An assembly for use in a drug delivery device includes a cartridge, a housing and a cartridge holder. The cartridge holder is releasably attached to the housing. The assembly includes a piston rod configured to move from a start position to an end position to dispense a dose of medication when the cartridge holder is attached to the housing, and the piston rod is configured to be reset to the start position when the cartridge holder is detached from the housing. The assembly further comprises a flexible element configured to axially fix the cartridge in the cartridge holder when the cartridge holder is attached to the housing. Furthermore, the assembly comprises a guiding element in threaded engagement with the piston rod. The flexible element is configured to fix the guiding element with respect to a housing of the drug delivery device when the cartridge holder is attached to the housing.
