An assembly for a drug delivery device, comprising: a housing having a distal end and a proximal end, a piston rod, a drive member, the drive member being moveable in a proximal direction with respect to the housing and with respect to the piston rod for setting a dose of a drug, at least one stop member which is provided on one of the piston rod and the drive member and which is radially moveable with respect to the one of the piston rod and the drive member on which it is provided, and at least one interaction member which is provided on the other one of the piston rod and the drive member. Furthermore, a piston rod for a drug delivery device is provided.