Abstract: A non-reusable syringe utilizes a liquid-tight seal between a sliding piston assembly within an elongated barrel. The piston assembly comprises a valve assembly comprising a forward valve and a rear valve that separate upon depression of the plunger for expulsion of the liquid contents. The rear valve that a seal with the barrel and the plunger for drawing in of liquid and the forward valve that forms a seal with the barrel and the plunger for expulsion of the liquid. During liquid expulsion, the two valves separate. A bypass is provided on a plug attached to the plunger which prevents a seal from forming without the rear valve being in position. Thus, when the plunger is retracted after the first use, no liquid may be drawn up into the barrel. The syringe may further include a safety barb and closed ends to prevent tampering or manipulation of the piston apparatus in an effort to eliminate the separation of the two valves.