A therapeutic foam for the treatment of, inter alia, varicose veins comprises a sclerosing solution foamed with a physiological gas such as carbon dioxide, oxygen or a mixture thereof. The foam has a nitrogen content of less than 0.8%. It may be generated using a pressurized canister system incorporating a fine mesh of micron dimensions through which the gas and sclerosing liquid are passed to make the foam. Alternatively, the foam may be generated by passing gas and solution between two syringes through a fine mesh. Techniques are described for minimizing the amount of nitrogen in a canister or syringe based product. A technique for generating and delivering foam simultaneously using a syringe based device is also disclosed.