Spider veins are abnormal tiny red vessels near the surface of the skin. They are unneeded for healthy blood flow and are often a result of minor trauma. When these vessels are injected with a sclerosing agent, a mild irritation of the injected vein occurs. The resulting inflammation causes most of the treated vessels to close down. When there is no blood flowing through these tiny veins, they wither away and disappear. More than one treatment is necessary for full removal for many patients. Sclerotherapy is an effective treatment for most spider veins.