Autopsy results released in June revealed Prince died of an accidental fentanyl overdose.
Officials told the Associated Press the singer had no prescriptions for controlled substances at the time.
Prince was found dead at 57 in an elevator inside his Paisley Park home in April.
According to the Star Tribune report, the musician weighed just 50kg (8 stone) at the time of his death and had significantly more than a fatal dose of fentanyl in his system.