A safety syringe adapted consists of an outer casing that supports a conventional hypodermic needle and an inner casing which supports a blunt cannula that is disposed inside the inner lumen of the hypodermic needle. The inner casing is keyed to the outer casing by one or more pins that engage helical tracks formed along the inner surface of the outer casing. Rotation of the inner casing relative to the outer casing advances the inner casing so that the blunt cannula is moved from a retracted position within the needle to an extended position in which the blunt end of the cannula extends beyond the tip of the needle.