A syringe includes a retractable needle assembly comprising a needle seat, a hypodermic needle, and a barrel. A hollow plunger is slidably mounted within the barrel. A seal is provided between the plunger and barrel to prevent air from passing into or out of the barrel. A piston is slidably mounted within the plunger such that movement of the plunger into the fill position locks the piston in place. A seal between the piston and the interior of the plunger prevents air or fluid from passing into the plunger. As the plunger is depressed the plunger moves in the barrel while in the piston is stationary. The relative axial movement of the plunger in relation to the piston causes a vacuum to be created within the plunger. The plunger in the fully depressed position releases the needle carrier and the vacuum pulls the needle carrier and needle into the hollow plunger.
