A medical injector, comprising: a body having a distal end and a proximal end; a displaceable plunger (164, 194) disposed in an interior of the body, the plunger having a plurality of spaced-apart ratchet teeth (168, 200, 202) disposed along the length thereof; at least one indexer having a pawl formed to engage the ratchet teeth of the plunger, wherein the indexer is configured to allow the plunger to displace distally toward the distal end of the body but not proximally toward the proximal end of the body; and an actuator having an engagement portion formed to engage one or more of the ratchet teeth, wherein the actuator is displaceable to a ready state, the engagement portion being displaced proximally relative to the plunger upon the actuator being displaced to the ready state, the indexer preventing proximal movement of the plunger thereby allowing the engagement portion to bypass one or more of the ratchet teeth upon the actuator being displaced to the ready state, and wherein the actuator is displaceable from the ready state to cause actuation of the medical injector, the displacement from the ready state causing distal displacement of the engagement portion with the engagement portion engaging one or more of the ratchet teeth and causing distal displacement of the plunger; characterized in that: the medical injector further comprises a plunger slider disposed on an exterior of the body to be operable by a user; wherein the plunger is slidably displaceable relative to the body, and rotatable relative to the body to an orientation in which the ratchet teeth are engaged or disengaged from the indexer and the actuator; and wherein the body comprises axial slots and the plunger slider has arms extending through the axial slots to engage the plunger, the plunger slider arms being slidable along the axial slots. a body having a distal end and a proximal end; a displaceable plunger (164, 194) disposed in an interior of the body, the plunger having a plurality of spaced-apart ratchet teeth (168, 200, 202) disposed along the length thereof; at least one indexer having a pawl formed to engage the ratchet teeth of the plunger, wherein the indexer is configured to allow the plunger to displace distally toward the distal end of the body but not proximally toward the proximal end of the body; and an actuator having an engagement portion formed to engage one or more of the ratchet teeth, wherein the actuator is displaceable to a ready state, the engagement portion being displaced proximally relative to the plunger upon the actuator being displaced to the ready state, the indexer preventing proximal movement of the plunger thereby allowing the engagement portion to bypass one or more of the ratchet teeth upon the actuator being displaced to the ready state, and wherein the actuator is displaceable from the ready state to cause actuation of the medical injector, the displacement from the ready state causing distal displacement of the engagement portion with the engagement portion engaging one or more of the ratchet teeth and causing distal displacement of the plunger; characterized in that: the medical injector further comprises a plunger slider disposed on an exterior of the body to be operable by a user; wherein the plunger is slidably displaceable relative to the body, and rotatable relative to the body to an orientation in which the ratchet teeth are engaged or disengaged from the indexer and the actuator; and wherein the body comprises axial slots and the plunger slider has arms extending through the axial slots to engage the plunger, the plunger slider arms being slidable along the axial slots. the medical injector further comprises a plunger slider disposed on an exterior of the body to be operable by a user; wherein the plunger is slidably displaceable relative to the body, and rotatable relative to the body to an orientation in which the ratchet teeth are engaged or disengaged from the indexer and the actuator; and wherein the body comprises axial slots and the plunger slider has arms extending through the axial slots to engage the plunger, the plunger slider arms being slidable along the axial slots.