Quercetin belongs to a group of plant pigments called flavonoids that give many fruits, flowers, and vegetables their color. Fruits and vegetables, particularly citrus fruits, apples, onions, parsley, sage, tea, and red wine, are the primary dietary sources of quercetin. Olive oil, grapes, dark cherries, and dark berries, such as blueberries, blackberries, and bilberries, are also high in flavonoids, including quercetin.

Scientists have long considered quercetin, and other flavonoids, important in cancer prevention. People who eat more fruits and vegetables tend to have lower risk of some types of cancer. And animal and test tube studies suggest that flavonoids do indeed have anti cancer properties.
Quercetin and other flavonoids have been shown in these studies to inhibit the growth of cancer cells from breast, colon, prostate, ovarian, endometrial, and lung tumors.  One study even suggests that quercetin is more effective than reservatrol in terms of inhibiting tumor growth. Another found that frequent intake of quercetin rich foods was associated with lower lung cancer risk.
Recent studies suggest that quercetin may slow the growth of cancer cells and help foster apoptosis, a form of natural cell death that doesn't happen in most cancer cells. Some studies in animals show that quercetin may help protect against certain types of cancer, particularly colon cancer.

In addition to this, preliminary studies have also suggested that quercetin may be effective in protecting against prostatitis (inflamed prostate) and heart disease.

Of course Quercetin is available in higher amounts in supplements than would be found in food. It can be found in capsule or tablet form ranging in doses from 50 milligrams (mg) to 500 mg. There is no recommended standard dose for quercetin so talk to your medical practitioner about dosage.
Read more here.