The invention relates to a medical drug delivery apparatus which comprises a cartridge (1) being closed at one end by a flexible membrane (2) and at the opposite end by a movable piston (10). The movable piston rod (19) is moved forward by a piston rod (20). In order to distribute the force from the piston rod (20) to the movable piston (10) a piston rod foot (30) is provided sandwiched between the piston rod (29) and the movable piston (10) The piston rod foot (30) provided between the piston rod (20) and the movable piston (10) has a plurality of arms (34) extending in a direction perpendicular to a longitudinal axis (X) of the cartridge (1) and which arms (34) engages the inner wall (4) of the cartridge (1) thereby obtaining geometric guidance or generating a force of friction between the piston rod foot (30) and the inner wall (4) of the cartridge (1).
