A non-reusable safety syringe comprising a cylinder (1); an injection needle (4) fixed on the front end of the cylinder (1); a plunger (6) which is slidable in the cylinder (1) from a position of maximum withdrawal for the filling of the syringe to a position of maximum insertion for the discharging of the syringe, and which is mounted on a manually movable stem (7) which projects beyond the rear end of the cylinder (1); a protective sleeve (12) fitted slidably longitudinally on the outside of the cylinder (1) so that it can be moved from a withdrawn inoperative position, in which the needle (4) projects beyond it, to a forward safety position, in which the protective sleeve (12) extends entirely around the needle (4), covering it completely, and with securing means (23, 25) to automatically retain the protective sleeve (12), so that it cannot be retracted with respect to the cylinder (1), in its forward safety position and the needle-holder (3) in the protective sleeve (12) in its forward position.