Varicose veins treatment. Gloved hand injecting a sclerosant into a patient's leg to treat varicose veins. The paths of the veins are marked in purple on the skin. A sclerosant is a substance which causes irritation of the veins' linings, leading to scarring and their subsequent destruction. The body naturally re-routes the blood supply around the affected area. Varicose veins are those in which the valves preventing the backflow of blood fail. This causes pooling of blood and enlarging of the veins, giving them an unsightly appearance. Varicose veins are usually far more apparent than the small examples here.