The protected needle assembly is provided with various enhancements for improved use. In one embodiment, a V-shaped slot is provided at the rear of the guide slot so that the retractable sheath can be locked in a retracted position. In another embodiment, a cam surface is provided at the front of the guide slot to direct the projection on the sheath into a position to lock of the sheath in an extended position. An additional inclined guide portion may be provided at the forward end of the guide slot for intramuscular injection use. The movable sheath may also be mounted on a syringe without need for a resilient bias or may incorporate a coiled spring or rubber strip to bias the sheath into an extended position. Another embodiment allows for multiple or universal use of the needle assembly.