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 summer, allowing Victorian festival-goers to test drugs for potentially deadly chemicals.
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