A power pack, for use in an autoinjector to deliver a drug from a container at an injection rate includes a power pack body configured to hold a liquefied gas and a flow regulator, the power pack body further including at least one outlet through which some of the liquefied gas can exit the power pack in a gaseous state such that, during use, the flow regulator will control a rate of exit flow such that the liquefied gas in the canister is maintained at substantially its vapor pressure, while the gas leaving the outlet applies an adaptively changing force to the drug in the container so that delivery of the drug will occur at about a constant injection rate.
