A drug delivery device being a needle-based pen-type injector, the drug delivery device comprising: - a cartridge unit, wherein the cartridge unit comprises a cartridge retaining member, the cartridge retaining member being provided to retain a cartridge therein, the cartridge having an outlet at its distal end, wherein medication is retained in the cartridge and a bung is retained within the cartridge, the bung being movable with respect to the cartridge, wherein movement of the bung with respect to the cartridge in a distal direction causes medication to be dispensed from the cartridge through the outlet of the cartridge during operation of the drug delivery device, and - a drive mechanism, the drive mechanism comprising a housing and a piston rod arrangement, the piston rod arrangement being retained in the housing, wherein the piston rod arrangement comprises a bearing and a piston rod body, the bearing being axially fixed to the piston rod body and the piston rod body being enabled to rotate relative to the bearing, wherein the bearing has an axis and comprises a first and a second surface (39, 20), which are spaced apart in the direction of the axis, and wherein the bearing comprises a third surface, wherein the third surface connects the first and the second surface (39, 20), wherein the cartridge unit and the drive mechanism are releasably secured to one another, wherein the cartridge retaining member is provided with a fixing member for attaching the cartridge unit to the drive mechanism, and wherein a proximal end side of the cartridge unit is secured to the drive mechanism at a distal end side of the housing, wherein the piston rod arrangement is configured for transferring force to the bung, thereby displacing the bung in the distal direction with respect to the cartridge to dispense a dose of medication from the cartridge, wherein an opening (41, 42) is provided in each of the first and the third surface (39, 40), and wherein the opening in the first surface is connected to the opening in the third surface, and wherein the second surface of the bearing is formed by a bung contact wall which is arranged to abut a proximal end face of the bung. - a cartridge unit, wherein the cartridge unit comprises a cartridge retaining member, the cartridge retaining member being provided to retain a cartridge therein, the cartridge having an outlet at its distal end, wherein medication is retained in the cartridge and a bung is retained within the cartridge, the bung being movable with respect to the cartridge, wherein movement of the bung with respect to the cartridge in a distal direction causes medication to be dispensed from the cartridge through the outlet of the cartridge during operation of the drug delivery device, and - a drive mechanism, the drive mechanism comprising a housing and a piston rod arrangement, the piston rod arrangement being retained in the housing, wherein the piston rod arrangement comprises a bearing and a piston rod body, the bearing being axially fixed to the piston rod body and the piston rod body being enabled to rotate relative to the bearing, wherein the bearing has an axis and comprises a first and a second surface (39, 20), which are spaced apart in the direction of the axis, and wherein the bearing comprises a third surface, wherein the third surface connects the first and the second surface (39, 20), wherein the cartridge unit and the drive mechanism are releasably secured to one another, wherein the cartridge retaining member is provided with a fixing member for attaching the cartridge unit to the drive mechanism, and wherein a proximal end side of the cartridge unit is secured to the drive mechanism at a distal end side of the housing, wherein the piston rod arrangement is configured for transferring force to the bung, thereby displacing the bung in the distal direction with respect to the cartridge to dispense a dose of medication from the cartridge, wherein an opening (41, 42) is provided in each of the first and the third surface (39, 40), and wherein the opening in the first surface is connected to the opening in the third surface, and wherein the second surface of the bearing is formed by a bung contact wall which is arranged to abut a proximal end face of the bung.