Abstract: A hypodermic syringe assembly which is self-sheathing in that use of the syringe activates a locking mechanism which automatically and irreversibly causes a hypodermic needle to be retracted into an outer sheath. As a result, the syringe assembly is automatically and permanently made safe to handle by the user. In addition, reuse of the syringe is prevented by the locking mechanism, in that the locking mechanism serves to puncture the syringe if an attempt is made to reuse the syringe. The syringe includes a tubular-shaped sheathing member having an apertured end through which the hypodermic needle extends when the syringe is in use. Telescopically disposed within the sheathing member is a tubular-shaped insert having one end which is open to receive a plunger, with the other end being provided with an aperture and boss for receiving the hypodermic needle.