Sclerotherapy: The injection of a chemical irritant into a vein to sclerose ('harden') it. The chemical irritates the lining of the vein, causing it to swell and the blood to clot. The vein turns into scar tissue that fades from view. Blood flow shifts to nearby healthy blood vessels. Sclerotherapy may be done to treat varicose veins, spider veins, hemorrhoids, and esophageal varices.