Canberra's free pill-testing service has found a dangerous substance linked to fatal overdoses in "fake" prescription drugs.
The ACT government issued a public health alert on Saturday after metonitazene was detected in tablets that were sold and wrongly described as oxycodone.
Oxycodone, often called oxy tablets, is a powerful sedative that has become a popular illicit drug.
Metonitazene is a nitazene, which is a class of highly potent, synthetic opioids.
Pill-testing service CanTEST said the substance was detected in yellow, circular tablets that had no markings or stamps.
It is the first time a nitazene has been found in counterfeit tablets in Australia, though New South Wales authorities issued an alert last week after detecting it in heroin.
Stephanie Stephens, the acting chief executive of Directions Health Services which oversees CanTEST, said the discovery had potentially saved lives.
"Had we not been able to test these samples, the person would have consumed them and otherwise had a potentially fatal overdose," she said.
Drug users urged to test pills
Nitazenes are often stronger than fentanyl, another prescription drug that is largely responsible for an epidemic of fatal overdoses in North America.
Ms Stephens said exposure to nitazenes could slow a user's breathing, make them drowsy or unconscious, and turn their skin blue or grey.
"We are really worried about these pills, circulating here in Australia, and potentially other opioids being contaminated with a nitazene, which is far stronger than what anyone is expecting," she said.
The ACT government is warning Canberrans who bought what they believed was oxycodone that the pills could harm and even kill them.
It advises anyone with yellow "oxy" tablets to dispose of them safely.
However, its alert does not apply to oxycodone received via a prescription from a pharmacy or health service.
Ms Stephens said Canberrans should test any illicit substances they had at CanTEST's city premises.
"Bring your drugs in to be checked, do not use them, they may contain nitazenes," she said.
Opioid users, including heroin users, should also ensure they have the drug naloxone on hand, which can reverse or reduce the effects of an overdose, she said.
The ACT is the only jurisdiction in Australia that has a legal pill-testing service, which allows people to anonymously check what substances are in their drugs before they use them.
CanTEST's site, at 1 Moore Street, is open from 1pm to 10pm on Thursdays and 6pm to 9pm on Fridays.
