Sclerotherapy effectively treats varicose and spider veins. It's often considered the treatment of choice for small varicose veins. Varicose veins occur when the veins swell and bulge, usually in the legs. This is due to weak vein walls that, in turn, weaken the vein valves. As a result, blood pools up the veins, causing them to swell and look different. The treatment procedure involves injecting a solution directly into the small vein. The sclerotherapy solution causes the vein to scar, forcing blood to reroute through healthier veins. The collapsed vein is reabsorbed into local tissue and eventually fades. After sclerotherapy, treated veins tend to fade within a few weeks, although occasionally it may take a month or more to see the full results. In some instances, several sclerotherapy treatments may be needed.