A pen injector 10 is disclosed for use with a cartridge or syringe 20 to deliver a plurality of single metered doses therefrom. The injector 10 comprises a body arranged to provide a conduit 42 which, in use, provides a fluid communication path between the cartridge 20, a delivery needle 30 and a mechanism 50 arranged to expel therapeutic material from the cartridge 20. The mechanism 50 is arranged to draw therapeutic material from the cartridge 20 into the conduit 42 by negative pressure and discharge a metered dose from the conduit 42 via said delivery needle 30. Preferably the body comprises non-return valves 44, 46 to prevent fluid flow from the conduit 42 to the cartridge 20 and from the needle 30 to the conduit 42. The body may further comprise a cartridge needle 48 to penetrate the septum 26 of the cartridge 20 upon assembly of the pen injector 10. Optionally, the mechanism 50 comprises a pump. The mechanism 50 may further comprise a displacement chamber 52 with a cylinder and a release button 60 arranged to move a piston 54 or membrane within the cylinder. A dose adjuster may also be provided.
