A drive mechanism (4) for a drug delivery device comprising—a lead screw (14) being threadedly coupled to an insert (16), the lead screw (14) being moveable with respect to the insert (16) in a dose delivery state, —a locking means (8, 18) being suitable for constraining rotational movement of the lead screw (14) with respect to the insert (16) in a dose setting state, the locking means (8,18) releasing the lead screw (14) in the dose delivery state in such a manner that the lead screw (14) is moveable with respect to the insert (16), —a drive member (22) being coupled to the lead screw (14), the drive member (22) moving axially with respect to the insert (16) during the dose setting state and moving helically with respect to the insert (16) during the dose delivery state, thereby driving the lead screw (14).
