A device (100) for delivering a medicament into a patient's body by injection into or through the patient's skin, including a main body having a top enclosure (116) and a bottom enclosure (104), a reservoir (160) disposed within the main body for containing the medicament, and an injection needle (152) for penetrating the skin of the patient, the needle (152) having a lumen and selectively communicating with the reservoir (160). The device also includes a pressurizing system (600, 140) for selectively pressurizing the reservoir (160). In a pre-activated position, the pressurizing system (600, 140) contacts and compresses the reservoir (160) to reduce the initial volume of the reservoir (160), thereby allowing for partial filling of the reservoir (160).