A medical injector is provided herein which includes a body; a slidable button at least partially disposed in the body; a displaceable plunger disposed in the body; a spring disposed to advance the plunger; and, a releasable retainer for retaining the plunger in a first state against force of the spring. Upon a predetermined extent of sliding movement of the button, the button engages the releasable retainer so as to cause the releasable retainer to release the plunger thus allowing the spring to advance the plunger. Advantageously, the subject invention provides a medical injector having an automated plunger drive which is triggered upon button activation, thereby minimizing premature or failed activations.