An autoinjector includes a main body having a container, the container having a drug therein, a hollow needle coupled to the container through which the drug can be delivered. The autoinjector also has a power source having a liquefied gas therein as a driver, a flow regulator, and at least one outlet through which some of the driver can exit the power pack in a gaseous state. The power source and container are coupled together for operation such that, when an injection is initiated, the flow regulator will control an exit rate of the driver such that the liquefied gas in the power source is maintained at substantially its vapor pressure and the power source will adjustably apply a force to deliver the drug via the hollow needle at a delivery rate that is a constant delivery rate.
