An injection assembly includes a housing with a barrel positioned within the housing. The barrel includes an axis extending from a proximal end to a distal end thereof and a chamber for storing medicine. An elongate anti-rotation shaft is rotationally fixed to the housing and has a non-circular cross-section. The injection assembly includes an inner screw having an inner screw opening to complementarily receive the anti-rotation shaft to prevent rotation of the inner screw with respect to the anti-rotation shaft but allow respective linear translation along the axis. An outer screw is positioned within, and rotatable with respect to, the housing and has an outer screw opening to threadably receive the inner screw, wherein rotation of the outer screw translates the inner screw along the axis and along the anti-rotation shaft. A plunger is coupled to the distal end of the inner screw and is moveable with respect to the chamber to push the medicine out of the chamber when the outer screw is rotated.
