An injection device for automatically combining two substances, including a body and a cartridge displaceably disposed within the body having a first chamber for containing a first substance, a second chamber for containing a second substance, a first stopper displaceably disposed within the cartridge separating the first and second chambers, a second stopper displaceably disposed within the cartridge defining an end of the second chamber. The device also includes a needle hub displaceably disposed within the body having threads thereon for connecting a pen needle to the injection device. The device additionally includes a plunger displaceably disposed within the body, a biasing member, a sleeve, biased by the biasing member and communicating the bias to the plunger, and an activation button radially displaceable with respect to the body to selectively release the sleeve to drive the plunger and automatically mix the first and second substances.
