A disposable injector with a housing (10) in which a mechanical spring energy reservoir, a cylinder/piston unit (100), a piston-actuating ram (60) and a trigger unit are arranged, wherein the spring energy reservoir comprises at least one pretensioned spring element, wherein at least part of the piston-actuating ram is positioned between the spring energy reservoir and the piston of the cylinder/piston unit, and wherein the piston (111) of the cylinder/piston unit is separately moveable via pump rod (140). The pump rod can be coupled releasably to the piston or to a piston rod of the piston. The cylinder of the cylinder/piston unit has a rear sealing element (105) that permits sterile closure of the cylinder interior located between the piston and the cylinder.