A pen injector apparatus comprising a dosing mechanism for delivering a plurality of single metered doses, the dosing mechanism comprising: a linearly activated dose delivery mechanism arranged to deliver a metered dose of therapeutic material from a cartridge; and a non-return mechanism, arranged to prevent the dose delivery mechanism from returning toward a primed position until a full metered dose has been delivered; wherein the non-return mechanism comprises: a track defining an actuation path and a return path; an engagement member arranged to be received within said track; the track and engagement member being relatively moveable such that the engagement member is arranged to move along the actuation path during actuation of the dose delivery mechanism and along the return path during priming of the dose delivery mechanism; at least a portion of the actuation path being provided with a toothed profile to prevent reverse movement of the engagement member in the actuation path; characterised in that the relative movement between the track and the engagement member is rotational movement, and in that the dose mechanism further comprises a cam (40,42) arranged to provide relative rotational movement between the track and the engagement member in response to the linear movement of the dose delivery mechanism. a linearly activated dose delivery mechanism arranged to deliver a metered dose of therapeutic material from a cartridge; and a non-return mechanism, arranged to prevent the dose delivery mechanism from returning toward a primed position until a full metered dose has been delivered; wherein the non-return mechanism comprises: a track defining an actuation path and a return path; an engagement member arranged to be received within said track; the track and engagement member being relatively moveable such that the engagement member is arranged to move along the actuation path during actuation of the dose delivery mechanism and along the return path during priming of the dose delivery mechanism; at least a portion of the actuation path being provided with a toothed profile to prevent reverse movement of the engagement member in the actuation path; a track defining an actuation path and a return path; an engagement member arranged to be received within said track; the track and engagement member being relatively moveable such that the engagement member is arranged to move along the actuation path during actuation of the dose delivery mechanism and along the return path during priming of the dose delivery mechanism; at least a portion of the actuation path being provided with a toothed profile to prevent reverse movement of the engagement member in the actuation path; characterised in that the relative movement between the track and the engagement member is rotational movement, and in that the dose mechanism further comprises a cam (40,42) arranged to provide relative rotational movement between the track and the engagement member in response to the linear movement of the dose delivery mechanism. the relative movement between the track and the engagement member is rotational movement, and in that the dose mechanism further comprises a cam (40,42) arranged to provide relative rotational movement between the track and the engagement member in response to the linear movement of the dose delivery mechanism.