Benefits
Antioxidant activity in lab studies
In test tube studies quercetin neutralizes free radicals. This has not been confirmed as a measurable antioxidant effect in people taking quercetin supplements.
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. That is a small change. The drop was significant in trials using 500 mg a day or more and in people whose CRP started below 3 mg/L, but the size of the drop was not linked to dose.
Immune support
In a single small trial of 40 trained male cyclists, 1,000 mg a day for 3 weeks before, during and 2 weeks after a hard 3-day training block cut colds in the following 2 weeks to 1 of 20 versus 9 of 20 on placebo. Natural killer cell activity, lymphocyte response and salivary IgA did not differ from placebo.
Nasal and eye comfort in pollen season
In a 4-week randomized placebo-controlled trial of 66 adults aged 22 to 78 with pollen allergy symptoms, 200 mg a day of quercetin as a phytosome (an enhanced-absorption form) improved several self-rated scores, including eye itching, sneezing, runny nose and sleep. This was a single small trial, and the result belongs to that form and dose.
Healthy blood pressure
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. The effect was seen at 500 mg a day or more (systolic down 4.45 mmHg) but not at lower doses, although a direct test of the dose difference was not significant.
Mechanism of action
Free radical scavenging (laboratory)
In test tube studies quercetin donates electrons to unstable molecules called free radicals. Whether this happens at the low blood levels reached after a supplement is not known.
Anti-inflammatory signaling
In laboratory studies quercetin dampens NF-kB signaling, a switch that turns on inflammatory messengers. In people, pooled randomized trials show a small fall in C-reactive protein of about 0.33 mg/L.
Histamine and mast cells (laboratory)
In laboratory work quercetin makes mast cells less likely to release histamine. This is the usual explanation offered for the pollen season results, but the human trial did not measure histamine.
Absorption
Quercetin is poorly absorbed from the gut. Phytosome forms bind it to lecithin to improve uptake, and the pollen season trial used such a form.
Clinical trials
Systematic review and meta-analysis of 7 placebo-controlled randomized trials (9 treatment arms, 587 people) of quercetin supplements. (Serban et al. 2016, J Am Heart Assoc)
587 adults across 7 trials.
Systolic blood pressure fell by 3.04 mmHg and diastolic by 2.63 mmHg versus placebo. At 500 mg a day or more the falls were 4.45 and 2.98 mmHg; below 500 mg a day there was no significant effect, although indirect comparison did not show a significant difference between dose groups.
Systematic review and meta-analysis of 7 randomized controlled trials (10 treatment arms) of quercetin and blood CRP, an inflammation marker. (Mohammadi-Sartang et al. 2017, Eur J Clin Nutr)
Adults across 7 trials.
CRP fell by 0.33 mg/L on average. The drop was significant in trials using 500 mg a day or more and in people starting with CRP below 3 mg/L. Meta-regression found no link between the change in CRP and dose or starting CRP.
Randomized double-blind placebo-controlled trial: 1,000 mg a day for 3 weeks before, during and 2 weeks after 3 days of 3-hour cycling sessions. (Nieman et al. 2007, Med Sci Sports Exerc)
40 trained male cyclists.
Upper respiratory infections in the 2 weeks after the training block: 1 of 20 on quercetin versus 9 of 20 on placebo (P=0.004). Exercise-induced changes in natural killer cell activity, lymphocyte proliferation, oxidative burst activity and salivary IgA did not differ from placebo.
4-week randomized placebo-controlled double-blind parallel-group trial of 200 mg a day of quercetin as Quercetin phytosome. (Yamada et al. 2022, Eur Rev Med Pharmacol Sci)
66 Japanese adults aged 22 to 78 with pollen allergy symptoms of the eyes and nose.
Several self-rated quality of life scores improved versus placebo, including eye itching, sneezing, nasal discharge and sleep disturbance. Only minor adverse effects were noted. The trial was small and tested one specific formulation.