A medical injector is provided herein having a body and a reservoir disposed in the body, the reservoir including at least first and second mixable components. At least one stopper is associated with the reservoir where distal advancement of the stopper over a predetermined distance causes mixing of the mixable components. A plunger is disposed in the body, along with a knob stem and a knob fixed to the knob stem so as to be rotatable therewith. A sleeve is telescopingly disposed over the knob stem, wherein the knob stem and the sleeve have cooperating elements formed thereon which selectively permit rotation of the knob stem to be transmitted to the sleeve so that the knob stem and the sleeve may rotate together. A spring is also provided for urging the sleeve distally. A releasable retainer releasably retains the sleeve against force of the spring. Rotation of the knob releases the sleeve and allows the spring to displace the sleeve distally. Distal movement of the sleeve causes distal movement of the plunger which causes distal movement of the stopper thus causing mixing of the mixable components. Advantageously, with the subject invention, a medical injector may be provided where autoreconstitution can be achieved by the turning of a knob.