An accidental overdose of fentanyl killed Prince at the end of the rock icon’s long battle with almost constant shooting hip pain and sore ankles. Chronic and excruciating pain was just one of the health problems that plagued the star. News reports say Prince, 57, was under treatment for withdrawal from opioid painkillers, anemia and fatigue when he was found dead April in an elevator at his home studio in Minnesota.

Many people had never heard of fentanyl until the Midwest Medical Examiner’s Office in Ramsey, Minnesota reported that Prince died from an apparently accidental self-administered dose. The synthetic opioid was designed in the 1950s as an effective alternative to morphine and meperidine. It is the strongest opioid approved for medical use in the United States, and it provides a measure of relief for many patients suffering from the crippling pain of advanced cancer.

The National Institute for Drug Abuse says fentanyl is 30 to 50 times more potent than heroin and 50 to 100 times more potent than morphine. And it has become a nightmare for public health officials. The Centers for Disease Control reports emergency rooms saw almost 4,600 seizures related to fentanyl in 2014.

The CDC and the Drug Enforcement Administration together confirmed 1,103 deaths related to the use of fentanyl for non-medical purposes from 2005 to 2007. For the most recent period in which data is available, late 2013 to early 2015, the number of deaths from fentanyl overdoses reported to the DEA alone climbed above 700.

Why is fentanyl so dangerous? The reason may be found in the tiny amount of dosage which can be lethal. Just a quarter of a milligram can be fatal, according to research published in the Annals of Pharmacotherapy. The standard aspirin tablet is 1,300 larger at 325 milligrams.

The DEA says the number of deaths from accidental overdose or misuse of fentanyl that has been properly prescribed is significant. However, the DEA says most fentanyl-related deaths can be traced to “abuse for its intense euphoric effects.” Drug dealers are mixing fentanyl with cocaine or heroin, and the practice is deadly. Fentanyl is a very dangerous substitute for heroin because it is much more potent than heroin, according to the DEA.

Prince appears to have kept secret from even his closest friends his dependence on painkillers he had started taking for hip pain. He resisted hip surgery until the mid-2000s, but when he did undergo a procedure to alleviate the pain, the operation failed. He was prescribed more pain pills.

Local authorities have asked the DEA for help in investigating how Prince obtained the drug. It’s not known if the fentanyl was prescribed for Prince, or, if so, why. The autopsy report provided no information beyond the finding of death from accidental overdose.