Devices and methods are provided for delivering an agent into a patient's body using an injection device that includes a drive module and an injector module coupled to the drive module. The drive module includes a canister and puncture mechanism in a first chamber, a plunger in a second chamber communicating with the first chamber, and an actuator that moves the puncture mechanism to cause a puncture pin thereon to penetrate a septum of the canister and cause the gas within the canister to flow through the first chamber around the canister, and into the second chamber. The injector module includes a piston slidably disposed within an agent chamber and coupled to the distal end of the plunger such that, when the plunger moves from a retracted position to an extended position, the piston is advanced within the agent chamber to deliver an agent therein into a patient's body.