'Eight Is Enough' Star Adam Rich Cause of Death, Fentanyl Overdose

Adam Rich, who starred in the hit series, "Eight Is Enough" died from an overdose that's become far too common in America -- the culprit was fentanyl.

The L.A. County Medical Examiner's autopsy report, obtained by TMZ, says Adam's died in an accidental overdose of the powerful opioid. He also had non-toxic levels of alcohol and lorazepam... the generic for Ativan.

TMZ broke the story ... the former child TV star was found dead back in January inside his L.A. area home. At the time, foul play was not suspected. The M.E.'s report says his body was in a "state of mild decomposition."

Law enforcement later told us, his death was likely the result of an overdose ... adding the manager at Adam's apartment building found his lifeless body inside the bathroom and there was also a white powder substance, believed to be drugs.

Rich, who was 54, struggled with drugs for years and was once arrested for allegedly trying to steal pills from a pharmacy ... leading investigators to initially believe he overdosed.

Now, we have a clear understanding of what exactly led to his death.

Adam will be best remembered for his role as Nicholas Bradford, the youngest son on "Eight Is Enough." He stayed on the series for its entire duration from 1977-1981.