Nick Menza: Drugs Not a Factor in Death

Nick Menza's onstage death had nothing to do with drugs, it was a massive heart attack ... TMZ has learned.

Law enforcement sources tell us autopsy results revealed heart disease in the 51-year-old, and also confirmed a working theory -- that this was not your stereotypical drug-related death. Officials searched the scene and interviewed people close to the former Megadeth drummer -- who died onstage Saturday -- and concluded Menza had no recent history of drug use.

Sources close to Nick say he had dabbled -- like many rock stars -- but we're told he'd been completely sober for 8 years.