What Quercetin is
Quercetin is a flavonol, a type of plant polyphenol (flavonoid). It occurs naturally in a range of foods such as onions, apples, berries, broccoli, citrus fruits, leafy herbs like cilantro and dill, tea, and red wine. Laboratory research suggests it may have antioxidant, anti-inflammatory, antiviral, and immune-modulating activity, but lab and test-tube findings do not establish clinical benefit. Quercetin has very low oral bioavailability (roughly 3-17%); per the NIH, taking it with sunflower lecithin can substantially raise absorption (reported as up to about 20 times in some research).
Commonly used for
General immune supportAntioxidantReducing inflammationStudied (with unclear results) for upper respiratory tract infections such as coldsStudied (inconclusively) for COVID-19
What the evidence shows
The available evidence does not establish that quercetin treats, cures, or prevents any illness. The NIH notes quercetin has been studied for reducing the risk or severity of upper respiratory tract infections, but "it's not clear whether it helps," and results across the few clinical trials have been mixed. For COVID-19, the NIH states the data are very limited and it is unclear whether quercetin reduces the risk or severity of disease because it has not been well studied. Marketing quercetin as a treatment, cure, or preventive for any disease is not supported by current evidence.
Typical dosage
There is no established or officially recommended supplement dose. Clinical trials referenced by the NIH have used 500 to 1,000 mg per day (sometimes combined with vitamin C or niacin), and the FDA considers up to 500 mg of quercetin per serving generally recognized as safe (GRAS) as an ingredient in foods and beverages. Because quercetin can interact with several medications and its benefits are unproven, talk with a clinician or pharmacist before starting it, especially if you take prescription drugs or have a health condition.
Safety
The NIH reports that no serious adverse effects have been reported in clinical trials of quercetin supplements using up to 1,000 mg/day for up to 12 weeks. Safety beyond these short study periods is not well characterized, and supplement quality is not tightly regulated. As with any supplement, people who are pregnant or breastfeeding and those with medical conditions should consult a healthcare provider before use.
Side effects
No serious side effects have been reported in studies of quercetin supplements at the doses tested. According to the NIH, quercetin supplements may, however, cause minor effects such as stomach pain, acid reflux, constipation, diarrhea, flatulence, and trouble sleeping. Data on long-term effects are limited, so it is reasonable to start cautiously and stop if you notice an adverse reaction.
Interactions & cautions
Quercetin may interact with several medications. The NIH notes it can affect drug-metabolizing enzymes (such as CYP3A4) and may raise blood levels of cyclosporine, pravastatin (a cholesterol medication), and fexofenadine (an antihistamine). It may add to the blood-pressure-lowering effect of antihypertensive medications, may interfere with drugs that suppress the immune system, and may inhibit platelet aggregation (relevant if you take blood thinners). Review all of your medications with a clinician or pharmacist before use.
This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Summarized from public health authorities; general reference, not medical advice. Talk to a licensed healthcare professional before starting any supplement.