A replaceable needle assembly is provided for a retractable, prefilled syringe comprising a barrel having a mounting member and a plunger, whereby the retractable needle can be replaced by a user without affecting the retraction mechanism. The replaceable needle assembly comprises a needle fitted to a retractable needle mount and a needle retainer. The needle mount is removably, screw-threadedly mountable to the mounting member of the barrel. The needle retainer comprises a plurality of fingers that releasably engage the retractable needle to thereby prevent inadvertent retraction of the retractable needle before engagement by the plunger. The plunger comprises a plunger seal comprising an outer member and an inner member that engages the needle for spring-driven retraction. At the end of retraction, a lock is formed between the plunger and barrel to prevent re-use of the syringe.