An injection pen, comprising: a housing including a retract nut having a retract nut opening having first flat sides (137, 138); a lead screw having second flat sides (80, 81), the lead screw being axially moveable in the housing; a dose set member for dose setting and dose correcting, the dose set member including teeth and an internal shoulder, and the dose set member being connected to the housing; a setback member including tabs, an enlarged head portion and teeth, the enlarged head portion being operatively connected to the internal shoulder of the dose set member during dose setting and dose correcting; a clicker operatively connected to the dose set member and the setback member; and a rotatable driver including grooves that are operatively connected to the tabs of the setback member the rotatable driver being operatively connected to the lead screw, wherein the setback member is axially moveable with respect to the driver, during dose setting and dose correcting, the driver is prevented from moving axially and rotating, characterized in that: the teeth of the setback member slide over teeth of the dose set member, arms of the clicker pass over at least one of the teeth of dose set member and the teeth of the setback member, and the setback member is rotatably locked by engagement of the tabs of the setback member with the grooves of the rotatable driver, and during an injection, a push button is depressed until the dose set member fully rotates back into the housing, the teeth of the setback member engage the teeth of the dose set member such that the dose set member and the setback member rotate together and the driver rotates with the setback member via engagement of the tabs in the setback member and the grooves in the driver to drive the lead screw, the retract nut is rotationally fixed to the housing, the engagement between the first flat sides (137, 138) of the retract nut opening and the second flat sides (80, 81) of the lead screw substantially prevents the lead screw from rotating and allows the lead screw to move only axially to push a stopper and dispense medication. a housing including a retract nut having a retract nut opening having first flat sides (137, 138); a lead screw having second flat sides (80, 81), the lead screw being axially moveable in the housing; a dose set member for dose setting and dose correcting, the dose set member including teeth and an internal shoulder, and the dose set member being connected to the housing; a setback member including tabs, an enlarged head portion and teeth, the enlarged head portion being operatively connected to the internal shoulder of the dose set member during dose setting and dose correcting; a clicker operatively connected to the dose set member and the setback member; and a rotatable driver including grooves that are operatively connected to the tabs of the setback member the rotatable driver being operatively connected to the lead screw, wherein the setback member is axially moveable with respect to the driver, during dose setting and dose correcting, the driver is prevented from moving axially and rotating, characterized in that: the teeth of the setback member slide over teeth of the dose set member, arms of the clicker pass over at least one of the teeth of dose set member and the teeth of the setback member, and the setback member is rotatably locked by engagement of the tabs of the setback member with the grooves of the rotatable driver, and during an injection, a push button is depressed until the dose set member fully rotates back into the housing, the teeth of the setback member engage the teeth of the dose set member such that the dose set member and the setback member rotate together and the driver rotates with the setback member via engagement of the tabs in the setback member and the grooves in the driver to drive the lead screw, the retract nut is rotationally fixed to the housing, the engagement between the first flat sides (137, 138) of the retract nut opening and the second flat sides (80, 81) of the lead screw substantially prevents the lead screw from rotating and allows the lead screw to move only axially to push a stopper and dispense medication. the teeth of the setback member slide over teeth of the dose set member, arms of the clicker pass over at least one of the teeth of dose set member and the teeth of the setback member, and the setback member is rotatably locked by engagement of the tabs of the setback member with the grooves of the rotatable driver, and during an injection, a push button is depressed until the dose set member fully rotates back into the housing, the teeth of the setback member engage the teeth of the dose set member such that the dose set member and the setback member rotate together and the driver rotates with the setback member via engagement of the tabs in the setback member and the grooves in the driver to drive the lead screw, the retract nut is rotationally fixed to the housing, the engagement between the first flat sides (137, 138) of the retract nut opening and the second flat sides (80, 81) of the lead screw substantially prevents the lead screw from rotating and allows the lead screw to move only axially to push a stopper and dispense medication.