Abstract: The present invention provides a clinical syringe to be rendered useless after being used once. It comprises a barrel with a needle fitting hub, a piston sealingly and slidably fitted within the barrel and a piston rod connected to the piston. The piston head is provided with a resilient detent plug with a slot thereon, and an injection opening is defined at a position of the barrel corresponding to the detent plug, the diameter of which is smaller than the inner diameter of the needle fitting hub. A U-shaped notch is punched at the top surface of the piston head adjacent the detent plug to form a U-shaped tongue. If the piston rod is to be pulled back after the syringe being used once, the piston head would be torn at the U-shaped notch due to the retaining of the detent plug by the bottom surface of needle fitting hub, rendering it incapable of drawing the injection fluid.