A disposable injector (4) having a two-chamber system (99), wherein at least one first chamber (105) is part of a piston-cylinder unit (100) that can be accommodated in the disposable injector (4), and wherein the second chamber (255) is part of a chamber (250) which is at least temporarily closed by means of a stopper (257) and inserted into a container adapter (200) detachably mounted on the disposable injector (4), or part of a piston-cylinder unit (254), and wherein the container adapter (200) comprises a connecting tube (242) initially closed by means of a cap (290). To this end, on the side oriented toward the second chamber, the stopper has a recess (271), into which at least one stopper channel (275) connecting the lateral surface (277) of the stopper to the recess (271) opens. After the container has been pushed in or activated, the connecting tube (242) connects the interior of the piston-cylinder unit to the interior of the container or of the piston-cylinder unit via the stopper channel and the tubular channel.