Bioflavonoids are a large family of substances found in food. They enhance health in many ways. One bioflavonoid is quercetin, derived from buckwheat and citrus fruits. It is often used to treat allergies. Quercetin appears to stabilize the membranes of cells that release histamine, one mediator of allergic reactions. Its action is preventive rather than symptomatic. The best way to use quercetin is to start taking it a week or two before the expected onset of the pollen season, continuing till the end. Other bioflavonoids include rutin and hesperidin. Citrus bioflavonoid complexes are often ingredients of dietary supplements.