The invention relates to a drive mechanism for use in a drug delivery device comprising: an internal housing having an external helical thread, which internal housing is provided within a main housing； a dose dial sleeve (70) having a helical thread engaged with the helical thread of the said internal housing； a drive sleeve (30) releasably connected to the said dose dial sleeve (70)； and a clutch means (60) located between the dose dial sleeve (70) and the drive sleeve (30)； When the dose dial sleeve (70) and the drive sleeve (30) are coupled, both are allowed to rotate with respect to the internal housing. When the dose dial sleeve (70) and the drive sleeve (30) are de-coupled, rotation of the dose dial sleeve (70) with respect to the internal housing is allowed, whilst rotation of the drive sleeve (30) with respect to the internal housing is not allowed, whereby axial movement of the drive sleeve (30) is allowed so that a force is transferred in the longitudinal direction to the proximal end of the drug delivery device.
