Sclerotherapy is a medical procedure used to eliminate varicose veins and spider veins. Sclerotherapy involves an injection of a solution directly into the vein. The solution irritates the lining of the blood vessel, causing it to collapse and stick together and the blood to clot. Over time, the vessel turns into scar tissue that fades from view. Sclerotherapy is a proven procedure that has been in use since the 1930s.