Citrin, SIHT rihn, is a chemical substance that belongs to a group of chemicals called flavonoids. Scientists do not know exactly what citrin is, or whether it is essential to the health of human beings. However, they have found that flavonoid substances help to control bleeding from the capillaries (tiny blood vessels) in the body. Citrin affects the capillary walls, making them less likely to hemorrhage. The citrin used in medicine is usually prepared from paprika and lemon peel. Citrin was once called vitamin P, but it is not a vitamin.