The invention relates to a medical drug delivery apparatus which comprises a cartridge (1) being closed at one end by a membrane (12) and at the opposite end by a movable piston (15). A piston rod foot (1) is provided for transferring the pressure from the piston rod (20) of the drug delivery apparatus and onto the piston (15). The piston rod foot (1) comprises a center part (2) abutting the piston rod (20) and an outer part (3) which center part (2) and outer part (3) are releasable coupled together such that the two (2, 3) can be released when a force above a certain threshold limit is applied to the outer part (3).