Paul Rodriguez is having some bad luck, because he keeps getting linked to fentanyl ... and things have already turned deadly.

TMZ has learned ... Paul's best friend who was found dead inside the comedian's home back in November died from a fentanyl overdose.

According to the Medical Examiner ... Paul's friend Donald Serrato died from the combined effects of fentanyl, PCP and methamphetamine.

We broke the story ... Paul told us his friend was staying with him for a few days when he died. Paul said he left the house one morning to go work on a Hollywood set and he called Donald around noon to see if he wanted to grab lunch and got no response. Paul says he called his GF to go check on Donald and she found him dead in the house.

Paul is now at war with the Burbank Police Department over a recent arrest for alleged narcotics possession. He was busted as part of a traffic stop involving a car a female friend was driving.

As we've told you ... cops say they searched the car and found drugs in the woman's purse ... and our law enforcement sources say officers found suspected fentanyl and police are currently running tests on the substance.

Paul, through his attorney, previously denied being in possession of any controlled substance in regards to his arrest.

We reached out to Paul's camp about his friend's fentanyl death ... so far no word back.