The powerful drug that killed Prince was found in mislabeled pills - and it's part of a much bigger problem with painkillers

Fentanyl, the drug that tragically killed musical genius Prince, is an opioid painkiller that's 50 times stronger than pure heroin.

The drug is legal and can be prescribed by a doctor for a variety of conditions - most often to treat severe pain.

Prince did not have a prescription for any such drugs in the year before he died, according to the Associated Press.

But while Prince's cause of death was originally listed simply as "self-administered fentanyl," new information from investigators is complicating that picture.

On August 22, investigators told the AP that one source of the fentanyl they found in Prince's body may have been fake pain pills which contained fentanyl but were labeled as something else.

The investigators told the AP they discovered two dozen pills in an Aleve bottle in Prince's home that had been labeled "Watson 385," a stamp used to ID pills that contain a mix of two other pain-relieving medications: hydrocodone (another powerful opioid painkiller) and acetaminophen (the active ingredient in Tylenol). When they tested the pills, at least one came up positive for fentanyl.

Counterfeit painkillers: A growing and deadly problem

Since fentanyl is so much stronger than hydrocodone, it ostensibly requires far less to deliver the same effect. As a result, counterfeit drug makers often swap whatever they're claiming to sell for fentanyl. The problem has worsened in recent years as deaths from drug overdoses related to painkillers continues to skyrocket. In July, the DEA released a report warning of counterfeit fentanyl pills disguised as common prescription drugs like Norco (hydrocodone), Percocet (oxycodone), and Xanax (alprazolam).

Still, fentanyl belongs to the larger class of drugs that includes these prescription drugs. And regardless of their individual strength, all of these drugs work by capitalizing on our body's natural pain-relief system, which - in high enough doses - can result in a surging sense of euphoria. Since 1999, overdose deaths involving opioid painkillers have quadrupled. In 2014 alone, more than 14,000 people died from overdoses involving the drugs.

Tests on Prince before his death didn't show any fentanyl in his system, suggesting that he wasn't a long-time fentanyl user and instead probably took the fatal dose sometime in the 24 hours before his death, the AP official said.