Quercetin is a plant pigment from the flavonoid group of polyphenols. It is found in many fruits, vegetables, leaves, seeds, and grains, such as capers, red onions, kale, apples, berries, and grapes. Quercetin has potent antioxidant and anti-inflammatory effects that might help reduce swelling, kill cancer cells, control blood sugar, and help prevent heart disease. It is most commonly used for conditions of the heart and blood vessels and to prevent cancer. Quercetin can also help stabilize the cells that release histamine in the body and thereby have an anti-inflammatory and antihistamine effect. It is available as an ingredient in dietary supplements, beverages, and foods. Quercetin supplements are available as pills or capsules, and they are often packaged with bromelain, an enzyme found in pineapple, because both are anti-inflammatories. The primary dietary sources of quercetin are fruits and vegetables, particularly citrus fruits, apples, onions, parsley, sage, tea, and red wine.