Abstract: A hypodermic syringe includes an elongate housing having rear and forward ends and a piston chamber which slidably supports the piston to define an innoculant storage space within the housing, a hollow needle supported by the plunger assembly and in communication with the storage space. The needle is extendable from the forward end of the housing and retractable into the housing during an injection stroke and a safety device is provided for preventing re-use of the syringe after completion of the injection stroke. The safety device includes an override chamber at the rear end of the housing and adjoining the piston chamber, the override chamber having a greater width than the piston chamber, and a displaceable diaphragm located at or adjacent the juncture of the piston and the override chambers to partition and seal the override chamber from the piston chamber, the diaphragm being displaced by the piston and the piston entering the override chamber at the end of the injection stroke.