A medication injection pen, comprising: a body; a first compartment defined by the interior of the body, proximal to a wall, housing a dose set knob, a setback member, a dose stop member, a driver, and a leadscrew; a second compartment defined by the interior of the body, distal to the wall, housing a threaded drive insert and an enlarged distal end of the driver, the wall being provided with an aperture sized to fit the main cylindrical body of the driver, but not the enlarged distal end, thus axially fixing the driver to the body; the dose set knob comprising at least one external thread and being threadedly connectable and rotatable with respect to said body which translates into axial movement to set a desired injection dose; the leadscrew being axially advanceable in a first direction, said first direction being that which expels medication from a cartridge; said leadscrew being axially movable with respect to said driver; the threaded drive insert being fixed to said body and comprising an aperture with threads disposed thereon, which are threadedly coupled to corresponding threads on the leadscrew; and the setback member being disposed between said driver and said dose set knob, wherein said setback member is a tubular member rotationally fixed to said driver and selectively rotationally engaged to said dose set knob, and wherein the enlarged distal end of said driver and said threaded drive insert form a unidirectional coupling, wherein during dose setting and dose correction, said unidirectional coupling substantially prevents said driver from rotating with said dose set knob, and during an injection, said setback member is rotationally locked to said dose set knob thereby overcoming said unidirectional coupling to allow said driver to rotate with said setback member and axially advance said lead screw in said first direction. a body; a first compartment defined by the interior of the body, proximal to a wall, housing a dose set knob, a setback member, a dose stop member, a driver, and a leadscrew; a second compartment defined by the interior of the body, distal to the wall, housing a threaded drive insert and an enlarged distal end of the driver, the wall being provided with an aperture sized to fit the main cylindrical body of the driver, but not the enlarged distal end, thus axially fixing the driver to the body; the dose set knob comprising at least one external thread and being threadedly connectable and rotatable with respect to said body which translates into axial movement to set a desired injection dose; the leadscrew being axially advanceable in a first direction, said first direction being that which expels medication from a cartridge; said leadscrew being axially movable with respect to said driver; the threaded drive insert being fixed to said body and comprising an aperture with threads disposed thereon, which are threadedly coupled to corresponding threads on the leadscrew; and the setback member being disposed between said driver and said dose set knob, wherein said setback member is a tubular member rotationally fixed to said driver and selectively rotationally engaged to said dose set knob, and wherein the enlarged distal end of said driver and said threaded drive insert form a unidirectional coupling, wherein during dose setting and dose correction, said unidirectional coupling substantially prevents said driver from rotating with said dose set knob, and during an injection, said setback member is rotationally locked to said dose set knob thereby overcoming said unidirectional coupling to allow said driver to rotate with said setback member and axially advance said lead screw in said first direction.