The present invention relates to a piston rod assembly for a drug delivery device, comprising: a piston rod adapted to be operably engaged with a piston of a cartridge containing a medicament, at least one adjusting member displaceably disposed at the piston rod with respect to the piston rod's long axis, and being interconnected with a distal end section of the piston rod, and at least one interlock means adapted to interact with the adjusting member and/or with the piston rod for mutually locking in position the adjusting member and the piston rod in an arbitrary relative position in order to compensate for tolerances of manufacture and/or of assembly.