Pen injection device and method of using same

Pen injection devices (10) and methods of using a pen injection device are disclosed. Pen injection devices include a sterile dual transfer spike assembly (150) including a dual transfer spike defining a fluidic pathway; a first cartridge assembly (12) including a first cartridge (20) containing a first substance; a second cartridge assembly (14) including a second cartridge (30) containing a second substance; a plunger rod (50) translatable in a first direction; and a biasing mechanism (60) configured to bias the plunger rod in a second direction opposite the first direction. Methods of using a pen injection device having a first container, a second container, a plunger rod, and a biasing mechanism include activating the pen injection device, thereby creating a fluidic pathway between the first container and the second container; and translating the plunger rod at least once, thereby transferring a first substance from one container to the other container to mix with a second substance.