A medicine injection pen includes a body, a drive member within the body and configured to move relative thereto upon actuation, a cartridge housing releasably engageable with the body and configured to retain a medicine cartridge, at least one sensor, and at least one detector. The medicine cartridge has a liquid medicine therein and includes a piston configured to slide therein. The drive member is configured to move relative to the body to urge the piston to slide within the medicine cartridge to dispense at least some of the liquid medicine from the medicine cartridge. The sensor(s) is configured to sense movement of the drive member relative to the body to enable determination of an amount of the liquid medicine dispensed. The detector(s) is configured to detect a gap between the drive member and the piston.