The invention comprises a syringe body (10) having a distal end (10B) fitted with an injection needle (14), at least one sheath (16, 18) inside which the body is positioned, and a protective cap (40) able to be retained relative to the syringe body (10) to cover the needle (14) and be separate from this body to release the needle. The device comprises a non-return unit (50) able to oppose a refitting of the cap (40) in its protection position once the needle has been released. This non-return unit, for example, comprises an elastic lip (50) joined to the sheath or the cap.