A drive mechanism for a drug delivery device which can be switched between a normal operation state and a reset state comprises of a housing having a proximal end and a distal end, a rotation member which is adapted to rotate in a first direction and in a second direction with respect to the housing, a drive member, wherein in the normal operation state the rotation member is coupled with the drive member so that the drive member follows rotational movement of the rotation member in the second direction, a piston rod, wherein in the normal operation state the piston rod is displaced in a distal direction with respect to the housing when the drive member rotates in the second direction with respect to the housing, a stop member, wherein in the normal operation state the stop member is coupled with the drive member so that the stop member prevents rotational movement of the drive member with respect to the housing in the first direction, and a separation member for switching the operation state, the separation member being adapted to move the stop member and the drive member in the distal direction with respect to the housing when the normal operation state is switched to the reset state, thereby decoupling the stop member and the drive member and decoupling the drive member and the rotation member.