A foam transfer device (600) is described, for use with aerosol canister apparatus for producing a sclerosant foam for the treatment of, inter alia, varicose veins. The device enables diversion of an initial quantity of below-specification foam from the canister to waste, e.g. to an integral waste chamber, before dispensing a further quantity of foam for use in treatment. The switching of the flow from the waste chamber to a different outlet (618) for use is accomplished without interrupting the flow from the aerosol canister since this would cause the foam to drop below specification again. The waste chamber may be transparent so that the foam entering it can be observed and a decision made by a user when to stop diverting foam to waste. Alternatively, the foam may be diverted automatically e.g. when a set time has elapsed or a set volume of foam dispensed. The foam for use is normally dispensed into a syringe for subsequent injection into a varicose vein of a patient.