Sclerotherapy is the treatment of choice for smaller veins where a catheter (ablation) cannot be negotiated into the vein. The procedure is performed by injection into the vein to treat smaller varicose veins and spider veins. The physician will inject a medication, or sclerosant, into problem veins. The injected sclerosant causes the veins to scar and collapse so blood can no longer fill them. Blood is redirected to normal, healthy veins, and the treated vein eventually dissolves into the body. Most people return to normal activities the same day.