A drive mechanism for a drug delivery device, comprising: - a housing (13, 17, 40), - a piston rod arranged inside 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) and engaging the piston rod, so that rotational movement of the drive member with respect to the housing (13, 17, 40) is converted into rotational movement of the piston rod 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) about a common rotation axis of the drive member, the piston rod and the rotation member, and characterised by - drive fingers of the drive member, the drive fingers being resilient or resiliently mounted such that they tend to splay, - a stop member rotationally locked with respect to the housing (13, 17, 40) or forming part of the housing (13, 17, 40), the drive member being arranged between the rotation member and the stop member, - a further unidirectional rotational clutch engaging the stop member with the drive member when a slider is in a first position, - the slider disposed at the drive member, the slider being axially movable between the first position, in which the slider keeps the drive fingers constrained, and a second position, in which the drive fingers are released, - a taper formed as part of the slider, the drive fingers being splayed apart and thus disengaged from the rotation member by means of the taper when the slider is moved from the first position to a second position, the stop member being disengaged from the drive member when the slider is in the second position, - a unidirectional rotational clutch (25, 54) engaging the drive member with the rotation member, the clutch being formed by the drive fingers when the slider is in the first position, the drive member and the rotation member being disengaged when the slider is in the second position, - a cartridge unit which is attachable to the housing (13, 17, 40) and removable from the housing (13, 17, 40), - the drive fingers acting on the slider in such a manner that the slider is moved from the first position to the second position when the unit is being removed from the housing (13, 17, 40), - a coupling sleeve of the slider, and - an insert sleeve of the unit, the insert sleeve acting on the coupling sleeve and keeping the slider in the first position when the unit is attached to the housing (13, 17, 40). - a housing (13, 17, 40), - a piston rod arranged inside 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) and engaging the piston rod, so that rotational movement of the drive member with respect to the housing (13, 17, 40) is converted into rotational movement of the piston rod 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) about a common rotation axis of the drive member, the piston rod and the rotation member, and characterised by - drive fingers of the drive member, the drive fingers being resilient or resiliently mounted such that they tend to splay, - a stop member rotationally locked with respect to the housing (13, 17, 40) or forming part of the housing (13, 17, 40), the drive member being arranged between the rotation member and the stop member, - a further unidirectional rotational clutch engaging the stop member with the drive member when a slider is in a first position, - the slider disposed at the drive member, the slider being axially movable between the first position, in which the slider keeps the drive fingers constrained, and a second position, in which the drive fingers are released, - a taper formed as part of the slider, the drive fingers being splayed apart and thus disengaged from the rotation member by means of the taper when the slider is moved from the first position to a second position, the stop member being disengaged from the drive member when the slider is in the second position, - a unidirectional rotational clutch (25, 54) engaging the drive member with the rotation member, the clutch being formed by the drive fingers when the slider is in the first position, the drive member and the rotation member being disengaged when the slider is in the second position, - a cartridge unit which is attachable to the housing (13, 17, 40) and removable from the housing (13, 17, 40), - the drive fingers acting on the slider in such a manner that the slider is moved from the first position to the second position when the unit is being removed from the housing (13, 17, 40), - a coupling sleeve of the slider, and - an insert sleeve of the unit, the insert sleeve acting on the coupling sleeve and keeping the slider in the first position when the unit is attached to the housing (13, 17, 40).