The present invention concerns a drive mechanism (3) for a drug delivery device (1), which comprises a piston rod (40) comprising a first piston rod member (7) and a second piston rod member (8). The drive mechanism (3) further comprises an adjustment member (9). The drive mechanism (3) has a first state in which the first and the second piston rod member (7, 8) are moveable with respect to each other by operating the adjustment member (9), thereby adjusting the length of the piston rod (40), and wherein the adjustment member (9) is arranged at least partially inside the second piston rod member (8). Moreover, the present invention concerns a method for assembling the drug delivery device (1).