Benefits
Healthy blood pressure
A pooled analysis of 7 randomized trials (587 people) found systolic pressure fell about 3.0 mmHg and diastolic about 2.6 mmHg versus placebo, with the clearest effect at 500 mg a day or more. Individual trials vary: 730 mg a day lowered pressure only in people with stage 1 high blood pressure, 150 mg a day lowered systolic pressure 2.6 mmHg in overweight adults, and a 1,002-person trial saw only a slight fall.
C-reactive protein, a marker of inflammation
A pooled analysis of 7 randomized trials found CRP fell by 0.33 mg/L on average, a small change, with significant results in trials at 500 mg a day or more and in people starting below 3 mg/L. Individual trials did not agree: CRP did not change versus placebo at 150 mg a day in 93 overweight adults or at 500 or 1,000 mg a day in 1,002 adults, so whether quercetin meaningfully lowers CRP is uncertain.
Immune defense during heavy training
In a 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 was followed by fewer upper respiratory infections (1 of 20 versus 9 of 20 on placebo), though immune cell measures did not differ. In 1,002 community adults, 12 weeks at 500 or 1,000 mg a day did not change infection rates overall.
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 (a form made for better absorption) improved several self-rated scores, including eye itching, sneezing, runny nose and sleep. This was a single small trial funded by the maker of that form, and the result belongs to that form and dose.
Antioxidant markers in human trials
In laboratory tests quercetin slows the oxidation of LDL cholesterol. In people the results differ: 730 mg a day for 4 weeks did not change blood or urine markers of oxidative stress, while 150 mg a day for 6 weeks lowered oxidized LDL in 93 overweight adults.
Mechanism of action
Antioxidant activity (laboratory)
In laboratory tests quercetin slows the oxidation of LDL cholesterol. In people, 730 mg a day for 4 weeks did not change blood or urine markers of oxidative stress, while 150 mg a day for 6 weeks lowered oxidized LDL in overweight adults.
Inflammation signaling
Experimental studies suggest quercetin has anti-inflammatory effects. In people, pooled trials show a small fall in CRP of about 0.33 mg/L, but a 12-week trial in 1,002 adults found no change in CRP and only a slight fall in IL-6 at 1,000 mg a day.
Histamine and mast cells (laboratory)
In cultured human mast cells, quercetin reduced the release of histamine, leukotrienes and other allergy messengers. None of the human trials cited here showed that this happens at the blood levels reached with supplements.
Absorption depends on the form
In 9 volunteers with an ileostomy, about 52% of the quercetin glucosides in fried onions was absorbed, versus 24% of pure quercetin. Phytosome forms are designed to raise uptake, and the pollen season trial used one. Onion extract has its own page on this site.
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. Conducted under assignment from, and paid for by, Indena, the maker of the phytosome, which supplied the test and placebo products; one author works for Indena Japan. (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.
Randomized double-blind placebo-controlled trial: 500 or 1,000 mg a day of quercetin for 12 weeks in soft chews that also contained vitamin C and niacin. Funded by grants from Coca-Cola and Quercegen Pharma, which were involved in designing the study but not in data analysis or writing; the chews were prepared with Quercegen Pharma, and one author sat on its science advisory board. (Heinz et al. 2010, Pharmacol Res)
1,002 community adults aged 18 to 85.
Upper respiratory infection sick days, severity and symptom scores did not differ from placebo for the whole group, or by sex, body mass index or age. In a subgroup of 325 adults aged 40 or older who rated themselves in the top half of the group for fitness, 1,000 mg a day was linked to 31% fewer sick days and 36% lower severity.
Randomized placebo-controlled trial: 500 or 1,000 mg a day of quercetin with added vitamin C and niacin for 12 weeks. Same research group, number of adults (1,002), doses and 12-week design as the infection report by Heinz and colleagues. The authors concluded the effect on risk factors was negligible. (Knab et al. 2011, J Am Diet Assoc)
1,002 community adults (60% women) of widely varying age and body mass index.
Mean arterial blood pressure fell slightly at both doses versus placebo, total cholesterol differed between the 500 mg group and placebo, and HDL cholesterol fell slightly at 1,000 mg a day. CRP and other inflammation markers did not change, except a slight fall in IL-6 at 1,000 mg a day.
Randomized double-blind placebo-controlled crossover trial of 730 mg a day of quercetin for 28 days. (Edwards et al. 2007, J Nutr)
41 adults: 19 with prehypertension and 22 with stage 1 hypertension.
Blood pressure did not change in the prehypertension group. In the stage 1 group, systolic pressure fell 7 mmHg, diastolic 5 mmHg and mean arterial pressure 5 mmHg. Plasma and urine markers of oxidative stress did not change.
Randomized double-blind placebo-controlled crossover trial: 150 mg a day of quercetin for 6 weeks per period with a 5-week washout. (Egert et al. 2009, Br J Nutr)
93 overweight or obese adults aged 25 to 65 with metabolic syndrome traits.
Systolic blood pressure fell 2.6 mmHg versus placebo and oxidized LDL fell. HDL cholesterol also fell, while total cholesterol, triglycerides, TNF-alpha and CRP did not change. Liver and kidney blood tests showed no adverse effects.