THE PLT WAS OBSVD TAKING MEDICATION WITHIN 2 HRS BFR FLT. A WITNESS IDENTIFIED 1 PILL AS DARVOCET-N 100, SINCE SHE TOOK THE SAME MEDICATION. WHEN THE PLT STARTED THE ENG, IT WENT TO A VERY HI RPM, AS IF THE THROTTLE WAS TOO FAR OPEN. WHILE TAXIING TO TAKEOFF, THE ACFT NEARLY HIT A PARKED ACFT, WENT OFF THE TAXIWAY & CAME ALARMINGLY CLOSE TO A TRACTOR. ON A LOCAL FLT TO A NEARBY ARPT AT NGT, THE PLT'S SPEACH WAS SLURRED & ERRATIC, & HE COULD NOT FIND THE DESTN ARPT. TWR PSNL AT THE DESTN ARPT IDENTIFIED THE ACFT BY USING THEIR BRITE RADAR & COULD CLEARLY SEE THE ACFT LGTS. THEY TRIED TO VECTORTHE ACFT TO THE ARPT, BUT THE PLT DID NOT FLW DRCTNS. SUBSEQUENTLY, THE ACFT IMPACTED THE GND NEAR THE EDGE OF A FLOODEDFIELD, WHILE DSCNDG WITH THE GEAR EXTENDED, APRX 5 MI SW OF THE ARPT. AN UNIDENTIFIED BARBITUATE WAS FND IN THE PLT'S GASTRIC CONTENTS. HIS DOCTOR HAD PRESCRIBED TALWIN & DARVOCET-N 100 FOR BACK PAIN. THE MEDICINE COULD HAVE CAUSED DIS- ORIENTATION & DROWINESS. TOX CHK OF HIS BLOOD WAS NEG FOR DRUGS, ALCOHOL & CARBON MONOXIDE. INSUF URINE FOR DRUG CHK.