An automatic injector adapted to receive a cartridge including a barrel, a needle, and a plunger seal, the cartridge defining a longitudinal axis, the injector comprising: a housing (52, 236), a cartridge carrier (126, 202) adapted to receive at least a portion of the cartridge, the cartridge carrier (126, 202) being disposed for movement relative to the housing (52, 236) in a direction parallel to the longitudinal axis of the cartridge, a plunger carrier disposed for movement relative to the cartridge carrier (126, 202), the plunger carrier including an internally threaded bore and an elongated plunger rod disposed to confront and impart axial movement to the plunger seal, at least one transfer instrument (150, 204) disposed to selectively couple the cartridge carrier (126, 202) to the plunger carrier for movement therewith, wherein each of the plunger carrier and the cartridge carrier (126, 202) includes an opening (154, 206, 152, 210) that selectively receives the at least one transfer instrument (150, 204) to removably couple the cartridge carrier (126, 202) to the plunger carrier, the cartridge carrier (126, 202) not be being coupled to the plunger carrier when the at least one transfer instrument (150, 204) is disposed in a first position within only one of the cartridge carrier opening (152, 210) and the plunger carrier opening (154, 206), and the cartridge carrier (126, 202) being coupled to the plunger carrier for movement therewith when the at least one transfer instrument (150, 204) is disposed in a second position within both the cartridge carrier opening (152, 210) and the plunger carrier opening (154, 206), a retainer disposed to selectively retain the at least one transfer instrument (150, 204) within at least one of the openings (154, 206, 152, 210) in either the cartridge carrier (126, 202) or the plunger carrier, wherein a retainer biasing element disposes and biases the retainer toward a position that at least partially covers the at least one of the openings (154, 206, 152, 210) when the plunger carrier is moved distally to deliver a drug from the cartridge, and an elongated drive device (114, 196) including an externally threaded drive screw, the internally threaded bore of the plunger carrier being complimentary to, and disposed to engage, the externally threaded drive screw, rotation of the threaded drive screw of the elongated drive device (114, 196) being disposed to provide movement of the plunger rod in a direction of travel parallel to the longitudinal axis of the cartridge, the direction of travel of the plunger rod being dependent upon the direction of the rotation of the threaded drive screw. a housing (52, 236), a cartridge carrier (126, 202) adapted to receive at least a portion of the cartridge, the cartridge carrier (126, 202) being disposed for movement relative to the housing (52, 236) in a direction parallel to the longitudinal axis of the cartridge, a plunger carrier disposed for movement relative to the cartridge carrier (126, 202), the plunger carrier including an internally threaded bore and an elongated plunger rod disposed to confront and impart axial movement to the plunger seal, at least one transfer instrument (150, 204) disposed to selectively couple the cartridge carrier (126, 202) to the plunger carrier for movement therewith, wherein each of the plunger carrier and the cartridge carrier (126, 202) includes an opening (154, 206, 152, 210) that selectively receives the at least one transfer instrument (150, 204) to removably couple the cartridge carrier (126, 202) to the plunger carrier, the cartridge carrier (126, 202) not be being coupled to the plunger carrier when the at least one transfer instrument (150, 204) is disposed in a first position within only one of the cartridge carrier opening (152, 210) and the plunger carrier opening (154, 206), and the cartridge carrier (126, 202) being coupled to the plunger carrier for movement therewith when the at least one transfer instrument (150, 204) is disposed in a second position within both the cartridge carrier opening (152, 210) and the plunger carrier opening (154, 206), a retainer disposed to selectively retain the at least one transfer instrument (150, 204) within at least one of the openings (154, 206, 152, 210) in either the cartridge carrier (126, 202) or the plunger carrier, wherein a retainer biasing element disposes and biases the retainer toward a position that at least partially covers the at least one of the openings (154, 206, 152, 210) when the plunger carrier is moved distally to deliver a drug from the cartridge, and an elongated drive device (114, 196) including an externally threaded drive screw, the internally threaded bore of the plunger carrier being complimentary to, and disposed to engage, the externally threaded drive screw, rotation of the threaded drive screw of the elongated drive device (114, 196) being disposed to provide movement of the plunger rod in a direction of travel parallel to the longitudinal axis of the cartridge, the direction of travel of the plunger rod being dependent upon the direction of the rotation of the threaded drive screw.