A device comprising a body, a sleeve mobile in relation to the body, and a lock enabling locking of the sleeve in a protective position; the sleeve can be moved between a non-protective position, in which an injection member is exposed, and the protective position; the lock comprising at least one flexible proximal leg. According to the invention, the at least one leg comprises at least one bearing surface and the device comprises at least one corresponding bearing surface, these respective bearing surfaces being able to bear against each other when a stress is exerted on the sleeve in a direction tending to return the sleeve toward the non-protective position.