A single use hyperdermic syringe (20) having a one way valve (56 62) to prevent refilling and a retractable needle (30). A needle connector (26) and a needle hub (28) with needle (30) form an assembly (40) with a cavity (52) for the passage of liquid. An elastically deformable sealing ring (56) and a moveable sealing member (62) in the cavity are configured to form a seal after a single use of the syringe against a flow of liquid into the syringe barrel (22). Locking pins (76) on the piston engage the needle connector when the barrel is emptied whereby the needle is retractable into the barrel by moving the plunger (24) in the proximal direction.
