Quercetin quer·ce·tin (ˈkwər-sə-tən) is a plant-derived flavonoid, specifically a flavonol, used as a nutritional supplement.

From in vitro studies, quercetin has demonstrated significant anti-inflammatory activity by inhibiting both manufacture and release of histamine and other allergic/inflammatory mediators. In addition, it exerts potent antioxidant activity and vitamin C-sparing action.

In vitro, quercetin shows some antitumor activity. Cultured skin and prostate cancer cells showed significant mortality (compared to nonmalignant cells) when treated with a combination of quercetin and ultrasound. Note that ultrasound also promotes topical absorption by up to 1,000 times making the use of topical quercetin and ultrasound wands an interesting proposition.

Recent studies have supported that quercetin may help men with chronic prostatitis, and both men and women with interstitial cystitis, possibly because of its action as a mast cll inhibitor.

Quercetin may have positive effects in combating or helping to prevent cancer, prostatitis, heart disease, cataracts, allergies/inflammations, and respiratory diseases such as bronchitis and asthma. It also has been claimed to have antidepressant properties, however any claim of quercetin action against neurological diseases should be treated with skepticism due to the fact that quercetin is a neurotoxin in vitro.

It has also been claimed that quercetin reduces blood pressure in hypertensive subjects. It also may be found in dietary supplements.

An 8-year study found that the presence of three flavonols — kaempferol, quercetin, and myricetin — in the person's normal diet were associated with a reduced risk of pancreatic cancer, a rare but frequently fatal disease, of 23 percent in current tobacco smokers. There was no benefit to people that have never smoked or that had previously quit smoking.

A laboratory study found that a dose of 12.5 to 25 mg/kg increased endurance in mice by causing more mitochondria to grow.

An in-vitro study on fat cells suggested that quercetin and resveratrol may have a synergistic anti-obesity effect.

Despite these indications of possible health benefits, quercetin has neither been confirmed as a specific therapeutic for any condition nor has it been approved by any regulatory agency. In fact, bioavailability studies show that ingested quercetin is extensively metabolized into non-active phenolic acids, with more than 96% of the ingested amount excreted within 72 hours, indicating actual physiological roles, if they exist, involve quercetin in only minute amounts.