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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.

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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).

How to take it

TimingSplit into two doses; timing does not otherwise matter.
With food?With a meal containing fat — plain aglycone is only about 1-2% absorbed.
Worth knowingA phytosome or EMIQ form at 100-250 mg delivers more quercetin to your blood than 1,000 mg of the plain powder, and stays under the 1 g/day level linked to kidney injury reports.

Interactions

With medications

  • ciclosporin

    Quercetin alters ciclosporin blood levels — animal pharmacokinetic work found a marked decrease in oral bioavailability, while its CYP3A4 and P-glycoprotein inhibition points the other way, so the direction is unpredictable. do not combine

  • statins and other CYP3A4 or P-glycoprotein substrates

    Quercetin can raise blood levels of these drugs. talk to your doctor before combining

  • quinolone antibiotics (ciprofloxacin, levofloxacin)

    Quercetin competes at DNA gyrase and may reduce the antibiotic's effect. separate by at least 2 hours, or pause quercetin during the course

  • warfarin and antiplatelet drugs

    Mild additive anticoagulant effect. mention it to your doctor and stop before surgery

With other supplements

  • iron

    Quercetin chelates iron and can reduce its absorption. separate by at least 2 hours

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does quercetin lower blood pressure?

Yes, modestly — quercetin has moderate evidence: a meta-analysis of 7 trials found systolic pressure fell by 3.0 mmHg and diastolic by 2.6 mmHg. The effect was concentrated in trials using at least 500 mg/day for 8 weeks or more.

Does quercetin help the immune system or act as a zinc ionophore?

The evidence is limited: trials of quercetin in COVID-19 were few, small and at high risk of bias, with no reliable effect on hard outcomes. The zinc-ionophore effect has been shown only in lab systems at concentrations oral quercetin cannot reach.

What is the best form and dose of quercetin?

Plain quercetin aglycone is only about 1-2% absorbed; blood pressure trials used 500-1,000 mg/day. Phytosome, EMIQ or quercetin-3-glucoside forms are several-fold better absorbed and typically dosed at 100-250 mg/day, taken with a meal containing fat.

What drugs interact with quercetin?

Quercetin can alter ciclosporin levels unpredictably, can raise levels of statins and other CYP3A4 drugs, and may reduce quinolone antibiotic effects — separate those by at least 2 hours. Avoid quercetin doses above 1 g/day, which have been linked to kidney damage in case reports.

Scientific references

Where to buy

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

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