Sclerotherapy was developed over 75 years ago and at present is one of the most prevalent treatments for spider and varicose veins. During the treatment, a sclerosing agent is injected into the vein through a tiny needle. The solution replaces the blood in the vein, generating irritation and corrosion to the walls of the vessel. The body then initiates the natural healing process and the vein begins to decompose, shrink, and eventually disappear.