The invention relates to a cylinder-piston unit (10) of a one-time injector having at least one penetration (25) connecting the cylinder interior (31) to a face end opening (29). The end face (12) of the cylinder-piston unit comprises a press-in area (60) surrounding the opening and a press-on area (80) surrounding the press-on area. The press-in area comprises an interior bar (62) bounding the opening, and an external bar (63). The bars bound a recess surrounding the interior bar, the depth thereof measured parallel to the center axis (15) of the cylinder-piston unit is at least a fourth of the width thereof in a plane oriented normal thereto. The press-on area is shifted, at least during an injection, against the injection direction (2), toward the end face (61) of the insertion area. Furthermore, a surface (81) of the press-on area projected into a plane normal to the center axis is at least as great as the area of the press-in area projected into the same plane. The present invention increases the operational reliability of a cylinder-piston unit of a disposable injector.
