A drug delivery device is provided comprising a housing comprising a distal end and a proximal end and a cartridge adapted and arranged to contain at least one dose of a drug, a bung being movably arranged within the cartridge. In an initial state of the device, the bung is arranged in a proximal end position with respect to the cartridge. The device further comprises a piston rod adapted and arranged to move the bung in the distal direction with respect to the cartridge for delivering a dose of the drug. The device further comprises a mechanism operable such that, after a dose delivery operation was completed, the piston rod is moved into the proximal direction with respect to the bung by a back off distance (D), wherein in the initial state, a distance between a distal end of the piston rod and a proximal end of the bung is greater than zero, with the distance between the piston rod and the bung in the initial state being defined by the back off distance (D). Furthermore, a method for assembling a drug delivery device is provided.
