Quercetin

Pharmacology & Research

Quercetin is the most widely distributed flavonol in the plant kingdom — the yellow pigment behind onion skin, caper buds and countless herb extracts. It is one of the most heavily supplemented and studied plant molecules, yet its clinical story is modest and, above all, governed by one fact: the aglycone is poorly and erratically absorbed, circulates almost entirely as conjugated metabolites, and reaches plasma levels far below those used in most cell studies ^{18,19,20Reference 18Hollman PC et al. · 1997Relative bioavailability of the antioxidant flavonoid quercetin from various foods in manView study →Reference 19Graefe EU et al. · 2001Pharmacokinetics and bioavailability of quercetin glycosides in humansView study →Reference 20Mohos V et al. · 2020Inhibitory effects of quercetin and its main methyl, sulfate, and glucuronic acid conjugates on cytochrome P450 enzymes, and on OATP, BCRP and MRP2 transportersView study →}. Read the human evidence with that in mind — the effects that survive meta-analysis are small and dose-gated.

1. Blood pressure & cardiovascular

Quercetin’s best-supported effect. A 2016 meta-analysis of 7 RCTs (n≈587) found supplementation lowered systolic blood pressure by about 3.0 mmHg and diastolic by 2.6 mmHg versus placebo — but the pre-specified subgroup analysis is the load-bearing detail: the reduction appeared only at doses ≥500 mg/day, with nothing below that ^{1Reference 1Serban MC et al. · 2016Meta-analysisEffects of quercetin on blood pressure: a systematic review and meta-analysis of randomized controlled trials (7 RCTs, n≈587)View study →}. Later syntheses — a 2020 meta-analysis and a 2023 umbrella review of cardiometabolic outcomes — reproduce a small systolic effect, while the diastolic effect is more fragile across analyses ^{2,3Reference 2Huang H et al. · 2020Meta-analysisEffect of quercetin supplementation on plasma lipid profiles, blood pressure, and glucose levels — systematic review and meta-analysisView study →Reference 32023Meta-analysisThe effects of quercetin supplementation on cardiometabolic outcomes — an umbrella review of meta-analyses of randomised controlled trialsView study →}. Mechanistically it fits: quercetin supports endothelial nitric-oxide signalling and inhibits ACE.

Gap: the effect is small (~3 mmHg), drawn from short trials (4–10 weeks) in overweight / metabolic-syndrome cohorts, with no hard-outcome (cardiovascular event) data ^{1,3Reference 1Serban MC et al. · 2016Meta-analysisEffects of quercetin on blood pressure: a systematic review and meta-analysis of randomized controlled trials (7 RCTs, n≈587)View study →Reference 32023Meta-analysisThe effects of quercetin supplementation on cardiometabolic outcomes — an umbrella review of meta-analyses of randomised controlled trialsView study →}.

2. Metabolic markers

Results here are genuinely mixed. For glucose, a 2019 meta-analysis found fasting glucose fell only with ≥500 mg/day for ≥8 weeks, and not at all for HbA1c or insulin resistance ^{4Reference 4Ostadmohammadi V et al. · 2019Meta-analysisEffects of quercetin supplementation on glycemic control among patients with metabolic syndrome and related disorders — systematic review and meta-analysis of RCTsView study →}. For lipids, some meta-analyses report small reductions in total and LDL cholesterol in subgroups ^{5,6Reference 5Sahebkar A · 2017Meta-analysisEffects of quercetin supplementation on lipid profile — systematic review and meta-analysis of randomised controlled trialsView study →Reference 6Tabrizi R et al. · 2020Meta-analysisThe effects of quercetin supplementation on lipid profiles and inflammatory markers among patients with metabolic syndrome and related disorders — systematic review and meta-analysis of RCTsView study →}, but a 2023 umbrella review concludes quercetin does not reliably change the lipid profile ^{3Reference 32023Meta-analysisThe effects of quercetin supplementation on cardiometabolic outcomes — an umbrella review of meta-analyses of randomised controlled trialsView study →}. For inflammation, pooled analyses find no robust overall effect on CRP or IL-6, with significant reductions surfacing only in subgroups (diagnosed disease, higher dose) ^{6,7,8Reference 6Tabrizi R et al. · 2020Meta-analysisThe effects of quercetin supplementation on lipid profiles and inflammatory markers among patients with metabolic syndrome and related disorders — systematic review and meta-analysis of RCTsView study →Reference 7Mohammadi-Sartang M et al. · 2017Meta-analysisEffects of supplementation with quercetin on plasma C-reactive protein concentrations — systematic review and meta-analysis of randomised controlled trialsView study →Reference 8Ou Q et al. · 2020Meta-analysisImpact of quercetin on systemic levels of inflammation — meta-analysis of randomised controlled human trialsView study →}.

Gap: almost everything is dose- and subgroup-dependent, and the headline finding is that the meta-analyses disagree with one another — best read as “may nudge fasting glucose or CRP in metabolically-ill people at ≥500 mg”, not as a glucose or lipid drug ^{3Reference 32023Meta-analysisThe effects of quercetin supplementation on cardiometabolic outcomes — an umbrella review of meta-analyses of randomised controlled trialsView study →}.

3. Uric acid & gout

A well-run but small signal. In a 2016 randomised, double-blind, placebo-controlled crossover trial, 22 pre-hyperuricaemic men took 500 mg/day for 4 weeks and saw plasma uric acid fall significantly (~26 µmol/L) with no change in fractional excretion — consistent with xanthine-oxidase inhibition rather than increased excretion ^{9Reference 9Shi Y · 2016RCTQuercetin lowers plasma uric acid in pre-hyperuricaemic males — randomised, double-blinded, placebo-controlled, cross-over trial (n=22)View study →}. Quercetin’s in-vitro affinity for xanthine oxidoreductase is in the same order as allopurinol’s.

Gap: a single small crossover in pre-hyperuricaemic (not gout) men; there are no trials in clinical hyperuricaemia or gout, and no flare or hard-endpoint data. Mechanistically coherent, clinically unproven ^{9Reference 9Shi Y · 2016RCTQuercetin lowers plasma uric acid in pre-hyperuricaemic males — randomised, double-blinded, placebo-controlled, cross-over trial (n=22)View study →}.

4. Antiviral & COVID-19

RCTs exist, but read them sceptically. A 2021 pilot trial reported faster viral clearance and symptom resolution in early COVID-19 with a quercetin phytosome ^{10Reference 10Di Pierro F et al. · 2021RCTPotential clinical benefits of quercetin in the early stage of COVID-19 — second, pilot, randomised, controlled, open-label clinical trialView study →}, and two 2023 meta-analyses pooling ~5 trials (~500 patients) report reduced hospitalisation and severity ^{11,12Reference 112023Meta-analysisThe effect of quercetin supplementation on clinical outcomes in COVID-19 patients — systematic review and meta-analysisView study →Reference 122023Meta-analysisQuercetin for the treatment of COVID-19 patients — systematic review and meta-analysisView study →}.

Gap: the underlying trials are small, mostly open-label (high bias risk), several from the same group, and typically used a bioavailability-enhanced phytosome (Quercefit) rather than plain quercetin, added on to standard care — so the meta-analyses inherit that low quality. No effect on ordinary colds / URTIs has been established for the isolated molecule. Hypothesis-generating, not practice-changing ^{11,12Reference 112023Meta-analysisThe effect of quercetin supplementation on clinical outcomes in COVID-19 patients — systematic review and meta-analysisView study →Reference 122023Meta-analysisQuercetin for the treatment of COVID-19 patients — systematic review and meta-analysisView study →}.

5. Allergy & mast-cell stabilisation

The anti-allergic reputation is built largely on mechanism, not clinical trials. In vitro, quercetin stabilises human mast cells and inhibits histamine and cytokine (IL-8, TNF) release — in one study more effectively than cromolyn ^{13Reference 13Weng Z et al. · 2012Quercetin is more effective than cromolyn in blocking human mast cell cytokine release and inhibits contact dermatitis and photosensitivity in humansView study →}. But direct human trials in allergic rhinitis are sparse and use derivatives: a 2009 trial of enzymatically-modified isoquercitrin (EMIQ, a better-absorbed quercetin glucoside) eased ocular symptoms of cedar pollinosis ^{14Reference 14Kawai M et al. · 2009Preventive effect of enzymatically modified isoquercitrin on ocular symptoms of Japanese cedar pollinosis (a quercetin glycoside derivative)View study →}, and a 2022 RCT of a quercetin-containing supplement reported reduced allergic reaction ^{15Reference 152022RCTEffects of repeated oral intake of a quercetin-containing supplement on allergic reaction — randomised, placebo-controlled, double-blind parallel-group studyView study →} — neither is the pure aglycone.

Gap: there is no robust RCT of isolated quercetin aglycone in seasonal or perennial allergic rhinitis. The mast-cell mechanism is real in vitro but has not been translated to clinical rhinitis endpoints with the plain molecule — despite heavy marketing on that basis ^{13,14Reference 13Weng Z et al. · 2012Quercetin is more effective than cromolyn in blocking human mast cell cytokine release and inhibits contact dermatitis and photosensitivity in humansView study →Reference 14Kawai M et al. · 2009Preventive effect of enzymatically modified isoquercitrin on ocular symptoms of Japanese cedar pollinosis (a quercetin glycoside derivative)View study →}.

6. Exercise performance

A case where good evidence supports a near-null effect. A 2013 meta-analysis (7 studies, 288 subjects) found endurance performance improved by 0.74% and VO₂max by 1.94% versus placebo — magnitudes the authors judged trivial, with any hint of benefit confined to untrained subjects ^{16Reference 16Pelletier DM et al. · 2013Meta-analysisEffects of quercetin supplementation on endurance performance and maximal oxygen consumption — meta-analysis (7 studies, n=288)View study →}.

Gap: early single small crossovers suggesting a ~4% VO₂max bump did not survive pooling. Not a meaningful ergogenic aid; the mid-range score reflects strong confidence that the effect is negligible, not uncertainty ^{16Reference 16Pelletier DM et al. · 2013Meta-analysisEffects of quercetin supplementation on endurance performance and maximal oxygen consumption — meta-analysis (7 studies, n=288)View study →}.

Mechanisms

Target / pathway	Effect	Relevant to
Radical scavenging / Nrf2 induction	↓ oxidative stress, ↑ cellular glutathione	metabolic markers, general antioxidant
eNOS / nitric-oxide support + ACE inhibition	vasodilation, lower vascular tone	blood pressure
Xanthine-oxidase inhibition	↓ uric-acid synthesis	uric acid & gout
Mast-cell / basophil stabilisation	↓ histamine, IL-8 & TNF release	allergy
NF-κB inhibition	↓ pro-inflammatory transcription	metabolic inflammation, antiviral rationale

Most of these are demonstrated at in-vitro concentrations (10–100 µmol/L of free aglycone) well above what oral dosing achieves in blood — see Pharmacokinetics below.

Pharmacokinetics

Quercetin’s defining pharmacological fact is poor, glycoside-dependent oral bioavailability — and the sugar attached to the molecule matters more than the dose. In the classic human comparison, the same nominal quercetin dose was absorbed best from onion glucosides (set at 100%), reaching only ~24% from the pure aglycone and ~17% from rutin (quercetin-3-O-rutinoside) ^{18Reference 18Hollman PC et al. · 1997Relative bioavailability of the antioxidant flavonoid quercetin from various foods in manView study →}. The reason is the absorption site: glucosides are cleaved and taken up high in the small intestine (a fast, ~1 h peak), whereas rutin’s rhamnose–glucose disaccharide resists small-intestinal enzymes and must reach the colon to be freed by gut bacteria — giving a delayed, blunted, highly variable peak ^{18,19Reference 18Hollman PC et al. · 1997Relative bioavailability of the antioxidant flavonoid quercetin from various foods in manView study →Reference 19Graefe EU et al. · 2001Pharmacokinetics and bioavailability of quercetin glycosides in humansView study →}. So “quercetin” is not one drug: isoquercitrin behaves nothing like rutin.

Whatever is absorbed is immediately conjugated, so free aglycone is essentially undetectable in blood — circulating quercetin is a mix of glucuronides, sulfates and the methylated metabolite isorhamnetin ^{19,20Reference 19Graefe EU et al. · 2001Pharmacokinetics and bioavailability of quercetin glycosides in humansView study →Reference 20Mohos V et al. · 2020Inhibitory effects of quercetin and its main methyl, sulfate, and glucuronic acid conjugates on cytochrome P450 enzymes, and on OATP, BCRP and MRP2 transportersView study →}. The conjugates clear slowly (terminal half-lives ~11–28 h), so plasma accumulates modestly with daily dosing, but total concentrations stay low — typically well under 1–2 µmol/L even after gram-level doses ^{19Reference 19Graefe EU et al. · 2001Pharmacokinetics and bioavailability of quercetin glycosides in humansView study →}. This is the crux of quercetin’s in-vitro-vs-in-vivo gap: much of its published bioactivity uses 10–100 µmol/L of free aglycone, one to two orders of magnitude above achievable plasma levels and in a form that barely exists in the body ^{20Reference 20Mohos V et al. · 2020Inhibitory effects of quercetin and its main methyl, sulfate, and glucuronic acid conjugates on cytochrome P450 enzymes, and on OATP, BCRP and MRP2 transportersView study →}.

Clinical trials

Quercetin has been tested in dozens of small investigator-led RCTs — chiefly in blood pressure, metabolic markers and, recently, COVID-19 — but as a cheap, off-patent food molecule it attracts little registered industry trial activity, and several “quercetin” trials actually use branded phytosome or glycoside formulations.

Last checked: July 2026.

Monoamine oxidase (MAO) inhibition

Quercetin is one of the better-characterised flavonoid monoamine-oxidase inhibitors — a moderate, MAO-A-preferring inhibitor with a reported IC50 around 11 µM against MAO-A ^{17Reference 17Herraiz T · 2018Monoamine oxidase-A inhibition and associated antioxidant activity in plant extracts with potential antidepressant actionsView study →}. This is sometimes invoked to explain the mood effects of quercetin-rich plants, but it is far weaker than the β-carboline inhibitors and, given quercetin’s low systemic and brain exposure, is more a footnote than a primary mechanism. See the natural MAO inhibitors guide for the full comparison.

Isolate vs. Plant Studies

Quercetin sits at an unusual crossroads: its human trials genuinely test the isolated molecule (or a supplement of it), yet its plant occurrence is almost entirely as something else. Two cautions follow.

First, the supplement is often not the plain aglycone. Several headline human trials — the allergy and COVID-19 studies especially — used better-absorbed derivatives (enzymatically-modified isoquercitrin) or a phytosome formulation rather than plain quercetin ^{10,14,15Reference 10Di Pierro F et al. · 2021RCTPotential clinical benefits of quercetin in the early stage of COVID-19 — second, pilot, randomised, controlled, open-label clinical trialView study →Reference 14Kawai M et al. · 2009Preventive effect of enzymatically modified isoquercitrin on ocular symptoms of Japanese cedar pollinosis (a quercetin glycoside derivative)View study →Reference 152022RCTEffects of repeated oral intake of a quercetin-containing supplement on allergic reaction — randomised, placebo-controlled, double-blind parallel-group studyView study →}. Their results describe those specific preparations, not necessarily generic quercetin powder.

Second, in plants quercetin is stored as glycosides, not free aglycone — rutin, isoquercitrin, quercitrin, hyperoside, spiraeoside — which absorb very differently ^{18,19Reference 18Hollman PC et al. · 1997Relative bioavailability of the antioxidant flavonoid quercetin from various foods in manView study →Reference 19Graefe EU et al. · 2001Pharmacokinetics and bioavailability of quercetin glycosides in humansView study →}. So a herb’s “quercetin content” is really its quercetin-yielding glycoside pool, and a whole-plant antioxidant effect can’t be assumed to be free quercetin. Across this database quercetin is named in the antioxidant flavonoid fraction of many herbs — bearberry, ginkgo, raspberry leaf, hibiscus, horsetail and others — almost always as part of a mixed extract and frequently as glycosides. A few monographs attribute specific activity to quercetin isolated from the plant (for example anti-psoriatic / antiproliferative activity from Smilax rhizome, or cytokine suppression by isolated quercetin), but most “quercetin” herb claims are whole-plant attributions. Read the supplement trials as evidence for the molecule (in a stated form), and the herb chemistry as evidence for the plants.

Prevalence in Nature

Quercetin is among the most ubiquitous of all plant flavonols, turning up across most flowering-plant lineages rather than a few families ^{21,22Reference 21Dabeek WM · 2019Dietary quercetin and kaempferol: bioavailability and potential cardiovascular-related bioactivity in humansView study →Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →}. The densest dietary sources cluster in a handful: capers (Capparis spinosa) by a wide margin, onion skin (Allium cepa), kale and other Brassica, leafy Apiaceae (dill, lovage), apple peel, Vaccinium berries (cranberry, lingonberry, blueberry), elderberry, buckwheat and tea ^{22,23Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →Reference 23U.S · 2018Department of Agriculture, Agricultural Research ServiceView study →}. It concentrates in the metabolically exposed, UV-stressed organs — leaves, flower buds, fruit skins and outer scales — rather than in flesh or roots (see the Content-by-Source chart) ^{23Reference 23U.S · 2018Department of Agriculture, Agricultural Research ServiceView study →}.

Crucially, in living tissue quercetin is stored as glycosides, not the free aglycone: rutin (quercetin-3-O-rutinoside), isoquercitrin (3-O-glucoside), quercitrin (3-O-rhamnoside), hyperoside (3-O-galactoside) and onion’s spiraeoside (4’-O-glucoside). The food-table figures are “total quercetin” measured after acid hydrolysis of these conjugates — the plant’s quercetin-yielding potential, not its free-aglycone content ^{21,22Reference 21Dabeek WM · 2019Dietary quercetin and kaempferol: bioavailability and potential cardiovascular-related bioactivity in humansView study →Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →}.

Biosynthetically it is a flavonol built through the phenylpropanoid → flavonoid pathway: phenylalanine is converted (via PAL / C4H / 4CL) to p-coumaroyl-CoA, which chalcone synthase condenses with malonyl-CoA to naringenin; flavanone 3-hydroxylase and flavonoid 3’-hydroxylase then give dihydroquercetin (taxifolin), and flavonol synthase desaturates it to quercetin ^{22Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →}. The pathway is strongly up-regulated by UV-B and other stress, consistent with quercetin’s role as a plant sunscreen and antioxidant — hence its accumulation in sun-exposed peels and outer leaves. There is essentially no natural non-plant source: animals and ordinary bacteria do not make it, and it is absent from meat, dairy and eggs; the only non-botanical “source” is engineered microbial fermentation in the lab ^{22Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →}.

Discovery & Synthesis

Quercetin takes its name from the oak: from quercetum (“oak forest”), itself from the genus Quercus. The name has been in use since about 1857, applied to the yellow flavonol pigment characterised from plants in the mid-nineteenth century. Nineteenth-century flavonoid nomenclature was tangled, though — quercetin, quercitrin and rutin were described and renamed by several chemists across the 1840s–60s — and no single primary publication or first isolator can be reliably pinned, so the etymology and approximate date are solid but the “who” is not ^{22Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →}.

Commercially, quercetin is almost never made by total synthesis (laboratory routes to the flavonol skeleton exist but are not cost-competitive). Instead it is produced by acid hydrolysis of rutin: rutin is boiled with dilute acid, cleaving its rhamnose–glucose sugar to free the quercetin aglycone. The rutin feedstock is extracted in bulk from the flower buds of the Japanese pagoda tree (Sophora japonica, ~15–20% rutin by dry weight — the dominant global source) and the Brazilian legume fava d’anta (Dimorphandra mollis) ^{22Reference 222024ReviewQuercetin as one of the most abundant represented biological valuable plant components with remarkable chemoprotective effects — review (occurrence, biosynthesis, manufacture)View study →}.

Patents: not yet researched (future patent-loop pass).

Toxicity & Safety

Oral quercetin is generally well tolerated: human trials have used up to ~1 g/day for weeks to months with adverse effects usually limited to mild GI upset, headache or tingling — and its poor absorption is, ironically, part of that safety margin. The most-cited toxicity signal is route-specific and does not apply to supplements: a 1996 Phase I oncology trial gave quercetin by intravenous bolus and hit dose-limiting renal toxicity at 1700 mg/m², with a measurable acute drop in glomerular filtration even at the recommended dose ^{24Reference 24Ferry DR et al. · 1996Clinical trialPhase I clinical trial of the flavonoid quercetin — pharmacokinetics and evidence for in vivo tyrosine kinase inhibition (intravenous; dose-limiting nephrotoxicity)View study →}. That is IV dosing that bypasses the gut and reaches plasma levels unattainable by mouth — not evidence that oral quercetin harms the kidneys.

The interactions matter more than the side effects, because quercetin is a promiscuous modulator of drug-handling enzymes and transporters. It inhibits CYP3A4 and P-glycoprotein and can disturb cyclosporine exposure — a real concern for transplant patients on that narrow-margin immunosuppressant ^{25,26Reference 25Choi JS et al. · 2004Effect of quercetin on the pharmacokinetics of oral cyclosporineView study →Reference 26Yu CP et al. · 2011Quercetin and rutin reduced the bioavailability of cyclosporine from Neoral, an immunosuppressant, through activating P-glycoprotein and CYP 3A4View study →}. A week of quercetin raised exposure to fexofenadine (a P-glycoprotein / OATP substrate) by roughly 50% in healthy volunteers ^{27Reference 27Kim KA et al. · 2009Short-term effect of quercetin on the pharmacokinetics of fexofenadine, a substrate of P-glycoprotein, in healthy volunteersView study →}. It displaces warfarin from serum albumin and inhibits CYP2C9, so caution and monitoring are prudent with anticoagulants ^{28Reference 28Poór M et al. · 2017Interaction of quercetin and its metabolites with warfarin — displacement of warfarin from serum albumin and inhibition of CYP2C9 enzymeView study →}, and its circulating conjugates themselves inhibit several CYPs plus the OATP, BCRP and MRP2 transporters ^{20Reference 20Mohos V et al. · 2020Inhibitory effects of quercetin and its main methyl, sulfate, and glucuronic acid conjugates on cytochrome P450 enzymes, and on OATP, BCRP and MRP2 transportersView study →}. A long-standing in-vitro caution — that high-dose flavonoids might antagonise fluoroquinolone antibiotics at their shared topoisomerase / gyrase target — remains theoretical rather than a documented clinical failure. Reasonable caution therefore applies to anyone on immunosuppressants, anticoagulants or multiple CYP3A4 / P-gp / OATP-substrate drugs, and to those with significant renal impairment.

Dosage

Human trials have most often used 500–1,000 mg/day of quercetin (usually the aglycone, sometimes a phytosome or glycoside form), and the recurring lesson is a dose threshold: blood-pressure, glucose and uric-acid effects appear only at ≥500 mg/day ^{1,4,9Reference 1Serban MC et al. · 2016Meta-analysisEffects of quercetin on blood pressure: a systematic review and meta-analysis of randomized controlled trials (7 RCTs, n≈587)View study →Reference 4Ostadmohammadi V et al. · 2019Meta-analysisEffects of quercetin supplementation on glycemic control among patients with metabolic syndrome and related disorders — systematic review and meta-analysis of RCTsView study →Reference 9Shi Y · 2016RCTQuercetin lowers plasma uric acid in pre-hyperuricaemic males — randomised, double-blinded, placebo-controlled, cross-over trial (n=22)View study →}. Divided or with-food dosing is common because absorption is low and glycoside-dependent.

These are doses used in research and are not a personal recommendation — appropriateness depends on the individual, their medications (see interactions above) and professional guidance. Note that better-absorbed forms (isoquercitrin, phytosome) are not interchangeable milligram-for-milligram with plain quercetin.

References