The present invention relates to a medicament injector including an applicator with a cartridge-accepting receptacle. The medicament injector includes interchangeable, pre-filled medicament-containing cartridges. Each cartridge includes at least one barrier member having a closed position in which access to the medicament is inhibited. The barrier member is moveable from the closed position to an open position, in which the medicament is accessible via the applicator, when the cartridge is inserted into the cartridge-accepting receptacle. The barrier member moves back to the closed position when the cartridge is removed from the cartridge-accepting receptacle. This allows for a plurality of pre-filled medicament cartridges to be selected on demand via a need-based scenario for quick assembly with the medicament injector for use in various clinical scenarios to reduce the bulk, errors, complexity, and steps in medicament delivery while increasing the versatility, safety, and speed of medicament delivery in time critical situations.