An apparatus for injecting an additive in a pressurized air conditioning or refrigeration system includes a cartridge having two ends, a first open end which connects to the system being charged and a second open end, and wherein the first open end has a threaded nozzle. A piston is sealably disposed with the second open end of the cartridge. An adapter is provided to sealably and releasably connect the cartridge to a service valve of the pressurized air conditioning or refrigeration system without the interposition of an intermediate hose assembly. The adapter is a fitting having a fitting body with a first end portion connectible to the threaded nozzle of the cartridge and a second end portion connectible to the service valve of the pressurized air conditioning or refrigeration system.