Quercetin has antioxidant and anti-inflammatory effects that might help reduce swelling, kill cancer cells, control blood sugar, and help prevent heart disease. Quercetin is most commonly used for conditions of the heart and blood vessels and to prevent cancer.
Quercetin is a plant pigment (flavonoid). It's found in many plants and foods, such as red wine, onions, green tea, apples, and berries.
It is also used for arthritis, bladder infections, and diabetes, but there is no strong scientific evidence to support most of these uses.