Quercetin

Quercetin is the base form of a number of other flavonoids, such as rutin and quercitrin, found in citrus fruit & onions. Quercetin forms the glycosides quercitrin and rutin together with rhamnose and rutinose, respectively.
Quercetin is found to be the most active of the flavonoids in studies, and many medicinal plants owe much of their activity to their high quercetin content. Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several initial processes of inflammation. For example, quercetin inhibits both the manufacture and release of histamine and other allergic/inflammatory mediators. In addition, it is a potent antioxidant and spares vitamin C.
Quercetin also shows anti-tumour properties. A study in the British Journal of Cancer showed that, when treated with a combination of quercetin and ultrasound at 20 kHz for 1 minute duration, skin and prostate cancers show a 90% mortality within 48 hours with no visible mortality of normal cells.
Recent studies have indicated that quercetin can help men with chronic prostatitis, and both men and women with interstitial cystitis, possibly because of its action as a mast cell 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 antidepressant properties.
Foods rich in quercetin include capers (1800mg/kg), lovage (1700mg/kg), apples (440mg/kg), tea (Camellia sinensis), onions (higher concentrations of quercetin occur in the outermost rings), red grapes, citrus fruits, broccoli and other leafy green vegetables, cherries, and a number of berries including raspberry, bog whortleberry (158 mg/kg, fresh weight), lingonberry (cultivated 74mg/kg, wild 146 mg/kg), cranberry (cultivated 83 mg/kg, wild 121 mg/kg), chokeberry (89 mg/kg), sweet rowan (85 mg/kg), rowanberry (63 mg/kg), sea buckthorn berry (62 mg/kg), crowberry (cultivated 53mg/kg, wild 56 mg/kg), and the fruit of the prickly pear cactus. A recent study found that organically grown tomatoes had 79% more quercetin than "conventionally grown".
A study by the University of Queensland, Australia, has also indicated the presence of quercetin in varieties of honey, including honey derived from eucalyptus and tea tree flowers.
Quercetin is contraindicated with antibiotics; it may interact with fluoroquinolones (a type of medicinal antibiotic), as quercetin competitively binds to bacterial DNA gyrase. Whether this inhibits or enhances the effect of fluoroquinolones is not entirely clear.
Quercetin is also a potent inhibitor of CYP3A4, an enzyme that breaks down most drugs in the body. As such, quercetin would be expected to increase serum levels, and therefore effects, of drugs metabolized by this enzyme.
Quercetin is therefore one of the supplements to be taken with caution, and preferably medical approval, if you are on conventional medication.