The present invention provides a delivery device comprising a fluid cartridge holder, housing and a piston drive mechanism that comprises a hollow piston rod and a drive-shaft. The hollow piston rod has an internal thread that mates with the external thread of the drive-shaft, forming a thread connection and being axially restrained in the proximal direction relative to the housing. The drive mechanism is in turn connected to the housing via a one way ratchet such that the piston rod is prevented from moving in the proximal direction.