A medical injector (152) has a body (154) with a displaceable plunger (164). The plunger (164) includes a plurality of spaced-apart ratchet teeth (168) disposed along the length thereof. At least one indexer (176) is provided to engage the plunger (164), wherein the indexer (176) is configured to allow the plunger (164) to displace distally toward a distal end of the body (154) but not proximally toward a proximal end of the body (154). The medical injector (152) also includes an actuator (158) having an engagement portion (178) formed to engage one or more of the ratchet teeth (168). The actuator (158) is displaceable to a ready state, the engagement portion (178) being displaced proximally relative to the plunger (164) with the actuator (158) being displaced to the ready state. The indexer (176) prevents proximal movement of the plunger (164), thereby allowing the engagement portion (178) to bypass one or more of the ratchet teeth (168) with the actuator (158) being displaced to the ready state. In addition, the actuator (158) is displaceable from the ready state to cause actuation of the medical injector (152). The displacement from the ready state causes distal displacement of the engagement portion (178) with the engagement portion (178) engaging one or more of the ratchet teeth (168) and causing distal displacement of the plunger (164) with the engagement portion (178). The plunger (164) is slidably displaceable relative to the body (154), and rotatable relative to the body (154) to selectively engage and disengage ratchet teeth (168) engaged with the indexer (176) and the actuator (158).
