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Quercetin: inflammatory markers, seasonal symptoms, absorption and safety

A small average drop in CRP across trials, a large null trial, one small seasonal-symptom study, and real drug interactions. Grade C.

Published Sources checked 12 sourcesEverstack research teamHow we work

Education only, not medical advice. Talk to your clinician before starting a supplement, especially if you are pregnant, take medication or have a condition.

Evidence grade: C (A strong, B good with gaps, C limited or mixed, D early)

Inflammatory markers: CRP, IL-6, TNF-alpha

A meta-analysis of seven randomized trials found quercetin lowered C-reactive protein by an average of 0.33 mg/L, with the effect seen at 500 mg a day or more [1]. That is a small change in a marker, not a health outcome.

The honest null: in the largest trial, 1,002 community adults took 500 or 1,000 mg a day (with vitamin C and niacin) for 12 weeks. CRP and inflammatory cytokines did not differ from placebo, apart from a slight IL-6 decrease at 1,000 mg, and the authors called the overall effect negligible [2]. The same trial found no reduction in upper respiratory infections [3]. In 50 women with rheumatoid arthritis, 500 mg a day for 8 weeks lowered TNF-alpha and some symptoms [4], but that is a disease population and a disease use we do not claim.

Seasonal symptoms

Human evidence is thin. In one double-blind trial, 24 adults with Japanese cedar pollen sensitivity took 100 mg of a quercetin glycoside (EMIQ) or placebo for 8 weeks; eye symptom plus medication scores were lower with EMIQ [5]. That is one small trial of a different form. MSK notes that the anti-allergy effects are from lab studies [6]. Quercetin is not a treatment for allergies, and we do not make a seasonal claim.

Absorption and dosing studied

Plain quercetin is poorly soluble and poorly absorbed, and absorption depends on the form [7]. A lecithin-based phytosome form reached plasma levels up to about 20 times higher than unformulated quercetin in 12 healthy volunteers, in a study run with the ingredient maker [8]. Trials have mostly used 500 to 1,000 mg a day of plain quercetin for 8 to 12 weeks [1][2].

Safety and drug interactions

Side effects in human studies were rare and mild, but there are no adequate safety data for long-term use above 12 weeks at 1,000 mg or more, and animal data raise concerns for people with kidney disease and estrogen-dependent cancers [9].

CYP3A4: quercetin inhibits CYP3A4 and CYP2C19 in lab studies, so it may raise levels of drugs cleared by those enzymes [6]. P-glycoprotein: in 12 healthy people, 500 mg three times a day for 7 days raised exposure to fexofenadine, a P-gp substrate, by 55% [10]. Anticoagulants: a man on stable warfarin had a raised INR after starting quercetin that normalized when he stopped [11]. Quinolone antibiotics: quercetin binds bacterial DNA gyrase, the enzyme these antibiotics target, in lab work [12]; effects in people are unknown, so do not combine with ciprofloxacin or similar drugs without asking your pharmacist.

Pregnancy and breastfeeding: supplemental doses have not been studied for safety, so do not use quercetin supplements if pregnant or breastfeeding [9]. Not for children.

Our grade and our claim

Grade C: a small, inconsistent signal on one marker, a large null trial, and limited seasonal data. The only claim we make is: supports a healthy inflammatory response.*

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Sources

  1. Mohammadi-Sartang et al. 2017, Eur J Clin Nutr. doi.org/10.1038/ejcn.2017.55
  2. Knab et al. 2011, J Am Diet Assoc. doi.org/10.1016/j.jada.2011.01.013
  3. Heinz et al. 2010, Pharmacol Res. doi.org/10.1016/j.phrs.2010.05.001
  4. Javadi et al. 2016, J Am Coll Nutr. doi.org/10.1080/07315724.2016.1140093
  5. Hirano et al. 2009, Allergol Int. doi.org/10.2332/allergolint.08-oa-0070
  6. Memorial Sloan Kettering, About Herbs: Quercetin. www.mskcc.org/cancer-care/integrative-medicine/herbs/quercetin
  7. Graefe et al. 2001, J Clin Pharmacol. doi.org/10.1177/00912700122010366
  8. Riva et al. 2018, Eur J Drug Metab Pharmacokinet. doi.org/10.1007/s13318-018-0517-3
  9. Andres et al. 2017, Mol Nutr Food Res. doi.org/10.1002/mnfr.201700447
  10. Kim et al. 2009, Eur J Clin Pharmacol. doi.org/10.1007/s00228-009-0627-6
  11. Patel et al. 2022, J Pharm Technol. doi.org/10.1177/87551225221125667
  12. Plaper et al. 2003, Biochem Biophys Res Commun. doi.org/10.1016/s0006-291x(03)01006-4

DOIs verified against Crossref and official pages checked on 2026-10-15.