Quercetin is a plant pigment (flavonoid). It is found in many plants and foods, such as red wine, onions, green tea, apples, berries, Ginkgo biloba, St. John's wort, American elder, and others. Buckwheat tea has a large amount of quercetin. People use quercetin as a medicine.Quercetin is most commonly taken by mouth to treat conditions of the heart and blood vessels. It is also used for arthritis, bladder infections, and diabetes. But there is limited scientific evidence to support these uses.How does it work?Quercetin has antioxidant and anti-inflammatory effects which might help reduce inflammation, kill cancer cells, control blood sugar, and help prevent heart disease.