Benefits
Antioxidant Activity (laboratory only)
In test tube studies quercetin can neutralize free radicals. None of the studies cited on this page measured oxidative stress or antioxidant status in a person, and none tested whether quercetin prevents cancer or heart disease.
Anti-Inflammatory Effects
A pooled analysis of 7 randomized trials found quercetin lowered C-reactive protein, a blood marker of inflammation, by 0.33 mg/L on average (P<0.001). That is a small change. In the same analysis the authors found no relationship between the dose used and the size of the CRP effect. No cited trial tested whether this helps arthritis or heart disease.
Immune Support
In a randomized trial of 40 trained male cyclists taking 1,000 mg a day for 3 weeks before a hard 3-day training block, 1 of 20 on quercetin developed an upper respiratory infection versus 9 of 20 on placebo (P=0.004). In that same trial natural killer cell activity, lymphocyte proliferation, oxidative burst activity and salivary IgA all showed no significant change. So fewer illnesses were counted in a small group of male athletes under unusual training stress, with no measurable change in any immune marker. No cited study tested quercetin against a specific virus.
Allergy Relief
In a 4-week randomized placebo-controlled trial of 66 people aged 22 to 78 with diagnosed pollen allergy, 200 mg a day of a quercetin phytosome (a specific enhanced-absorption form, not plain quercetin) improved several self-reported scores including eye itching, sneezing, nasal discharge and sleep. The mast cell and antihistamine explanation comes from laboratory work, not from this trial. The trial was small and the result belongs to that formulation and that dose.
Heart Health
A pooled analysis of 7 randomized trials in 587 people found quercetin lowered systolic blood pressure by about 3.0 mmHg and diastolic by about 2.6 mmHg. Dose was decisive: 500 mg a day or more lowered systolic pressure by 4.45 mmHg (P=0.007), while doses under 500 mg a day changed nothing (P=0.273). No cited study showed any change in cholesterol or any effect on artery disease.
Brain Health
None of the studies cited on this page measured memory, thinking or any brain condition in people. There is no human evidence here for a brain benefit, and quercetin should not be presented as protecting against any neurological disease.
Exercise Performance
The one exercise study cited here, in 40 trained male cyclists at 1,000 mg a day, did not show reduced exercise-induced immune or inflammatory changes; all of those markers were unchanged. What it did find was fewer upper respiratory infections during a heavy training block. Endurance performance claims are not supported by the references on this page.
Mechanism of action
Free Radical Scavenging (laboratory)
In test tube studies quercetin donates electrons to unstable molecules called free radicals. This is laboratory chemistry. No study cited on this page measured oxidative stress, nerve growth or brain tissue in a person.
Anti-Inflammatory Effects
In laboratory studies quercetin dampens NF-kB signaling, a switch that turns on inflammatory messengers. What has actually been measured in people is modest: pooled randomized trials show C-reactive protein falling by about 0.33 mg/L. Nothing cited here tested brain inflammation or any neurological condition.
Histamine and Mast Cells (laboratory)
In laboratory work quercetin makes mast cells less likely to release histamine, which is the usual explanation offered for the hay fever results. No study cited on this page measured mood, anxiety or depression, and quercetin should not be presented as relieving either.
Immune Support
Quercetin is a flavonoid. It contains no beta-glucans or immune polysaccharides, so this pattern recognition receptor mechanism does not apply to it. In the one human trial cited here, quercetin did not change natural killer cell activity, lymphocyte proliferation, oxidative burst activity or salivary IgA, even though fewer participants fell ill.
Absorption
Quercetin is poorly absorbed, which is why it is often taken with food or sold in enhanced-absorption forms. None of the studies cited on this page looked at gut bacteria, the gut lining or stomach health, and the H. pylori claim has no support among the references listed here.
Clinical trials
Single-center, open-label, randomized controlled trial at King Edward University, Lahore, Pakistan, in early-stage COVID-19 patients receiving quercetin vs standard care. (Di, Front Pharmacol)
Early COVID-19 outpatients.
Reported small changes in some clinical and inflammation measures versus usual care. Nobody was blinded, the sample was small, and this study is not among the four references listed on this page, so it cannot be checked. It was research in people already diagnosed and being treated for an infection. Quercetin is not a treatment or a preventive for COVID-19 or any other infection.
2-week, open-label, randomized controlled trial in 42 outpatients with mild COVID-19 comparing standard care vs standard care + quercetin. (Di)
42 mild COVID-19 outpatients.
Reported smaller symptom scores and inflammation markers. It was open-label with only 42 participants, industry funded, and it is not among the references cited on this page, so it cannot be checked. Quercetin is not a treatment for COVID-19.
Randomized, double-blind, placebo-controlled trial of Quercetin Phytosome™ (500 mg/day) for 2 months in adults with chronic fatigue symptoms.
Chronic fatigue patients.
Reported small improvements on fatigue questionnaires. No reference for this trial is listed on this page and the number of participants is not given, so the result cannot be checked. Quercetin phytosome is a lecithin-based form and is not the same thing as the plain quercetin in most products.
Double-blind, randomized controlled trial of quercetin 500 mg/day for 8 weeks in women with active RA. (J Am Coll Nutr)
Women with active RA.
Reported small reductions in disease activity and morning stiffness. This was research in women already diagnosed with rheumatoid arthritis, an autoimmune disease managed with prescription medicines. It is not evidence that quercetin treats arthritis and it says nothing about what quercetin does for a healthy person. No reference for this trial is listed on this page.
Evidence review and pooled analysis of 20 clinical trials (1,164 participants) examining quercetin in metabolic syndrome components.
Pooled across 20 clinical trials.
Reported small changes in blood pressure, blood fats and inflammation markers. No reference for this review is listed on this page, so the 20 trials and 1,164 participants cannot be verified. The blood pressure part overlaps with the meta-analysis that is cited here, which found about a 3 mmHg reduction and only at doses of 500 mg a day or more.
Double-blind, randomized controlled trial of quercetin (500 mg/day) in women with T2DM. (J Res Med Sci)
Women with T2DM.
Reported small changes in blood sugar and blood fat measures. This was research in women already diagnosed with type 2 diabetes, and it is not evidence that quercetin manages, treats or prevents diabetes. No reference for this trial is listed on this page, so it cannot be checked.
Phase 1/2 clinical trial testing dasatinib + quercetin (1,000 mg/day) in patients with diabetic kidney disease. Outcomes: senescent cell burden, kidney function. (EBioMedicine)
Diabetic kidney disease patients.
Reported small reductions in markers of cellular senescence and inflammation. Any effect belongs to the combination, not to quercetin on its own: dasatinib is a prescription cancer drug and the study design cannot separate the two. It was an early-phase study in people with diabetic kidney disease, and no reference for it is listed on this page. This is experimental research and is not a reason to take a quercetin supplement.
Randomized, double-blind, placebo-controlled trial of quercetin (500 or 1,000 mg/day) vs placebo in adults for URI prevention/management. (— or related)
Adult URI participants.
This card gives no reference at all, and its description does not match the one respiratory trial actually cited on this page, which was 40 trained male cyclists at 1,000 mg a day. Treat it as unverified. The verifiable result is that cyclist trial: 1 of 20 on quercetin got an infection versus 9 of 20 on placebo, with no change in any immune marker measured.
Randomized, double-blind, placebo-controlled trial in 60 male physical education students examining 500 mg quercetin + vitamin C vs placebo. Outcomes: oxidative stress, athletic performance.
60 physical education students.
Because quercetin was given together with vitamin C, any change cannot be credited to quercetin alone. No reference for this trial is listed on this page, so the 60 participants and the results cannot be checked, and it does not support an antioxidant claim for quercetin by itself.