A drive mechanism (4) for a drug delivery device comprising—a piston rod (14), —a trigger member (9) coupled with the piston rod (14) in such a manner that rotational movement between the piston rod (14) and the trigger member (9) is constrained in a first state, the trigger member (9) being elastically deformable in such a manner that the piston rod (14) is rotationally moveable with respect to the trigger member (9) in a second state, —a trigger button (10) being suitable for activating the deformation of the trigger member (9).
