Variable fill drug delivery devices for delivering a therapeutic agent to a patient are provided. A variable fill drug delivery device includes a drug container to store a variable amount (or user-selectable amount) of a therapeutic agent. A plunger of the variable fill drug delivery device is positioned in the drug container. An infusion engine of the variable fill device is coupled to the plunger. The infusion engine retains the plunger prior to activation of the variable fill drug delivery device and releases the plunger after activation of the variable fill drug delivery device. After activation, the infusion engine drives the plunger from a first position within the drug container to a second position within the drug container to expel the variable amount of the therapeutic agent from the drug container for delivery to a patient.
