Home – All FAQs – How Does Foam Sclerotherapy Work? How Does Foam Sclerotherapy Work? Home – All FAQs – How Does Foam Sclerotherapy Work? The sclerosant in foam sclerotherapy is made up of a drug called sodium tetradecyl sulphate (STS). Sclerosants like STS have been used for many years to inject varicose veins. STS is a licensed drug which is prescription only and extremely safe. It can only be prescribed by a doctor and administered under the doctor’s supervision. There is a dosage safety limit restricting how much of the drug we can administer in any one session. The drug works by killing the cells that form the skin that lines the vein wall. Without its skin, the vein dies off. Sclerosants work quite well for small veins, especially below the knee where good compression can be applied; however, if the underlying reflux is not dealt with, the varicose veins will just come back. Mixing the sclerosant with air or CO2 into a foam leads to a more effective closure of the veins.