Injection devices are disclosed for injections of substances. The disclosed devices separately may hold two separate substances, e.g., solid, liquid or gas substances, in separate compartments which are interrupted so that the separate substances come into contact during the activation of the device. One disclosed device includes a needle for injection and two substances that are mixed with only a few actions by the operator of the device. The device may or may not activate mixing by the removal of a sleeve which triggers compartment interaction through movement of pins, pegs, springs, wires, string, or other mechanisms which allow the materials held in separate chambers to come into contact with each other. Operator error during mixing and checking the dosage of substances to be injected is avoided with the disclosed devices. Also, the needle is maintained in a sterile state because the needle is held internally of the device until the sleeve is removed and the device begins to activate.