A retractable syringe, plunger and releasable needle retaining assembly are provided. The retractable syringe typically has a glass barrel and is prefilled with fluid contents before use. The releasable needle retaining system comprises a retractable needle, a needle seal, a retaining member and an ejector member that is operable to release the retractable needle from the retaining member. The retaining member has a mating surface for mounting to a complementary mating surface of an interior wall of a syringe barrel. The plunger comprises a plunger outer, a plunger rod frangibly connected to a controlling member, a spring and a unitary plunger seal capable of engaging the retractable needle, wherein the plunger rod, plunger outer and the controlling member co-operate to releasably maintain the spring in an initially compressed state. After delivery of fluid contents of the syringe, the plunger forces the ejector member to release the retractable needle from the retaining member. Decompression of the spring at the end of depression of the plunger facilitates retraction of the retractable needle when engaged with the unitary plunger seal. Dual locking systems prevent re-use of the syringe after needle retraction.本發明為一種預先注滿可縮回注射器、活塞、針頭及其組合方法，其中活塞包含一活塞桿、一活塞殼、一控制元件和一復位元件，其中活塞桿、活塞殼和控制元件以可釋放方式共同維持一復位元件在初始能量狀態，而該針頭係可縮回，藉此組合成預先注滿可縮回注射器，防止注射器之重複使用或被使用過的注射器針頭刺傷。
