A white powder sold as cocaine has been laced with a deadly opioid 100 times more potent than heroin.
The Department of Health issued an urgent warning on Tuesday to Victorians over the white powder being sold in Melbourne.
The warning follows multiple incidents across the city associated with people who ingested the powder believing it to be cocaine.
"The product appears to produce strong adverse effects such as loss of consciousness, respiratory depression, and life-threatening hypoxia (insufficient oxygen for normal functioning)," the alert warns.
"Know the signs of opioid overdose and carry naloxone."
Naloxone is a drug that can temporarily reverse opioid overdose.
Cocaine and protonitazene are very different substances, according to the warning.
"Cocaine produces stimulant effects, whereas protonitazene produces sedative effects and may lead to opioid overdose," it says.
Mixing alcohol or other prescription drugs with protonitazene such as Xanax or Valium, can also increase the risk of overdose.
The warning comes a week after the Victorian government announced it would legalise pill testing from this summer, allowing festival-goers to test drugs for potentially deadly chemicals.
Acting Premier Ben Carroll was alarmed by the warning and said it was one of the reasons the government had green-lit the pill-testing trial.
"Unfortunately and sadly, drugs are laced with chemicals that young people don't know they're laced with," he told reporters.
"That is concerning and I'll follow up with Victoria Police."
Pill testing will operate at 10 festivals and other events over an 18-month period, with a Melbourne fixed site to open in mid-2025.
Asked if the warning would prompt the trial's roll out to be sped up, Mr Carroll said the government was working with a range of partners to develop it.
The services will become permanent at the end of the trial.