Know2eat

Quercetin

Isolated Compounds

What quercetin may help with

Ranked by how strong the human evidence is — best supported first.

Best supportedHealthy blood pressureLimited evidence· 3 sources

Then, in order

About Quercetin

A flavonol pigment found in onions, capers, apples and tea, sold as an isolate extracted from Japanese pagoda tree flower buds or dyer's broom. Aglycone quercetin is poorly water-soluble and poorly absorbed.

Quercetin is among the most-studied flavonoids in the laboratory and among the least convincing in people, and the reason is absorption: the aglycone sold in capsules is absorbed far less efficiently than the quercetin glycosides in onions, and what does get in is rapidly conjugated. The exercise-immunity literature is the strongest part of the record — trials in athletes report small reductions in upper respiratory symptoms after heavy training — and even there the effect sizes are modest and inconsistent. During the pandemic quercetin was heavily promoted as a zinc ionophore on the strength of a cell-culture paper; the human trials that followed were small and poorly controlled. What quercetin does do reliably is inhibit drug-metabolising enzymes and transporters, which makes it one of the more interaction-prone supplements on the shelf.

Sold as
Capsule (dihydrate), Phytosome complex, Powder, Combination with bromelain or vitamin C
Standardised to
Percentage quercetin; phytosome and enzymatically modified isoquercitrin forms absorb several-fold better than the plain aglycone — a product without this marker may not resemble what was studied.
Also known as
Quercetin dihydrate, Quercetin phytosome, Sophora japonica extract

Regulation: Sold as a dietary supplement in the US with no pre-market efficacy review. It was widely marketed for respiratory infection during the COVID-19 pandemic and drew regulatory warning letters over those claims.

What the research used

The amounts, forms and durations published trials of Quercetin actually administered.

These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.

  • Studied for Exercise-related respiratory symptoms

    500–1000 mg

    Limited

    Capsule, daily, often with vitamin C · 2–12 weeks

    The dose used in athlete trials. Co-ingestion with vitamin C or bromelain is standard in these studies, so the effect cannot be attributed to quercetin alone.

  • Studied for Blood pressure trials

    500–1000 mg

    Limited

    Capsule, daily or divided · 4–10 weeks

    Meta-analyses suggest any effect is confined to doses above 500 mg and to people who already have raised blood pressure. Individual trials are small.

  • Studied for Long-term tolerability studies

    1000 mg

    Limited

    Daily · Up to 12 weeks

    The longest well-documented human exposure at supplemental doses. Safety beyond about three months at gram doses is not established.

This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.

When it was taken

Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.

None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.

  • With food

    Not enough

    Taken with a meal containing some fat for tolerability and on the basis that lipids improve solubilisation of a poorly soluble aglycone. No trial has compared fed with fasted dosing on an outcome.

    Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.

The evidence in depth

Best supported: Limited

What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.

Healthy blood pressure

Heart Health
Limited evidence

Pooled analyses report a small reduction in systolic blood pressure, mostly in people with hypertension and at doses of 500 mg or more. Individual trials are small and heterogeneous.

Limited evidence. Early human work, or mostly lab and animal research.

View the 3 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Immune function

Immune Health
Limited evidence

Trials in athletes undertaking heavy training report modest reductions in the number of days with upper respiratory symptoms. Results in untrained people and in general populations are inconsistent, and the trials usually combine quercetin with other compounds.

What may be going on: Quercetin modulates NF-κB signalling and mast-cell mediator release in laboratory systems. The concentrations used in those experiments are well above what oral dosing achieves in blood.

Limited evidence. Early human work, or mostly lab and animal research.

View the 2 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Inflammatory markers

Inflammation
Limited evidence

Some trials report reductions in C-reactive protein and TNF-α, others report nothing. The heterogeneity between trials is large enough that pooled estimates should be read cautiously.

Limited evidence. Early human work, or mostly lab and animal research.

View the 2 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Exercise performance

Muscle & Physical Performance
Not enough evidence

Early trials suggested small endurance improvements, and subsequent better-controlled work in trained athletes found little or nothing. The effect appears largest in untrained participants, which is a common pattern for weak ergogenic aids.

Not enough evidence. Not enough human research to say anything useful yet.

View the 4 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Things to know

Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.

  • Interaction

    Inhibits CYP enzymes and drug transporters

    Interacts withCYP3A4, CYP2C8 and CYP2D6 substrates; P-glycoprotein substrates; ciclosporin; digoxin

    Quercetin inhibits CYP3A4, CYP2C8 and CYP2D6 and the P-glycoprotein and OATP transporters in human studies, which can raise blood levels of medicines that depend on them. Documented or predicted interactions include ciclosporin, some statins, midazolam, digoxin, fexofenadine and several chemotherapy agents. This is one of the strongest reasons to check with a pharmacist before adding it to a medication list.

  • Interaction

    Antibiotic and anticoagulant interactions

    Interacts withQuinolone antibiotics, warfarin, antiplatelet drugs

    Quercetin has been reported to interact with quinolone antibiotics through competition at the same bacterial target, and its mild antiplatelet activity in laboratory work makes it worth flagging to anyone on anticoagulants.

  • Too much

    Kidney concerns at high doses

    Headache and tingling in the limbs are the commonest complaints. More importantly, high-dose quercetin has been associated with kidney damage in animal work and there are case-level concerns in people with existing kidney disease, who are not a group in whom it has been studied.

  • Worth knowing

    Thyroid effects reported

    Interacts withLevothyroxine and thyroid function

    Quercetin inhibits thyroid cell function in laboratory and animal studies and there are reports of altered thyroid hormone levels. Anyone with thyroid disease or on levothyroxine should raise it with their prescriber rather than assume it is inert.

  • Worth knowing

    Supplement form absorbs worse than food form

    The glycosides in onions and apples are absorbed several times more efficiently than the aglycone in most capsules. A 500 mg capsule does not put five hundred times more quercetin into the blood than a serving of onions, and specialised phytosome or isoquercitrin forms exist precisely because of this problem.

In the food catalogue

The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.

ApplesQuercetin sits mainly in the skin, which is why peeling removes most…CapersWeight for weight the most concentrated common food source.OnionsThe richest everyday source, and in a glycoside form that is absorbed…

Evidence ratings on this page: 3 limited evidence, 1 not enough evidence.

Supplements are sold in the US without pre-market review of efficacy or safety. Nothing on this page is a recommendation to take Quercetin, or a dose to follow.