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Quercetin and inflammatory markers: what human trials show

A meta-analysis of seven randomized trials found a small average drop in CRP at 500 mg a day or more, but the largest community trial (about 1,000 adults) was essentially null on CRP and cytokines. Seasonal and disease-population signals are thin or not transferable. Grade C.

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Editorial section. Research summaries are not product claims and contain no product links or offers. A summary is never proof that any product works.

The question
What do randomized human trials show for quercetin on C-reactive protein and related inflammatory markers, and does a phytosome form change the picture?
Study design
A systematic review and meta-analysis of seven randomized trials on plasma CRP (Mohammadi-Sartang 2017); a 12-week randomized community trial of 500 or 1,000 mg a day (with vitamin C and niacin) in 1,002 adults (Knab 2011) with a paired upper-respiratory analysis (Heinz 2010); an 8-week trial of 500 mg a day in 50 women with rheumatoid arthritis (Javadi 2016); an 8-week double-blind trial of 100 mg of a quercetin glycoside (EMIQ) in 24 adults with Japanese cedar pollen sensitivity (Hirano 2009); a pharmacokinetics study of a lecithin phytosome versus unformulated quercetin in 12 healthy volunteers (Riva 2019); a glycoside absorption study (Graefe 2001); a safety review (Andres 2018); a P-glycoprotein interaction study with fexofenadine (Kim 2009); a warfarin case report (Patel 2022); lab work on DNA gyrase binding (Plaper 2003); and Memorial Sloan Kettering About Herbs notes on quercetin. [1][2][3][4][5][6][7][8][9][10][11][12]
What they found
The CRP meta-analysis reported an average reduction of 0.33 mg/L with quercetin, mainly at doses of 500 mg a day or more. That is a small change in a blood marker, not a clinical outcome. In Knab 2011, CRP and inflammatory cytokines did not differ from placebo apart from a slight IL-6 decrease at 1,000 mg; the authors called the overall effect negligible, and Heinz 2010 found no reduction in upper respiratory infections in the same cohort. In women with rheumatoid arthritis, 500 mg a day for 8 weeks lowered TNF-alpha and some symptoms; that is a disease population and a disease use we do not claim. Hirano 2009 reported lower eye symptom plus medication scores with EMIQ versus placebo in one small seasonal trial of a different form. Plain quercetin is poorly absorbed; a phytosome form reached plasma levels up to about 20 times higher than unformulated quercetin in Riva 2019, a study run with the ingredient maker. [1][2][3][4][5][6][7][8][9][10][11][12]
Limitations
The largest and most relevant null (Knab 2011 / Heinz 2010) outweighs the small average CRP drop in the meta-analysis for community adults. Most marker trials were short (8 to 12 weeks). Javadi 2016 enrolled people with rheumatoid arthritis, so its symptom results do not support a general wellness claim. Hirano 2009 used EMIQ, not plain quercetin or phytosome, in 24 people. Riva 2019 was industry-tied (Indena phytosome) and measured absorption, not inflammatory outcomes. Safety data above 12 weeks at 1,000 mg or more are inadequate, and animal data raise concerns for kidney disease and estrogen-dependent cancers (Andres 2018).
What this does not mean
These studies do not show that quercetin is a treatment for allergies, arthritis, infections or any disease. Our claim is structure/function only: supports a healthy inflammatory response. Drug interactions matter: quercetin can inhibit CYP3A4 and CYP2C19 in lab work; 500 mg three times a day for 7 days raised fexofenadine exposure by 55% via P-glycoprotein (Kim 2009); a case report linked quercetin to a raised INR on stable warfarin (Patel 2022); and lab work shows binding to bacterial DNA gyrase, the target of quinolone antibiotics (Plaper 2003), so do not combine with ciprofloxacin or similar drugs without asking a pharmacist. Avoid in pregnancy and breastfeeding; not for children; ask a clinician first if you have kidney disease, an estrogen-sensitive condition, or take prescription drugs.
Sources
  1. Mohammadi-Sartang M et al. Eur J Clin Nutr. 2017;71(9):1033-1039. doi.org/10.1038/ejcn.2017.55
  2. Knab AM et al. J Am Diet Assoc. 2011;111(4):542-549. doi.org/10.1016/j.jada.2011.01.013
  3. Heinz SA et al. Pharmacol Res. 2010;62(3):237-242. doi.org/10.1016/j.phrs.2010.05.001
  4. Javadi F et al. J Am Coll Nutr. 2017;36(1):9-15. Rheumatoid arthritis population; disease use not claimed. doi.org/10.1080/07315724.2016.1140093
  5. Hirano T et al. Allergol Int. 2009;58(3):373-382. EMIQ glycoside, not plain quercetin. doi.org/10.2332/allergolint.08-oa-0070
  6. Riva A et al. Eur J Drug Metab Pharmacokinet. 2019;44(2):169-177. Industry-tied phytosome absorption. doi.org/10.1007/s13318-018-0517-3
  7. Graefe EU et al. J Clin Pharmacol. 2001;41(5):492-499. doi.org/10.1177/00912700122010366
  8. Andres S et al. Mol Nutr Food Res. 2018;62(1). doi.org/10.1002/mnfr.201700447
  9. Kim KA et al. Eur J Clin Pharmacol. 2009;65(11):1067-1072. doi.org/10.1007/s00228-009-0627-6
  10. Patel R et al. J Pharm Technol. 2022;38(6):347-350. Warfarin case report. doi.org/10.1177/87551225221125667
  11. Plaper A et al. Biochem Biophys Res Commun. 2003;306(2):530-536. Lab DNA gyrase binding. doi.org/10.1016/s0006-291x(03)01006-4
  12. Memorial Sloan Kettering Cancer Center, About Herbs: Quercetin. www.mskcc.org/cancer-care/integrative-medicine/herbs/quercetin

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