CONTINUED FLIGHT BY THE PILOT INTO INSTRUMENT METEOROLOGICAL CONDITIONS (IMC) AND HIS INABILITY TO MAINTAIN CONTROL OF THE AIRCRAFT AFTER BECOMING SPATIALLY DISORIENTED. CONTRIBUTING FACTORS WERE HILLY/MOUNTAINOUS TERRAIN, WEATHER CONDITIONS, THE PILOT'S LACK OF INSTRUMENT EXPERIENCE, AND HIS IMPAIRMENT FROM A PRESCRIPTION DRUG THAT WAS NOT APPROVED FOR USE WHILE FLYING.