Injection devices are known for the self administration of medicament by patients in which medicament is typically contained within a cartridge located within the injection device. It is a problem that injection devices should be small enough to fit into a jacket pocket or a hand bag without difficulty. Also, the injection device must be of a 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bung within the cartridge to be moved both to a maximum dispense position within the cartridge and to be fully withdrawn from the cartridge to allow for replacement of the cartridge.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 in which a second piston (42) is successively moved in relation to a first end of a medicament cartridge (8) containing a medicament selectively to drive a bung (10) into the medicament cartridge (8), the drive mechanism comprising a means (36) defining a passageway (38), a first piston (40) and the second piston (42), in which the passageway (38) has a first end and a second end, the first piston (40) closing the first end of the passageway (38) and a first end of the second piston (42) closing the second end of the passageway (38) to define a chamber in the passageway (38), a non-compressible medium being contained within the chamber such that a first relative movement in a first direction at the first end of the passageway (38) results in a second relative movement in a direction opposite the first direction at the second end of the passageway (38).