Syringes are available in both Leur Slip and Leur Lock attachments. The Slip type of syringe simply slides onto the needle, and when compressed will fit snugly. The Leur Lock syringe requires that the needle be twisted/ screwed onto the end of the syringe. It will not have the ability to slide off. This locking mechanism makes the device safer for use with liquids that provide high back pressure. (such as thicker penicillin).