 systematic review and meta-analysis of quercetin in COVID-19 patients found trials were few, small and at high risk of bias, with no reliable effect on hard outcomes. The zinc-ionophore mechanism has been demonstrated in cell-free and cell-culture systems only, at concentrations not reachable with oral dosing.

  • Allergy and mast cell stabilization Limited evidence

    Quercetin inhibits histamine release from mast cells in vitro, but controlled human allergy trials are few, small and mostly used combination products, so an independent effect of quercetin on allergic rhinitis symptoms is not established.

Dosage & safety

Studied doseBlood pressure trials used 500-1,000 mg/day of quercetin aglycone for 8-12 weeks. Exercise studies used 1,000 mg/day. Plain quercetin aglycone is poorly absorbed (roughly 1-2%); phytosome (lecithin), enzymatically modified isoquercitrin (EMIQ) and quercetin-3-glucoside forms have several-fold higher bioavailability and are typically dosed at 100-250 mg/day. Take with food containing fat.
SafetyGenerally well tolerated up to 1,000 mg/day for 12 weeks; headache and tingling in the extremities are the most reported effects. Doses above 1 g/day have been linked to kidney damage in case reports, and intravenous quercetin has caused nephrotoxicity. It inhibits CYP3A4 and P-glycoprotein and can meaningfully raise levels of cyclosporine, certain statins, and some chemotherapy drugs; it may also reduce the effect of quinolone antibiotics by competing at DNA gyrase. It has mild anticoagulant activity — stop before surgery. Avoid in pregnancy and lactation due to lack of data, and use caution in people with existing kidney disease or hypothyroidism (high doses may interfere with thyroid peroxidase).

Scientific references

Where to buy

California Gold Nutrition, Quercetin Phytosome Quercefit®, 250 mg, 60 Veggie Capsules

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