Abstract: A safety syringe employs a mechanical logic arrangement to control the movement of a spring-biased protective shield that covers the needle except during an injection, and which limits the number of injection cycles. The mechanical logic arrangement can include a series of parallel gated tracks, and a track follower pin disposed on the slide portion of the protective shield. There are one-way gates and redirection gates, which can be formed as flaps with a radial hinge in respect to the barrel axis. An alternative arrangement can employ a rotary cam. The number of injection cycles can be two or more, and the penetration depth for a given injection cycle can be controlled.