Quercetin

What is quercetin and what does it do in the body?

Quercetin is a secondary plant metabolite found in small amounts in fruits and vegetables. Most of the quercetin and the similar flavonoid kaempferol in plants is bound to sugar groups rather than free; the type of sugar it is bound to determines absorption and therefore biological effect. ^[10] Glucoside-bound forms found in onions appear to have the highest bioavailability in humans; the foods with the highest quercetin concentrations are not necessarily the best-absorbed sources. ^[10]

Absorbed quercetin is rapidly metabolized in the liver and is found in circulation as methyl, glucuronide, and sulfate metabolites; in human studies, bioactivity is assessed by measuring these metabolites in blood and urine. ^[10] Quercetin has no required function in the body; it is not a nutrient with a defined deficiency like a vitamin or mineral. The concepts of “requirement” and “deficiency” therefore do not apply to quercetin; the only question under discussion is whether additional supplementation changes a measurable outcome.

In the introduction of the study by Heinz and colleagues, it is reported that quercetin showed antipathogen activity against various viruses and bacteria in cell culture, whereas small-scale studies in humans gave conflicting results on infection rates. ^[3] A culture finding is not a human outcome; the sections below address only human studies.

What forms exist in supplements, and how does absorption vary?

The safety review by Andres and colleagues states that single-ingredient quercetin supplements are mostly marketed as free quercetin aglycone and that daily doses often go up to 1,000 mg, which exceeds usual dietary intake. ^[2]

The systematic review by Liu and colleagues (31 human intervention studies; with blood or urine measurements) summarizes the factors affecting absorption: the bioavailability of quercetin-3-O-oligoglucosides is 2 times that of quercetin-3-O-glucoside, 10 times that of quercetin-3-O-rutinoside, and about 20 times that of quercetin aglycone. Compared with the aglycone, the cyclodextrin complex of the glucoside showed a 10.8-fold increase; increases between 20 and 62-fold were reported for fenugreek galactomannan/lecithin encapsulation and lecithin phytosome forms. Dietary fat and fiber increased bioavailability by about 2-fold. ^[9] The review contains a conflict-of-interest statement reporting a consulting and grant relationship of one of the authors with two different commercial organizations. Whether the increase in blood level makes a difference in clinical outcome is not the subject of this review.

Does quercetin help recovery after exercise?

The 2023 meta-analysis by Rojano-Ortega and colleagues examined the effect of quercetin on exercise-induced muscle damage, muscle soreness and inflammation, antioxidant capacity, and oxidative stress markers. The 13 included studies had a total of 249 participants ranging from sedentary to well-trained; some concerns for risk of bias were found in all studies, and all but one used 1,000 mg per day. Quercetin reduced muscle soreness at 0/24 hours after exercise (SMD −1.33; p = 0.03), creatine kinase level at 24/48 hours (SMD −1.15; p = 0.02), and post-exercise oxidative stress (SMD −0.92; p = 0.03); there was no effect on IL-6 level. The authors interpreted the result for young men ranging from sedentary to well-trained. ^[1] The evidence therefore covers young men; this meta-analysis reports no result for women, older adults, and patients. The authors declared no conflict of interest. ^[1]

For endurance performance, the 2011 meta-analysis by Kressler and colleagues pooled 11 studies (254 people); the median study duration was 11 days and the median dose was 1,000 mg/day. A significant effect in favor of quercetin was found when VO2max and endurance performance were taken together (effect size 0.15; 95% CI 0.02–0.27); the authors assessed the magnitude as between trivial and small and reported that it corresponded to an improvement of about 2% compared with placebo. When the VO2max and endurance performance subgroups were compared with each other, there was no significant difference (p = 0.69). The PubMed record has an erratum and a commentary for this article; the percentage value is taken from the corrected abstract. ^[7]

Recovery measures (muscle soreness, creatine kinase) are short-term biomarkers and subjective scores; they do not directly measure long-term training adherence or performance.

Does quercetin affect immunity and infections?

General population. Heinz and colleagues conducted a double-blind RCT following 1,002 people aged 18–85 for 12 weeks with two quercetin doses (500 and 1,000 mg/day) or placebo; participants recorded upper respiratory tract infection (URTI) symptoms daily. When all participants were analyzed together, and separately by sex, body mass index, and age categories, there was no significant group difference in URTI outcomes. In a separate analysis, among 325 people aged 40 and over who rated their own fitness level in the upper half of the whole group, URTI severity was 36% lower (p = 0.020) and total sick days 31% lower (p = 0.048) in the 1,000 mg group. ^[3] This is a subgroup finding, not the main result; the authors summarized the overall finding as “no significant effect on URTI rates and symptoms.”

After intensive exercise. In the study by Nieman and colleagues, 40 trained male cyclists received quercetin (1,000 mg/day) or placebo before, during, and for two weeks after cycling three hours a day for three days. There was no difference in exercise-related immune measurements (natural killer cell activity, lymphocyte proliferation, neutrophil oxidative burst, salivary IgA); in the two weeks after exercise, URTI occurred in 1 of 20 people in the quercetin group and 9 of 20 in the placebo group (p = 0.004). ^[4] It is a single, small study; it does not change the result of the large study in the general population.

Is there evidence for quercetin in COVID-19?

The 2023 systematic review by Cheema and colleagues pooled six RCTs examining quercetin in COVID-19 patients with a random-effects model. Quercetin reduced the risk of intensive care admission (OR 0.31; 95% CI 0.10–0.99) and hospitalization frequency (OR 0.25; 95% CI 0.10–0.62); it did not reduce all-cause mortality and the rate of “non-recovery.” The authors say that the benefit may be particularly in the absorption-enhanced phytosome form and state that the findings need to be confirmed by large-scale RCTs. ^[8] The upper end of the confidence interval is very close to 1 for the intensive care outcome; pooling six trials is not considered sufficient evidence for a treatment recommendation.

Does quercetin affect blood pressure and blood lipids?

The 2016 meta-analysis by Serban and colleagues pooled 7 placebo-controlled RCTs reporting blood pressure (9 arms, 587 patients): systolic blood pressure −3.04 mmHg (95% CI −5.75 to −0.33), diastolic −2.63 mmHg (95% CI −3.26 to −2.01). By dose, the decrease was significant in the ≥500 mg/day groups and not significant in the groups under 500 mg/day; however, the indirect comparison test between doses showed no significant difference. The authors stated that clinical importance should be investigated separately. ^[5]

The 2020 meta-analysis by Huang and colleagues collected 17 RCTs (896 participants): systolic −3.09 mmHg (95% CI −4.59 to −1.59), diastolic −2.86 mmHg (95% CI −5.09 to −0.63). Lipid profile and glucose levels did not change significantly; in subgroups lasting ≥8 weeks and including parallel-design studies, significant changes in HDL cholesterol and triglyceride levels were reported. ^[6] Subgroup findings are not the main result. A difference of a few mmHg in blood pressure is a surrogate measure; the effect on cardiovascular events was not measured in these meta-analyses.

Is quercetin safe? Interactions and risk groups

According to the review by Andres and colleagues, in published human intervention studies, side effects after quercetin supplementation were rarely reported and remained mild. The same review states that there are not enough scientific data to assess the safety of high-dose (≥1,000 mg) use beyond 12 weeks. ^[2]

Based on animal studies, the review lists two possible critical safety points: increased nephrotoxic effects in damaged kidneys and promotion of tumor development, particularly in estrogen-dependent cancers. In addition, animal and human studies examining single-dose or short-term quercetin administration have shown interactions between quercetin and some drugs that change drug bioavailability; the review discusses possible risk groups on the basis of these results. ^[2] Some of these findings are animal data and have not been shown as clinical outcomes in humans; even so, people taking regular medicines, people with kidney disease, and people with a history of estrogen-dependent cancer are among the risk groups the review points to.

What is the regulatory status in Türkiye?

In Türkiye, quercetin falls under the Ministry of Agriculture and Forestry’s food supplement legislation; it is not a medicine. In the September 21, 2026 version of the Ministry’s Restricted Substances List for Food Supplements, the sources listed for quercetin are: Sophora japonica L. (at least 95% purity), Dimorphandra mollis, onion (Allium cepa, at least 95% purity), motherwort (Leonurus cardiaca), and Capparis species (at least 95% purity). The upper limit in the ages 11 and over column is written as 500 mg/day; the publication date is January 22, 2016 and the update date is February 19, 2018. ^[11]

Related substances appear in separate rows in the same list: 300 mg/day for rutin (75 mg/day when used together with quercetin), 750 mg/day for isoquercetin, and 100 mg/day for dihydroquercetin (taxifolin); all are in the ages 11 and over column. ^[11] These upper limits are daily intake limits in the regulatory document; they are not evaluated on the same scale as doses such as the 1,000 mg/day used in studies, and comparison is not a recommendation. This guide does not recommend products or places of purchase.

Evidence table: claim, population, and limits

This guide does not recommend individual doses; the figures above are reports of study results.