A medical injector is provided herein which includes first and second body portions rotatably coupled together； a displaceable plunger disposed in at least one of the first and second body portions； a spring disposed to advance the plunger； and, a releasable retainer for retaining the plunger in a first portion against force of the spring. The releasable retainer releases the plunger upon a predetermined extent of relative rotation between the first and second body portions, 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 rotation of injector body portions, thereby minimizing premature or failed activations.
