A plunger (13) for a syringe (11), comprising a cylindrical hollow body having a closed base and an opening located at the opposite base of said hollow body, wherein a seat accessible through said opening is provided to receive the head of a rod or a pusher (15), said seat being equipped with engagement means for retaining said head inside the seat at the end of an axial translational or an axial translational and rotary coupling movement. The application further concerns the rod intended to be associated with said plunger and a syringe or a phial using the plunger.