An injection device for automatically combining two substances, comprising: a body; a cartridge displaceably disposed within the body and 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 and separating the first and second chambers, and a second stopper displaceably disposed within the cartridge and defining an end of the second chamber; a plunger displaceably disposed within the body; a biasing member; a needle hub displaceably disposed within the body, the needle hub having threads thereon for connecting a pen needle to the injection device; 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; and characterized: wherein the sleeve comprises a central portion and a radial arm extending radially from the central portion; wherein the activation button comprises: a user interface surface; and a J-shaped cantilevered arm extending from the user interface surface, the J-shaped cantilevered arm having an opening; wherein the activation button is configured to be radially displaceable from a pre-activated position, in which the J-shaped cantilevered arm retains the radial arm against the bias of the biasing member, to a depressed position to release the radial arm, wherein upon radial displacement of the activation button to the depressed position, the opening of the J-shaped cantilevered arm is configured to align with the radial arm, permitting displacement of the sleeve. a body; a cartridge displaceably disposed within the body and 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 and separating the first and second chambers, and a second stopper displaceably disposed within the cartridge and defining an end of the second chamber; a plunger displaceably disposed within the body; a biasing member; a needle hub displaceably disposed within the body, the needle hub having threads thereon for connecting a pen needle to the injection device; 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; and characterized: wherein the sleeve comprises a central portion and a radial arm extending radially from the central portion; wherein the activation button comprises: a user interface surface; and a J-shaped cantilevered arm extending from the user interface surface, the J-shaped cantilevered arm having an opening; wherein the activation button is configured to be radially displaceable from a pre-activated position, in which the J-shaped cantilevered arm retains the radial arm against the bias of the biasing member, to a depressed position to release the radial arm, wherein upon radial displacement of the activation button to the depressed position, the opening of the J-shaped cantilevered arm is configured to align with the radial arm, permitting displacement of the sleeve. wherein the sleeve comprises a central portion and a radial arm extending radially from the central portion; wherein the activation button comprises: a user interface surface; and a J-shaped cantilevered arm extending from the user interface surface, the J-shaped cantilevered arm having an opening; wherein the activation button is configured to be radially displaceable from a pre-activated position, in which the J-shaped cantilevered arm retains the radial arm against the bias of the biasing member, to a depressed position to release the radial arm, wherein upon radial displacement of the activation button to the depressed position, the opening of the J-shaped cantilevered arm is configured to align with the radial arm, permitting displacement of the sleeve. a user interface surface; and a J-shaped cantilevered arm extending from the user interface surface, the J-shaped cantilevered arm having an opening; wherein the activation button is configured to be radially displaceable from a pre-activated position, in which the J-shaped cantilevered arm retains the radial arm against the bias of the biasing member, to a depressed position to release the radial arm, wherein upon radial displacement of the activation button to the depressed position, the opening of the J-shaped cantilevered arm is configured to align with the radial arm, permitting displacement of the sleeve.