A drug delivery device comprising: - a cartridge unit comprising a cartridge holder carrying a drug cartridge having a cartridge body and a piston structure (25, 29), and - a dose expelling unit comprising ∘ a housing extending along a longitudinal axis, ∘ a piston rod for pressurising the drug cartridge, the piston rod comprising a contact surface adapted to contact the piston structure (25, 29), and a non-self-locking thread, ∘ a nut member rotationally fixed with respect to the housing and engaged with the non-self-locking thread, and ∘ a piston rod drive member axially fixed with respect to the housing and rotatable about the longitudinal axis, wherein the piston rod drive member and the piston rod are rotationally interlocked, such that a rotation of the piston rod drive member in a first direction is associated with distal movement of the piston rod through the nut member, and a rotation of the piston rod drive member in a second direction is associated with proximal movement of the piston rod through the nut member, characterised in that the nut member carries a first coupling part of a unidirectional ratchet coupling (65, 68r), and the piston rod drive member is operatively coupled with a second coupling part of the unidirectional ratchet coupling (65, 68r), wherein in a pre-assembled state the nut member assumes a first nut member position relative to the housing and the piston rod drive member in which the first coupling part and the second coupling part are disengaged and in an assembled state the nut member assumes a second nut member position relative to the housing and the piston rod drive member in which the first coupling part and the second coupling part are engaged to render the piston rod drive member unidirectionally rotatable in the first direction, and wherein the nut member is configured to undergo irreversible displacement from the first nut member position to the second nut member position during assembly of the dose expelling unit and the cartridge unit in response to a converging relative axial motion between the housing and the cartridge holder, after contact between the piston structure (25, 29) and the contact surface has been established. - a cartridge unit comprising a cartridge holder carrying a drug cartridge having a cartridge body and a piston structure (25, 29), and - a dose expelling unit comprising ∘ a housing extending along a longitudinal axis, ∘ a piston rod for pressurising the drug cartridge, the piston rod comprising a contact surface adapted to contact the piston structure (25, 29), and a non-self-locking thread, ∘ a nut member rotationally fixed with respect to the housing and engaged with the non-self-locking thread, and ∘ a piston rod drive member axially fixed with respect to the housing and rotatable about the longitudinal axis, ∘ a housing extending along a longitudinal axis, ∘ a piston rod for pressurising the drug cartridge, the piston rod comprising a contact surface adapted to contact the piston structure (25, 29), and a non-self-locking thread, ∘ a nut member rotationally fixed with respect to the housing and engaged with the non-self-locking thread, and ∘ a piston rod drive member axially fixed with respect to the housing and rotatable about the longitudinal axis, wherein the piston rod drive member and the piston rod are rotationally interlocked, such that a rotation of the piston rod drive member in a first direction is associated with distal movement of the piston rod through the nut member, and a rotation of the piston rod drive member in a second direction is associated with proximal movement of the piston rod through the nut member, characterised in that the nut member carries a first coupling part of a unidirectional ratchet coupling (65, 68r), and the piston rod drive member is operatively coupled with a second coupling part of the unidirectional ratchet coupling (65, 68r), wherein in a pre-assembled state the nut member assumes a first nut member position relative to the housing and the piston rod drive member in which the first coupling part and the second coupling part are disengaged and in an assembled state the nut member assumes a second nut member position relative to the housing and the piston rod drive member in which the first coupling part and the second coupling part are engaged to render the piston rod drive member unidirectionally rotatable in the first direction, and wherein the nut member is configured to undergo irreversible displacement from the first nut member position to the second nut member position during assembly of the dose expelling unit and the cartridge unit in response to a converging relative axial motion between the housing and the cartridge holder, after contact between the piston structure (25, 29) and the contact surface has been established.