Abstract: An injector pen is provided that includes a cartridge housing 2, a dose selector housing 10 and a nut 18 for coupling the two housings 2,10 together. The dose selector housing 10 defines an intermediate axial position, where the nut 18 can be temporarily retained in the dose selector housing 10 to form a sub-assembly suitable for transport to a different location. During final assembly of the injector pen, the cartridge housing 2 securely engages with the nut 18 and displaces the nut 18 from the intermediate axial position to a final axial position.

Abstract: A fixed dose injector pen is provided that includes a transmission element that is rotated to set and deliver a dose of a drug from a multi-dose cartridge. A circular ratchet defines a small number of engagement positions around its circumference and selectively transmits the rotation of the transmission element to a piston rod driver. The circular ratchet transmits the rotation of the transmission element to the piston rod driver only when the transmission element is rotated in a dose delivery direction, and only if the transmission element has previously been rotated in an opposite, dose setting direction through an angle sufficient to reach one of the engagement positions. An axially moving injector element can be used to drive rotation of the transmission element via a helical coupling. When the piston rod has advanced to a predetermined axial position, a last dose lock-out mechanism maintains the circular ratchet in engagement.