Sclerotherapy is a medical procedure whereby a chemical, the sclerosant, is injected into a vein to entirely obliterate it. It is commonly mixed with air to make a foam like substance that tend to stay in the vessel more firmly. The sclerosant damages the innermost lining of the vessel, resulting in a clot that blocks the blood circulation in the vein. To prevent back-flow, most veins have valves that only allow blood only to flow in the direction of the heart. When these valves become incompetent, veins become enlarged and bulging (varicose). Smaller veins that feed these varicose veins can also become enlarged and appear as red or blue spider veins in the skin.Varicose veins can lead to a chronic swelling condition of the leg called venous insufficiency. Venous insufficiency predisposes a person to blood clots and skin ulceration. The destruction of these types of veins can be desirable both medically and cosmetically.