A drive mechanism for a drug delivery device, comprising: - a housing (13, 17, 40), - a unit which is attachable to the housing (13, 17, 40) and removable from the housing (13, 17, 40), - a drive member arranged inside the housing (13, 17, 40), the drive member being rotatable with respect to the housing (13, 17, 40), - a rotation member arranged inside the housing (13, 17, 40), the rotation member being rotatable with respect to the housing (13, 17, 40), - a clutch engaging the rotation member with the drive member when the unit is attached to the housing (13, 17, 40), - a stop member, - a further clutch engaging the stop member with the drive member when the unit is attached to the housing (13, 17, 40), - a separating member (50, 54, 60), the separating member (50, 54, 60) causing a separation of the rotation member, the drive member and the stop member when the unit is removed, thereby disengaging the clutch and the further clutch, and - a coupling feature of the separating member (50, 54, 60), the coupling feature being provided to move the separating member (50, 54, 60) when the unit is being attached to the housing (13, 17, 40) or is being removed, characterized in that - the clutch unidirectionally rotationally engages the rotation member with the drive member when the unit is attached to the housing (13, 17, 40), - the stop member is rotationally locked with respect to the housing (13, 17, 40) or forms part of the housing (13, 17, 40), - the further clutch unidirectionally rotationally engages the stop member with the drive member when the unit is attached to the housing (13, 17, 40), - the separating member (50, 54, 60) is disposed between the rotation member and the stop member, and - the coupling feature is provided to engage the unit. - a housing (13, 17, 40), - a unit which is attachable to the housing (13, 17, 40) and removable from the housing (13, 17, 40), - a drive member arranged inside the housing (13, 17, 40), the drive member being rotatable with respect to the housing (13, 17, 40), - a rotation member arranged inside the housing (13, 17, 40), the rotation member being rotatable with respect to the housing (13, 17, 40), - a clutch engaging the rotation member with the drive member when the unit is attached to the housing (13, 17, 40), - a stop member, - a further clutch engaging the stop member with the drive member when the unit is attached to the housing (13, 17, 40), - a separating member (50, 54, 60), the separating member (50, 54, 60) causing a separation of the rotation member, the drive member and the stop member when the unit is removed, thereby disengaging the clutch and the further clutch, and - a coupling feature of the separating member (50, 54, 60), the coupling feature being provided to move the separating member (50, 54, 60) when the unit is being attached to the housing (13, 17, 40) or is being removed, - the clutch unidirectionally rotationally engages the rotation member with the drive member when the unit is attached to the housing (13, 17, 40), - the stop member is rotationally locked with respect to the housing (13, 17, 40) or forms part of the housing (13, 17, 40), - the further clutch unidirectionally rotationally engages the stop member with the drive member when the unit is attached to the housing (13, 17, 40), - the separating member (50, 54, 60) is disposed between the rotation member and the stop member, and - the coupling feature is provided to engage the unit.