A retractable needle safety syringe (11) with plunger locking mechanism is disclosed. Locking means (36) are provided in the body (12) of the syringe to engage the plunger (13) once it is retracted after use, so that the syringe is rendered incapable of being used further. The locking means is preferably provided by a twist lock feature. A region of weakness (24) is also provided in the plunger so that after the needle is retracted into the body of the syringe, and the plunger engaged with the locking means, the plunger is broken off so it cannot be depressed again.
