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 .