L.A. County Medical Examiner documents show Jason died from the "effects of fentanyl." No other factors are listed in the report ... but it does say it was an accident.

TMZ broke the story ... Jason was found unresponsive in his bathroom on February 16. Law enforcement sources at the time told us there were items nearby that were consistent with heroin or opioid use ... including burnt tin foil and other paraphernalia.

Based on the ME's report ... the opioid would be fentanyl, and not heroin.

Jason -- known in Hollywood as the voice of Mikey Blumberg on the long-running Disney Channel show, "Recess" -- had battled addiction in the past. In 2011, he was busted and charged for allegedly having heroin on him. Those charges were eventually dropped.

He went on to co-found a charity called Cure Addiction Now, an org he was heavily involved with running. The voice actor was also known for being the grandson of the late Hollywood billionaire moguls, Marvin and Barbara Davis.

Jason was 35.

RIP