An automatic injection device including a housing, a chamber with a first compartment and a second compartment, and a seal structure between the compartments. The seal structure is initially in a sealing condition that seals the first compartment from the second compartment, where the seal includes a plug and an outer sealing member. The plug is slidably movable within the outer sealing member to convert the seal structure from the sealing condition to a mixing condition by opening a path between the first compartment and the second compartment. The automatic injection device also includes a needle assembly and an activation assembly. Activation of the activation assembly causes (1) pressurization of the first compartment, (2) the seal structure to convert from the sealing condition to the mixing condition, and (3) the first and second medicament components to be mixed and forced through the needle assembly.
