Injection devices are known for the self administration of a medicament by patients in which the medicament is contained within a cartridge located within the injection device. It is a problem that such injection devices should be small enough to fit into a jacket pocket or a hand bag without difficulty. Concurrently, the injection device must be of size that enables a piston or similar used to drive a cartridge bung within the cartridge to be moved both to a maximum dispense position within the cartridge and to be fully withdrawn from the cartridge to allow for replacement of the cartridge. A drive mechanism for an injection device is disclosed comprising a piston member 10, a semi rigid belt 18 connected to the piston member 10, a belt drive means to selectively engage with the semi rigid belt 18 to drive the semi rigid belt 18, and a dose setting mechanism to control the selective engagement of the semi rigid belt in which the semi rigid belt 18 includes a track incorporated therein and the belt drive means comprises a tooth 50 for selective engagement with the track to drive the belt 18 only when there is relative movement between the semi rigid belt 18 and the tooth 50 in a first direction. An injection device comprising such a drive mechanism is also disclosed.