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 bottom enclosure (104) that has a top surface including a button guide latch (268); a reservoir (160) disposed within the main body for containing the medicament; an injection needle (152) for penetrating the skin of the patient, the needle (152) having a lumen and communicating with the reservoir (160) when the device (100) is activated; a pressurizing system (140, 144) for pressurizing the reservoir (160) when the device (100) is activated; and an activator button (128) movably disposed on the main body and movable from a pre-activated position to an activated position. The activator button (128) includes an activation arm (228). When the activator button (128) moves from the pre-activated position to the activated position, an end of the activation arm (228) engages with the button guide latch (268) and prevents return movement of the activator button (128).