A pen injection device designed for use with dual or multi-chamber cartridges (14), is provided with an extended length plunger (24) having a non-drive portion (24&prime) ON its forward end and the usual screw-threaded drive portion (24&prime) ON its rearward end. This arrangement allows the plunger to be pushed forwardly whilst the dosing drive mechanism is disengaged to effect a reconstitution movement of the rearward bung (18&prime) in the cartridge. Thereafter the threaded portion ON the plunger (24&prime) engages the drive mechanism to allow metered dosing.
