SIXTY people have died in the UK in the past eight months after taking strong painkiller fentanyl.
Doctors give the opioid to cancer patients, often in the form of a patch, lollipop or nasal spray.
But it is 50 times stronger than heroin and is sometimes mixed with the Class A drug.
And the National Crime Agency has warned heroin dealers are "playing Russian roulette" with users' lives by mixing it.
Some tested batches contained a variant named Carfentanil, which is 10,000 times stronger than morphine and used to tranquillise elephants.
The 60 UK victims were mostly men and a range of ages over 18. At least 70 other deaths are awaiting test results.
Home Secretary Amber Rudd has vowed to tackle the "terrible blight" of fentanyl and its "devastating" effects.
She said: "We know of the damage that it's caused in the US and Canada and the terrible loss of life we've seen around it.
"In the UK we've just launched a new drug strategy and we're going to look in that about what else we can do to stop the spread of fentanyl in the UK.
Singing superstar Prince died after an accidental overdose when he self-administered Fentanyl in April 2016.
Fentanyl is an opioid painkiller which means it works by mimicking the body's natural painkillers, called endorphins, which block pain messages to the brain.
Typical symptoms of a fentanyl overdose include slow and difficult breathing, nausea and vomiting, dizziness and increased blood pressure.