And Monday, a source close to the investigation says U47700 was part of the mixture. The potent painkiller is a synthetic opioid, eight times stronger than morphine.
And, it took more time and more testing to detect.
"You may have to send it out to a more sophisticated lab that actually is looking for these sorts of drugs," said Dr. Cody Wiberg with the Minnesota Board of Pharmacy.
Investigative sources told reporter Beth McDonough that Prince may have thought he was taking a legitimate painkiller, like hydrocodone or fentanyl, that unknowingly also had U-47700 in it.
The pills often look just like other medications. Plus, Wiberg says U47700 can be resistent to the life-saving antidote Narcan.
Sources say when Princes plane made an emergency landing in Illinois less than a week before his death, paramedics gave him not one but two Narcan shots,
"You might have to administer multiple doses to try to reverse the effects," said Wiberg.
Because U-47700 is not considered a controlled substance by state or federal agents, it's not regulated. The Drug Enforcement Administrations says it tends to be produced overseas in China or Eastern Europe. It's widely available, easily accessible and affordable, about $40 online.
Wiberg says the drug is on his radar, because it can cause sedation to the point a person stops breathing, "people that use synthetic drugs are playing Russian Roulette, that's because another problem with these drugs is you simply don't know what you get."
Prince was known to have chronic hip pain. We've reported he was being treated for an addiction to painkillers.
As for U-47700, several states have moved to ban the drug. That means, anyone who possesses or sells it, would be subject to criminal penalties.
Updated: July 19, 2016 06:59 AM
Created: July 18, 2016 10:14 PM
Copyright 2016 - KSTP-TV, LLC A Hubbard Broadcasting Company