Abstract: A single use syringe wherein the essential concept is to create a valve within the orifice that leads from the drug chamber into the chamber leading to the needle. The valve consists of a conical washer supported within the chamber by inclined barbs formed on its outer perimeter. The barbs will only allow the washer to move in one direction; toward an orifice at the needle end of the chamber. Initially the washer is positioned so that it does not impede fluid flow in either direction. Engaging the washer but not attached to it is an expansion plug. The expansion plug can absorb fluid and expand to force the washer against the opening leading to the needle, to thereby occlude fluid flow. The process is slow, taking at least several minutes, thereby permitting the syringe to be used once for its intended purpose before occlusion occurs.