An auto-inj ector comprising: a first sub-assembly comprising: a chamber for holding a fluid, said chamber comprising an inner surface and an exit aperture and further comprising an injection needle in fluid communication with the exit aperture; a stopper movably disposed within the chamber and having an outer surface substantially in contact with the inner surface about its perimeter; and an adapter adapted to transfer fluid into the chamber, the adapter comprising means for moving the stopper through the chamber and a conduit for transferring fluid into the chamber when the stopper is moved away from the exit aperture; wherein the adapter is adapted to receive a fluid container and transfer the fluid from the container to the chamber; and wherein the conduit for transferring fluid into the chamber comprises a hollow fluid transfer needle adapted to engage the fluid container to transfer fluid from the fluid container through the hollow fluid transfer needle as the stopper is moved away from the exit aperture; and wherein the auto-injector further comprises a second sub-assembly comprising: a releasable drive mechanism, said drive mechanism comprising an elongate shaft which is driven against the stopper upon activation of the drive mechanism; wherein the releasable drive mechanism is, upon activation, adapted to: (a) move the chamber and the injection needle from a retracted position in which the needle is wholly inside a housing of the auto injector to an extended position in which the needle is at least partially outside the housing; and (b) subsequently move the stopper within the chamber toward the exit aperture to expel fluid out of the injection needle; and wherein the adapter is configured to be inserted within the elongate shaft. a chamber for holding a fluid, said chamber comprising an inner surface and an exit aperture and further comprising an injection needle in fluid communication with the exit aperture; a stopper movably disposed within the chamber and having an outer surface substantially in contact with the inner surface about its perimeter; and an adapter adapted to transfer fluid into the chamber, the adapter comprising means for moving the stopper through the chamber and a conduit for transferring fluid into the chamber when the stopper is moved away from the exit aperture; wherein the adapter is adapted to receive a fluid container and transfer the fluid from the container to the chamber; and wherein the conduit for transferring fluid into the chamber comprises a hollow fluid transfer needle adapted to engage the fluid container to transfer fluid from the fluid container through the hollow fluid transfer needle as the stopper is moved away from the exit aperture; and wherein the auto-injector further comprises a second sub-assembly comprising: a releasable drive mechanism, said drive mechanism comprising an elongate shaft which is driven against the stopper upon activation of the drive mechanism; (a) move the chamber and the injection needle from a retracted position in which the needle is wholly inside a housing of the auto injector to an extended position in which the needle is at least partially outside the housing; and (b) subsequently move the stopper within the chamber toward the exit aperture to expel fluid out of the injection needle; and