THE PILOT DIVERTED HIS ATTENTION FROM OPERATION OF THE HELICOPTER, FAILED TO MAINTIAN PROPER ROTOR RPM, AND FAILED TO MAINTAIN ALTITUDE (CLEARANCE) ABOVE THE GROUND, WHILE MANEUVERING AT LOW ALTITUDE. A CONTRIBUTING FACTOR WAS HIS IMPAIRMENT FROM USE OF A PRESCRIPTION DRUG (NORDIAZEPAM OR DIAZEPAM), WHICH WAS NOT COMPATIBLE WITH FLYING.