An injector (10) for injecting a medicament into a patient. The injector includes a container (18) defining a first chamber (22), which contains a fluid therein, and a second chamber (23). The injector also includes an injection conduit (126) configured for directing the fluid fired from the container into the patient. A transfer mechanism is operable by a user to transfer the fluid from the first chamber to the second chamber in a first stage of operation, and a firing mechanism is operable by the user for firing the fluid from the second chamber through the injection conduit in a second stage of operation. An energy source (62) is in powering association with the firing mechanism to drive firing mechanism in the first and second stages.