Drug delivery devices and related methods of assembly are disclosed. The drug delivery device may include a main housing having an exterior surface releasably attachable to a patient, a container disposed in an enclosed space defined by an interior surface of the main housing, and a drive assembly. The container may include a reservoir containing a drug and a stopper. The drive assembly may include drive housing and a tether which slidably engages and is pulled taut against a guide surface of the drive housing. A first end of the tether may be wound around a capstan which is mounted rotatably relative to the drive housing. A second end of the tether may be operably connected to a stopper biasing member. The tether may initially retain the stopper biasing member in an energized state. When released, the stopper biasing member may expand to move the stopper through the reservoir.
