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"Yeah, affected me way more than that much heroin would normally do. So yeah, I believe it had fentanyl in it for sure."
That's Mark, a former heroin user.
He believes the substances he once took were contaminated with fentanyl.
Fentanyl is a powerful drug that's ravaged the United States and Canada and left tens of thousands dead every year.
But now, an influx is feared of a new drug class: nitazines.
Mark says that could be very damaging if it becomes widely available in Australia.
"Very very scary. Fentanyl is scary enough with how much stronger it is compared to Fentanyl. If that gets mixed in with the supply in this country the overdose deaths will skyrocket."
Harm Reduction Victoria Chief Executive Sione Crawford says nitazines are even more potent, and threaten the lives of unsuspecting users.
"Around 25 to 50 times stronger than Fentanyl and Fentanyl is about 25 to 50 per cent times stronger than heroin."
The lab-manufactured drugs were developed and tested in the 1950s by pharmaceutical companies.
But unlike fentanyl, they did not proceed to therapeutic use because of safety concerns.
Last month the coroner warned at least 16 overdose deaths since 2021 in Victoria alone involved nitazines, many due to the synthetic opioids being used to cut cheaply drugs like heroin and MDMA.
These developments - along with the number of overdose deaths and jail sentences for drug use in Australia - have prompted calls at the World Health Summit in Melbourne for global decriminalisation.
Former New Zealand Prime Minister Dr Helen Clark is the chair of the Global Commission on Drug Policy.
"We are talking about human beings. We are talking about those who may be our siblings, our cousins, our parents - people who have a need for harm reduction. And you don't reduce harm by throwing people into prison."
Dr Clark says there are encouraging signs globally, with even once hardline south-east Asian countries like Malaysia and Thailand unveiling sweeping drug law reforms.
Sione Crawford says drug reform is sorely needed to save lives.
"The other thing drug decriminalisation will allow us to do things like drug-checking and having safer drug supply programs. We can't do that without changing the way we deal with drugs."
