Quercetin belongs to a group of plant pigments called flavonoids that give many fruits, flowers, and vegetables their colors. It has antioxidant and anti-inflammatory properties. It also may reduce symptoms of allergies, asthma, hay fever, and hives. It may help protect against heart disease and cancer as well. Fruits and vegetables are the primary dietary sources of quercetin, particularly citrus fruits, apples, onions, parsley, olive oil, grapes, tea, red wine, dark cherries, and dark berries such as blueberries, blackberries, and bilberries. Quercetin supplements are available as pills or capsules.