Abstract: The invention is a single use injection device (1) having an injection cylinder (2) for receiving a liquid medium, an injection needle (6), and a piston (4) which can slide inside the injection cylinder (2) and which is movably linked to a piston rod (5) which extends in the opposite direction to the injection needle (6). An anti-reuse device (7) which is actuated by the medium moved by the piston (4) is arranged in an inlet area (37) of the injection cylinder (2) that faces the injection needle (6) and has a switching member (22) with a sealing section (48) having a larger cross-sectional surface than the cross-section of an opening (33) provided in a retaining member (21) and whose sealing section (48) is retained in a rest position with a circular ring-shaped retaining member (21) which is elastically deformable in the radial direction on the side of the retaining member (21) that faces the piston (4).