Quercetin
EVIDENCE: WEAKAlso sold as: quercetin dihydrate, quercetin aglycone, isoquercetin
Quercetin is a flavonoid found in onions, apples and tea. Human trials at 500–1,000 mg a day found fewer illness days after hard exercise and small blood-pressure drops; its anti-allergy effects are mostly shown in lab studies.
Compiled by iSupplementReviews editorial team ·Researched 2026-10-16 · 4 sources
- Type
- Plant flavonoid
- Studied dose
- 500–1,000 mg/day
- Best form
- quercetin dihydrate or more absorbable forms (isoquercetin, phytosome)
- Our rating
- WEAK — Mostly small, animal, or traditional evidence
What is quercetin?
Quercetin is a flavonoid, a plant pigment, found in onions, apples, capers, berries and tea. Supplements concentrate it far beyond food amounts.
Other names and forms
Quercetin dihydrate (common, poorly absorbed), isoquercetin and quercetin phytosome (better absorbed).
Where does quercetin come from?
Plant foods; supplement quercetin is usually extracted from the Japanese pagoda tree or fava beans.
How does quercetin work in the body?
In lab studies quercetin stabilizes mast cells, which release histamine, and acts as an antioxidant (Mlcek et al., 2016). Absorption from standard quercetin is low, which may limit these effects in people.
What is quercetin claimed to do?
Ease seasonal allergies, support immunity and breathing, and support heart health.
What does the research show?
| Claim | Evidence | Dose studied | Verdict |
|---|---|---|---|
| Illness after intense exercise | RCT in cyclists | 1,000 mg/day | Early positive |
| Blood pressure | Meta-analysis | ≥500 mg/day | Small positive |
| Allergy symptoms | Mostly lab/animal | — | Insufficient |
Illness after exercise
Quercetin reduced illness in the two weeks after intense exercise. Nieman et al. (2007) gave trained cyclists 1,000 mg a day around three days of hard training. Immune measures didn’t change.
Blood pressure
Doses of 500 mg a day or more lowered blood pressure slightly in pooled trials (Serban et al., 2016).
What dose was studied?
| Goal | Dose | Duration |
|---|---|---|
| Post-exercise illness | 1,000 mg/day | 3 weeks around training |
| Blood pressure | 500–1,000 mg/day | 4–12 weeks |
Side effects
Headache and tingling occasionally; kidney concerns only at very high intravenous doses.
Drug interactions
- Quinolone antibiotics: quercetin may reduce their effect (moderate).
- Drugs processed by CYP3A4 or CYP2C19: may raise levels; clinical relevance unknown (moderate).
- Warfarin: one case report of a raised INR (moderate).
Who should not take quercetin?
People with kidney disease, and those who are pregnant (no data at supplement doses).
What the authorities say
Memorial Sloan Kettering’s About Herbs entry says the anti-inflammatory findings come from lab work and that clinical trials have not been conducted for allergy and inflammation. No regulator approves quercetin for allergies.
What to know about quercetin
Quercetin has limited human evidence at 500–1,000 mg a day, mostly for post-exercise illness and blood pressure. Its allergy reputation rests largely on lab work.
Frequently asked questions about Quercetin
Does quercetin help allergies?
It may calm mast cells in lab studies, which is why it's sold for seasonal allergies. Good human trials for allergy symptoms are scarce.
Does quercetin prevent colds?
In one trial of trained cyclists, 1,000 mg a day around three days of intense exercise reduced illness afterward. Evidence in the general population is limited.
Is quercetin safe?
Up to about 1,000 mg a day for 12 weeks appears well tolerated. It may interact with some antibiotics and drugs processed by the liver.
References
- [1]Mlcek J et al. — Quercetin and Its Anti-Allergic Immune Response (Molecules, 2016)
- [2]Nieman DC et al. — Quercetin reduces illness but not immune perturbations after intensive exercise (Med Sci Sports Exerc, 2007)
- [3]Serban MC et al. — Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials (J Am Heart Assoc, 2016)
- [4]Memorial Sloan Kettering Cancer Center — Quercetin (About Herbs database)
Products we reviewed containing Quercetin
Based on recorded labels or seller-described formulas. Inclusion does not establish a match to the strains, preparations or amounts studied here; check each review's disclosure limits.