An injection device comprising a) a housing, b) a cartridge containing a preparation or precursor components for the preparation and having a front part and a rear part defining a general cartridge axis, an outlet for the preparation arranged at the front part and at least one movable wall arranged between the front part and the rear part, a displacement of which wall causes the preparation to be moved towards or expelled through the outlet, c) a piston rod, having a first end and a second end, at least partly contained within the housing and being operable to engage and displace the movable wall at its first end and d) actuating means within the housing arranged to move at least the rod. According to the invention the device comprises gripping means connected to the housing and arranged to hold the cartridge, or an enclosure therefore, with its front part distal and its rear part proximal to the housing, the actuating means being arranged to move the gripping means so as to move the cartridge, with its rear part leading, relative the housing, locking means able to lock the cartridge against axial movements relative to the housing, hereby defining a locked position for the cartridge, the actuating means being able to move the piston rod between at least one retracted position and at least one extended position, and the relative positions between the piston rod in its retracted position and the cartridge in its locked position being such that the piston rod extends into the cartridge. The invention also relates to a method for operating the device.