An injector device comprises a housing having a distal end and a proximal end a medicament reservoir a stopper for expelling a medicament out of the medicament reservoir a cavity having an expandable volume which is arranged to move the reservoir or the stopper in a distal direction when the cavity is at least partially inflated with a fluid and a fluid reservoir which is configured to dispense the fluid in to the cavity wherein the cavity comprises a flexible tube, and the fluid reservoir is configured to dispense the fluid into the flexible tube.
