Fentanyl, prescribed by doctors for cancer treatment, can be made illicitly and is blamed for a spike in overdose deaths in the United States.
It's 25 to 50 times more potent than heroin and 50 to 100 times more potent than morphine, according to the US Drug Enforcement Administration, reported CNN.
Prince, whose full name was Prince Rogers Nelson, died April 21 at age 57, after being found unresponsive in an elevator at Paisley Park, his home and recording studio in Chanhassen, Minnesota.
"How injury occurred: The decedent self-administered fentanyl," the report said.
For manner of death, a box was marked for "accident."
The report didn't specify how the drug was taken and if the fentanyl was prescribed or illegally made.