An injector cap including a sleeve and a blocking means, wherein the sleeve is mounted slidably on a portion of an injection device, e.g., on an ampoule support, is forced by a pre-tensioning means into a first position in which it covers a cannula carried by the injection device, and can be slid against the pre-tensioning means into a second position in which the cannula emerges from the end of the sleeve, and wherein the blocking means non-detachably blocks the sleeve against sliding when it has been returned from the second position to the first position.
