How is sclerotherapy performed? January 24, 2023 Before your sclerotherapy begins, Dr. Downey or Dr. Bullocks will prepare your skin by thoroughly cleaning it and confirming the areas which you would like to be treated. She will then begin to inject your desired varicose veins and spider veins with the sclerosing agents. These sclerosing agents will close off the entrance of the vein, redirecting the blood into healthier, more viable veins in the area. Each treatment session typically takes anywhere between 15 minutes and an hour to complete, depending on the size of the treated area.