The sudden and apparently mysterious death of Prince that left the music world in mourning back in April was the result of an accidental overdose on a highly potent painkiller. Known as fentanyl, the substance is said to be 10 times stronger than heroin and 80 times stronger than morphine.

According to a medical report released by the Midwest Medical Examiner’s Office, the revered singer died of “fentanyl toxicity,” with the drug having been “self-administered.” As such, the manner of death has been put down as “accidental.”

Fentanyl is a synthetic opioid, which means it acts on the same brain receptors as the body’s natural opioids like endorphins and endomorphins, which help to relieve pain. Because of its extreme strength, fentanyl is occasionally prescribed to people suffering from severe pain who have failed to respond to weaker drugs such as oxycodone or morphine.

The substance is also sometimes mixed with street drugs like heroin in order to increase their potency. This has led to a number of accidental overdose deaths, as users are often unaware that they are taking fentanyl and therefore take way more than the safe dosage.

Because it is easy and cheap to synthesize, fentanyl and its derivatives have also been used to create a number of other street drugs – going by names like China White and TNT – which have been associated with several fatal overdoses.

That Prince was found with fentanyl in his system raises a number of questions, with some media reports claiming the Purple Rain star was addicted to prescription painkillers, though this has yet to be confirmed.