A drug delivery device (1) comprising or adapted to receive a drug-filled cartridge (20), comprises a housing (220, 510), a piston rod (410) adapted to engage and axially displace a piston (22) in a loaded cartridge in a distal direction, the piston rod comprising a first thread (413) and a second thread (411). The device further comprises a drive member (420, 421) comprising a drive thread (423) in engagement with the piston rod first thread (413), and a nut portion (430) comprising a propulsion thread (431) arranged rotationally locked relative to the housing and in engagement with the piston rod second thread (411). The drive member (420, 421) is arranged to rotate during expelling of a dose of drug, with the threaded engagement (423, 413) between the drive member and the piston rod causing the piston rod to rotate, and the threaded engagement (431, 411) between the nut portion and the piston rod causing the piston rod to move in the distal direction.
