A pen injector apparatus is disclosed for use with a cartridge to deliver a plurality of single metered doses therefrom. The injector apparatus includes a body arranged to provide a conduit which, in use, provides a fluid communication path between the cartridge and a delivery needle and a mechanism arranged to expel therapeutic material from the cartridge. The mechanism is arranged to draw therapeutic material from the cartridge into the conduit by negative pressure and discharge a metered dose from the conduit via the delivery needle.
