Prince also had a dangerously low red blood cell count, indicating he had been ill, Minneapolis ABC affiliate KSTP-TV said, citing two unnamed law enforcement officials.
A spokeswoman for the local medical examiner's office that conducted a post-mortem examination of Prince declined to confirm the reports.
The cause of Prince's death remained undetermined. The medical examiner's office said in late April the autopsy and toxicology results could take weeks.
The news reports came after federal authorities said on Wednesday they were joining the investigation into Prince's death.
The Drug Enforcement Administration and U.S. Attorney's office in Minnesota will bring federal resources to the local investigation and expertise on the illegal use and trafficking of prescription drugs, the U.S. Attorney's office said.
Also on Wednesday, a lawyer for a California addiction doctor said Prince's representatives had contacted the doctor the evening before his death, adding that the doctor had planned to visit Prince for a "life-saving mission."
The 57-year-old Prince was found dead on April 21 at his Paisley Park home-studio complex in a Minneapolis suburb. Prescription opioid medication was found at the scene, a law enforcement source told Reuters.
Dr. Howard Kornfeld, who runs Recovery Without Walls, a clinic in Mill Valley, California, planned to fly to Minnesota on April 22, his lawyer William Mauzy said. In the meantime, his son, Andrew Kornfeld, a clinic staff member, traveled to Minnesota on April 21 for an initial discussion.
When Andrew Kornfeld arrived at Paisley Park, Prince was not available, Mauzy said. A staff member found the artist unconscious in an elevator, and Kornfeld called 911.