A dispensing mechanism for delivering a dosage of medicament including a housing having a proximal-distal axis; a plunger rod having a screw thread and configured to advance along the proximal-distal axis relative to the housing; a user-operable push button moveable along the proximal-distal axis relative to the housing; a twist driver threadably engaged with the push button such that movement of the push button towards the distal end of the housing causes rotation of the twist driver about the proximal-distal axis; and a driver engaged with the twist driver such that rotation of the twist driver causes rotation of the driver, the driver also engaged with a portion of the plunger rod such that rotation of the driver causes rotation of the plunger rod, advancing the plunger rod along the proximal-distal axis relative to the housing.