Abstract: The present invention is related with an improved syringe having a novel piston design. The syringe comprises an outer tube or cylinder made of, for example, plastic material, narrowed at its outlet for fitting a detachable hollow needle. Inside the outer tube the novel piston is slidable fitted including at its forward end a coupling defined by a coupling bolt and a retaining head which form part of the piston body. To the coupling a rubber bulb is rotatably fitted including a longitudinal hole for allowing fluids flow from tube to the needle and from the piston to the tube. The forward end of the piston includes an orifice defining fluid communication between the inner volume of the hollow piston and the inner volume of the tube. During the operation the piston can be rotated so as to align the hole of said rubber bulb and said orifice of the piston forward end for defining a fluid channel from the syringe to the hollow needle.