Apparatus for combining and subcutaneously injecting a mixture of drug and other liquid such as a buffering solution, uses cartridges with cylinders containing the drug and other liquid, and pistons pushable toward discharge ends of the cylinders for discharging the liquids. A drive unit housing with motor therein is activated by computer to linearly move a motor plunger. A multiple cartridge holder receives cartridges with the drug and other liquid and has a mixing chamber with inlets connected to the discharge ends for receiving a mixture of drug and other liquids. Micro-tubing is connected to an outlet of the chamber and has a handpiece with subcutaneous injection needle at its opposite end for receiving and injecting the mixture. Plural cartridge plungers are engages via a motion transmission to the motor plunger and move into open ends of the cartridges to push the pistons and discharge liquids from the cartridges into the mixing chamber.
