Sclerotherapy is an in-office treatment for spider veins, the thin red or purple veins on the skin’s surface. It can also be used to treat some small varicose veins, the swollen veins that are often visible under the skin. A provider injects a sclerosing solution directly into the vein, which irritates the lining of the vessel, causing it to scar and collapse. This forces the blood to re-route through healthier veins, while the damaged vein fades away over the next two to six months.