In one aspect, a medical injector is provided herein which includes a body； a displaceable plunger disposed in the body； a spring disposed to advance the plunger； a releasable retainer for retaining the plunger in a first state against force of the spring； and, a sleeve disposed on the body, the sleeve being slidable relative to the body. Sliding movement of the sleeve over a predetermined extent relative to the body results in the releasable retainer releasing the plunger thereby allowing the spring to advance the plunger. Advantageously, the subject invention provides a medical injector having an automated plunger drive which is triggered upon sliding movement of the sleeve, thereby minimizing premature or failed activations.
