A drive mechanism (1) for use in a drug delivery device (100) is provided. The drive mechanism (1) comprises a housing (10, 11, 12, 14, 15) with a proximal and a distal end and a piston rod (22) which is rotatable with respect to the housing (10, 11, 12, 14, 15). The drive mechanism (1) further comprises a nut member (31) which is selectively rotationally lockable with respect to the housing (10, 11, 12, 14, 15), wherein the drive mechanism (1) is switchable between a setting mode of operation and a dispensing mode of operation. The drive mechanism (1) is configured such that in the setting mode of operation, the nut member (31) and the piston rod (22) are rotatable with respect to the housing (10, 11, 12, 14, 15), and in the dispensing mode of operation, the piston rod (22) is rotatable with respect to the housing (10, 11, 12, 14, 15) and the nut member (31) is rotationally locked with respect to the housing (10, 11, 12, 14, 15), wherein, when the piston rod (22) rotates, the piston rod (22) moves distally with respect to the nut member (31).