A retractable syringe comprises a cutting head mounted on a plunger of the syringe. When the plunger is advanced to expel fluid from the syringe, the cutting crown cuts a retention mechanism which holds a needle unit of the syringe, allowing a drive mechanism to drive the needle unit into the syringe. The cutting crown is shaped to reduce the force which has to be applied to cut the retention mechanism. Furthermore, a disposable needle assembly for attachment to a syringe unit comprises a needle and connector element for connecting the needle to the syringe unit. The connector element encircles the needle and has a thread on its outer surface for mating with the syringe unit. When the needle assembly is mated with the syringe, a central bore of substantially constant bore extends from the tip of the needle into the interior chamber of the syringe.