An injector for injecting a medicament in a patient. The injector includes a container comprising a fluid chamber containing a first volume of a medicament, and an injection conduit associated with the fluid chamber for defining a fluid pathway therefrom to inject the medicament from the fluid chamber through the injection conduit to an injection location. The injector also includes a firing mechanism associated with the fluid chamber for expelling the medicament from the fluid chamber through the injection conduit, arid a volume-control mechanism operable to control a fraction of the first volume of medicament that is injected when the firing mechanism is actuated to inject the medicament.