The present invention provides for retractability of a hypodermic needle and for locking the retracted needle into position by making a limited number of alterations to a standard design of hypodermic syringe. Engagement means are provided on the head of the plunger for engaging and reversibly locking into a complementary recess on the hub housing the needle. The hub is threadedly received into an opening at the forward end of the syringe body. By employing the engagement means on the end of the plunger, the plunger may be used to remove and retract the needle from its fully deployed position into the interior of the body of the syringe. Alternatively, the engagement means can be used to replace the needle into position for reuse. Means for locking the retracted plunger into position include a spiral channel in the body of the syringe which receives one or more elastic radial vanes formed on the plunger. The spiral channel and radial vanes act in the same manner as the threads of a screw in order to reversibly lock the plunger into position. The spiral channel is provided with an end wall which provides a stop mechanism to prevent the plunger from being completely withdrawn once it has been inserted into the body of the syringe.