Quercetin

What It Does

Mast cells release histamine and cytokines in allergic reactions. In cultured human mast cells, quercetin blocked cytokine release more effectively than cromolyn, a prescription mast cell stabilizer, and it reduced contact dermatitis and photosensitivity reactions in a small human test (Weng 2012). That is the basis for its use in seasonal allergies, where controlled trials are lacking.

The firmest human finding is a small effect on blood pressure. A 2016 meta-analysis of seven trials found that quercetin at 500 mg a day or more lowered systolic pressure by about 3 mm Hg (Serban 2016). For infection, 1,000 mg daily for 12 weeks did not reduce respiratory infections in 1,002 adults; a subgroup of older, physically fit participants had fewer sick days, which is a hypothesis rather than a result (Heinz 2010).

During the pandemic, computer docking studies showed quercetin binding a SARS-CoV-2 protease, and small open-label trials of quercetin phytosome reported faster recovery (Di Pierro 2021). Open-label designs cannot separate a real effect from expectation and differences in care, and no placebo-controlled trial has confirmed a benefit.

Quercetin inhibits CYP3A4, CYP2C8, and P-glycoprotein in laboratory studies and may raise levels of drugs that depend on them. High-dose intravenous quercetin caused kidney toxicity in an early cancer trial (Ferry 1996), so people with kidney disease should avoid high oral doses.

Forms and Bioavailability

Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.

Labels column: number of labels named as quercetin products that list this form. A product can list more than one form, and some labels do not state a form at all.

The pure aglycone is poorly soluble and less well absorbed than the quercetin glucosides in onions. Lecithin-bound quercetin phytosome produced up to about twentyfold higher plasma levels than the aglycone in a crossover study funded by its maker (Riva 2019). Enzymatically modified isoquercitrin adds glucose units that make quercetin water soluble, and it reached much higher plasma levels than the aglycone in humans (Murota 2010). Pairing quercetin with bromelain is common; no human data show that bromelain improves absorption.

Typical Dosing

Trials used 500-1,000 mg daily of quercetin aglycone, or lower doses of enhanced-absorption forms, for up to 12 weeks.

Food sources: capers, red onions, kale, apples with skin, berries, tea.

What to Look For

If you want to try quercetin, a phytosome at 250-500 mg or plain quercetin at 500 mg once or twice daily covers the trial range. Check any prescription drugs for CYP3A4 or P-glycoprotein interactions first.

Red flags: COVID-19 prevention or treatment claims, "zinc ionophore" marketing built on cell studies, and immune bundles with undisclosed amounts. Green flags: the form named, milligrams of quercetin per serving, and third-party testing.

Products on Titrate

3,011 of the 214,745 labels in the NIH Dietary Supplement Label Database list quercetin as an active ingredient (1,502 on market). 495 are sold as quercetin products by name.

Top on-market labels, ranked by label completeness and simplicity:

1. Mega NAC + Quercetin Body Kitchen · 600 mg
2. Mega Quercetin Body Kitchen · 1 g
3. Quercetin Herbadiet · 400 mg as standardized extract
4. Super Quercetin 1000 mg SuperSmart · 1 g as anhydrous, extract
5. Nettles Quercetin 350 mg Eclectic Herb · 350 mg

Open supplement search

Quercetin does not have its own search category yet. Each product above links to its full label in the NIH database.

Related Tools

• Enzymatically Modified Isoquercitrin vs Quercetin
• Enzymatically Modified Isoquercitrin vs Quercetin Phytosome
• Quercetin vs Quercetin Phytosome
• All quercetin form comparisons
• All supplement guides

References

Linus Pauling Institute Micronutrient Information Center, Oregon State University. Flavonoids.

Ferry DR, Smith A, Malkhandi J, et al. Phase I clinical trial of the flavonoid quercetin: pharmacokinetics and evidence for in vivo tyrosine kinase inhibition. Clin Cancer Res. 1996;2(4):659-668.

Heinz SA, Henson DA, Austin MD, Jin F, Nieman DC. Quercetin supplementation and upper respiratory tract infection: a randomized community clinical trial. Pharmacol Res. 2010;62(3):237-242.

Murota K, Matsuda N, Kashino Y, et al. alpha-Oligoglucosylation of a sugar moiety enhances the bioavailability of quercetin glucosides in humans. Arch Biochem Biophys. 2010;501(1):91-97.

Weng Z, Zhang B, Asadi S, et al. Quercetin is more effective than cromolyn in blocking human mast cell cytokine release and inhibits contact dermatitis and photosensitivity in humans. PLoS One. 2012;7(3):e33805.

Serban MC, Sahebkar A, Zanchetti A, et al. Effects of quercetin on blood pressure: a systematic review and meta-analysis of randomized controlled trials. J Am Heart Assoc. 2016;5(7):e002713.

Riva A, Ronchi M, Petrangolini G, Bosisio S, Allegrini P. Improved oral absorption of quercetin from quercetin phytosome, a new delivery system based on food grade lecithin. Eur J Drug Metab Pharmacokinet. 2019;44(2):169-177.

Di Pierro F, Derosa G, Maffioli P, et al. Possible therapeutic effects of adjuvant quercetin supplementation against early-stage COVID-19 infection: a prospective, randomized, controlled, and open-label study. Int J Gen Med. 2021;14:2359-2366.