A pen-type drug injection device is disclosed which comprises a housing having a longitudinal axis, a threaded piston rod, a guide member for guiding the piston rod in a drug dispense operation, and a cartridge holder for receiving a cartridge, wherein the cartridge holder is detachable from the housing and wherein the guide member comprises an iris type aperture defined by a plurality of threaded pivotable segments. The drug delivery device is configured such that the aperture increases in diameter when the cartridge holder is being detached from the housing, such that the piston rod is free of the threaded engagement with the guide member and can be reset or pushed back to its initial position, and decreases in diameter when the cartridge holder is attached, such that the threads on segments reengage the piston rod.