Sclerotherapy is a non-invasive, outpatient procedure to treat spider veins and varicose veins. The procedure injects a mild chemical solution, called a sclerosant, into a feeder vein. The sclerosant irritates the blood vessel, causing it to collapse. The blood flow is then redirected to other healthy veins in the area. Over time, the body will absorb the collapsed vein. Patients are able to return to normal activities immediately after the procedure.