A drive mechanism of a drug delivery device is presented comprising of a piston rod extending in an axial direction to be operably engaged with a piston of a cartridge, the piston rod being adapted to exert distally directed pressure to the piston for expelling a liquid medicament from the cartridge. A housing component to accommodates the piston rod and has a guiding member extending in a lateral plane and being engaged with piston rod for guiding the same in axial direction relative to the housing component. At least one actuation lock is used to engage with the piston rod to inhibit a proximally directed movement of the piston rod relative to the housing component.
