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′) on its forward end and the usual screw-threaded drive portion (24′) 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′) in the cartridge. Thereafter the threaded portion on the plunger (24′) engages the drive mechanism to allow metered dosing.