The invention relates to a system for injecting a fluid through or into the human skin, optionally by needle-free injection or by injection with a needle, with (i) an injection device (110) which includes an ampoule (112) with an ampoule body (114) forming a chamber for receiving the fluid to be injected; an ampoule piston (118) which is axially movable and sealingly guided in the chamber; an outlet (124) disposed at the distant end of the ampoule (112) for the fluid to be injected, wherein the ampoule (112) is rounded in the region of the outlet (124) and the rounded region forms a skin contact surface (126) for the needle-less injection through or into the human skin; and at least one first coupling element (128) of a coupling device which is arranged at the distal end of the ampoule (112) on the ampoule body (114) and configured for sealingly and reversibly attaching a needle attachment (150), and (ii) a needle attachment (150) with a cannula part (152) comprising a needle (156); and an adapter part (154) proximally connected to the cannula part, wherein the adapter part (154) has at least one second coupling element (162) of the coupling device, the coupling device configured for sealing and reversible attachment on the ampoule (112) of the injection device (110), characterized in that the needle attachment (150) is constructed in two parts, wherein the needle attachment (150) can be reversibly disassembled into the cannula part (152) and the adapter part (154).