Multiple dosage injector with rotating screw dosage system

A dispensing mechanism, comprising:

• 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 user-operable push button such that movement of the user-operable push button along the proximal-distal axis relative to the housing causes rotation of the twist driver about the proximal-distal axis;
• 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;
• and
• an anti-reverse mechanism including:
• at least one ratchet integrally formed on an internal surface of the housing;
• and
• a flexible column integrally formed with a sidewall defining a side opening of the user-operable push button, the flexible column having a flexible column protrusion at a proximal end thereof,
• wherein as the user-operable push button moves along the proximal-distal axis, the flexible column protrusion engages the at least one ratchet and restricts movement of the user-operable push button to one direction during a firing motion and a resetting motion.