Spider veins are caused by a hereditary vascular wall weakness. The venous valves stop working properly, causing the veins to become congested with blood. These veins become prominent and visible in the form of reddish-violet lines. The thighs and lower legs are most commonly affected. Foam sclerotherapy is a modern method for treating fine to medium-sized spider veins. A drug is injected as a foam, which displaces the blood from the veins and subsequently adheres to the vein wall. The number of required treatments varies between patients but depends mainly on the severity of the spider veins and quality of the connective tissue.