An assembly for a drug delivery device is provided the assembly comprising a cartridge adapted and arranged to contain at least one dose of a drug, a bung being slidably arranged within the cartridge, a housing comprising a distal end and a proximal end, and a piston rod adapted and arranged for moving the bung in the distal direction with respect to the housing for dispensing a dose of the drug. The assembly further comprises a drive member and arranged for transferring movement to the piston rod for dispensing the dose, and an engagement member configured to mechanically cooperate with the piston rod and with the housing. The engagement member is moveable between a reset position, where the engagement member is allowed to rotate with respect to the housing, and an operating position, where the engagement member is rotationally locked with respect to the housing, wherein the assembly is configured such that the engagement member is rotated and simultaneously axially displaced during movement of the engagement member between the operating position and the reset position. The engagement member and the housing are threadedly engaged with one another, wherein at least one of the engagement member and the housing comprises a thread and the other one of the engagement member and the housing comprises an interaction. Furthermore, the use of an engagement member for a drug delivery device is described.
