Sclerotherapy effectively treats spider veins of the thighs and legs. It’s often considered the treatment of choice for small varicose veins. Sclerotherapy involves injecting a solution directly into the veins. The sclerotherapy solution causes the vein to scar and collapse, 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 up to a month to see the full results. In some instances, several sclerotherapy treatments may be needed.