The present invention relates to a torsion spring driven injection device for delivering set doses of a liquid drug. The injection device mechanism is encapsulated in a housing which also stores the cartridge containing the liquid drug. A needle cannula which is in liquid communication with the interior of the cartridge and which has a distal tip is shielded by a telescopically movable needle shield is used to inject the set dose. The needle cannula is preferably used for multiple injections and cleaned between subsequent injections. The drive mechanism further comprises a nut member engaging the piston rod and releasable coupled to the housing to operate as a pressure relief mechanism. The telescopically movable needle shield covering the distal tip of the needle cannula is formed to lock the nut member to the housing during injection.