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 while 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.
