An injection device for dispensing medication from a medicament cartridge is disclosed having a needle assembly at one end and a piston disposed opposite the needle assembly wherein advancement of the piston within the cartridge discharges medication from the needle assembly. The injection device may include a housing assembly and a driving assembly adapted to drive the piston and thereby discharge medication from the needle assembly. A cartridge retainer may be securable to the housing assembly and adapted to retain the cartridge. Securing the cartridge retainer to the housing assembly with a retained cartridge may move the piston of the cartridge into contact with the driving assembly and perform a priming procedure. The cartridge retainer may be securable to the housing assembly by rotationally engaging a helical thread with a helical groove. A ratcheting mechanism may be used to prevent the rotational disengagement of the cartridge retainer from the housing assembly.
