According to the L.A. County Medical Examiner's Office -- the comedian's adult son's official cause of death was toxicity from fentanyl, ketamine and cocaine. The agency lists his death as accidental.

TMZ broke the story ... Dex passed away in November after his girlfriend called 911 ... and first responders found him unresponsive in the bathroom of an L.A. home.

Our sources say paramedics tried reviving him, but to no avail ... he was pronounced dead on the scene.

Days later, Dana said Dex died of an accidental drug overdose ... penning an emotional tribute to his son, praising his musical talents and calling him a "beautiful person."

In the note shared alongside wife Paula, they added, "To anyone struggling with addiction or who loves someone struggling with addiction, you are in our hearts and prayers."

The father and son duo were extremely close ... with Dex following in his father's comedy footsteps -- getting his feet wet in the stand-up world over the past few years.

Dana recently got back to work by hopping on his and David Spade's podcast -- where he said laughter would help him heal from the tragedy in his family.