A medicine injection device comprises a cartridge having a cartridge barrel and a plunger having a plunger shaft slidable within the cartridge barrel. The cartridge may have, e.g., a pre-filled medicine. The medicine injection device further comprises a body having a proximal end, a distal end, and a cavity in which the cartridge barrel is disposed. The proximal end of the body may optionally form a finger grip. The medicine injection device further comprises a plunger locking mechanism attached to the proximal end of the body. The plunger locking mechanism comprises a member with an aperture slidably engaged with the plunger shaft. The locking mechanism is configured for preventing the plunger from being removed from the cartridge barrel. For example, the plunger shaft may include a locking stop, e.g., a flange, that engages with the aperture in an interference arrangement.
