a system for injecting dna - based medications into humans is provided by the present invention . the system includes a needle - free injector with an injection orifice of approximately 0 . 004 inches for supplying dna - based medication at an initial pressure of from 3900 to 4300 psi , and then immediately declining to a level of about 2800 to 3800 psi , and then immediately cutting off pressure to terminate the injection . the injector includes an annular adapter for spacing the injection orifice from the skin of the patient . the adapter includes an abutment against which the injector is disposed so that the orifice is spaced approximately 0 . 76 - 1 . 0 inch from the skin of the patient , the adapter having an inner diameter at the distal end of approximately 0 . 50 - 0 . 70 inches .