A self-retracting syringe has an injection assembly and a retraction assembly. The injection assembly has a housing, a spring rest, and a spring that is restrained initially in compression between the housing and the spring rest. A plunger rod is disposed inside the coil of the spring. A coupler in contact with the spring rest and the plunger rod releasably couples the plunger rod to the spring rest. When a latch is released, the plunger rod moves distally to urge a seal forward, which seal movement causes a hypodermic needle to extend and a medicament to be expelled through the hypodermic needle. A retraction assembly returns the hypodermic needle into the syringe body after the medicament is expelled. Embodiments of retraction assembly provide an inlet through which fluid may be introduced into the syringe through said hypodermic needle. Methods for filling the syringe are also described.