A drive mechanism for a drug delivery device is presented that can be switched between a normal operation mode and a reset mode. A piston rod is displaced in a distal direction to deliver a dose in a normal operation mode and is displaced in the proximal direction in a reset mode. The drive mechanism also has a drive member that is rotationally moveable with respect to the housing, wherein in the normal operation mode, the drive member is coupled with the piston rod so that rotational movement of the drive member in the rotation direction with respect to the housing is converted into movement of the piston rod in the distal direction with respect to the housing, and wherein, in the reset mode, the piston rod is decoupled from the drive member.