A disposable injector with a housing (10) in which are arranged at least one mechanical spring energy reservoir (50), at least one cylinder/piston unit (100), at least one piston-actuating ram, and at least one trigger unit. For this purpose, the housing has at least one inverted draw hook, which has at least one support surface respectively in the region of its free end. The spring-loaded piston-actuating plunger rests on the support surface, whereby the contact zone placed between the draw hook and the piston-actuating plunger represents a variable-speed gear pair thrusting the draw hook radially outwards. The locking position of the draw hook is secured by an actuating element positioned in a locked position. The actuating element has a triggering position which effects radially outwards aligned retreat of the draw hook when the piston-actuating plunger is released.