a body having proximal and distal ends and a cavity therein for receiving a medical cartridge;
a needle cannula on the distal end of the body having a proximal portion extending proximally into the cavity, and a distal portion extending distally beyond the distal end of the body;
a plunger assembly substantially permanently attached to the proximal end of the body for distally directing a piston in a cartridge received in the cavity to deliver medication from the cartridge through the needle;
a shield slidably attached to the body;
cooperating detents and detent pockets on the body and shield to temporarily hold the shield in a etracted position wherein the distal portion of the needle is exposed, to temporarily hold the shield in an intermediate guarded position wherein the distal portion of the needle is substantially covered, and to substantially permanently lock the shield in a fully extended position distally beyond the intermediate position.