A syringe including a barrel, a needle subassembly holder positioned in the barrel, and a needle subassembly positioned in the holder. The needle subassembly includes a needle hub releasably connected to the holder and a needle connected to the hub. Further, the syringe includes a piston slidably received by the barrel. The piston includes a piston body and a piston head having a releasable portion temporarily covering a hatchway. The piston also includes a valve positioned proximal of the hatchway and movable between blocking and passing positions. A user may push the piston distally beyond an injected position in which the piston head contacts the holder causing the holder to move distally and the hub to contact the barrel, causing the barrel to bow, thereby storing potential energy. When the potential energy reaches a threshold amount sufficient to overcome forces holding the hub in the holder, the barrel rebounds toward its un-bowed shape thereby propelling the needle hub and needle proximally past the releasable portion and the valve and into the piston body. The valve then automatically returns to the blocking position thereby ensuring that the needle subassembly does not move proximally out of the piston body.