On Sunday, NSW Health issued a warning: several drug users in Sydney were being poisoned by acetyl-fentanyl - a highly dangerous substance which can be lethal in even small doses - after believing they were taking cocaine.

Dr Mary Ellen Harrod, the Chief Executive of NSW Users and AIDS Association, says it's a major concern.

"Fentanyl has largely not been in the drug supply here... it poses significantly higher risks if people don't know they're having it.

"It's hard to know whether this is the start of what happened in the United States."

NSW Health's warning reminded users that acetyl-fentanyl, in powdered form, is "indistinguishable" from cocaine.

The situation is so serious that harm minimisation experts are urging Sydneysiders to be extra cautious - and carry an opioid antidote called Naloxone.

Skip Instagram Post FireFox NVDA users - To access the following content, press 'M' to enter the iFrame.

What is naloxone?

When someone has an opioid overdose, their breathing and heart rate often slows, which can lead to death. It happens to 3 people every day in Australia.

Possible signs of an opioid overdose: quick onset of very slow, difficult or shallow breathing, there may also be snoring

cold and clammy skin and low body temperature

confusion

fainting

slow heartbeat

muscle twitching

gurgling sound in the throat from vomit or saliva

bluish skin and nails because of low oxygen

coma and death Source: YourRoom

And while you may think of heroin as the main cause for opioid overdoses in Australia, it isn't at all: pharmaceutical opioids (yes, the ones prescribed to patients by their doctors) make up 70 per cent of opioid-induced deaths in Australia.

Naloxone is a drug that essentially reverses the effects of an overdose - it's a highly effective antidote that allows users to breathe normally again.

And the good news is that it's currently free for Australians under a pilot program running in three states - New South Wales, South Australia and Western Australia.

Anyone in those states can ask their local pharmacy for free naloxone - and if they don't have it, customers can ask for them to order in stock.

Dr Mary Ellen Harrod from the NUAA, which runs the harm reduction program DanceWize in NSW, says it's reasonable for any person, especially loved ones of drug users, to carry Naloxone.

It would be fantastic if everyone carried it. It's a decent, good samaritan thing to do.

The antidote comes in a few different forms including a nasal spray, which is so easy to use that harm reduction programs in the US are even teaching six-year-olds how to administer it.

All it takes is for someone to take the nasal spray and "open, insert, squirt". It works quickly, but only if someone is having an overdose - meaning it has no effect on other people.

Getting free Naloxone as part of the trial is important to show lawmakers that the drug is needed, Dr Harrod said.

"If people who are party-drug users go in and start asking for this product, it will become much more available for all opioid users.

"If nobody takes it up, then it's more likely the program won't continue."

Dr Harrod says ahead of a big weekend in Sydney with the Mardi Gras parade on Saturday, it's important to remember how to reduce harm when taking drugs.

"Don't use drugs alone, look after the people you're using with, if it is possible to test your drugs - that's also a good idea. Get help early [by calling an ambulance], and if you can, carry Naloxone."

Dr Harrod says drug testing kits that can pick up fentanyl are uncommon, but NUAA is working on developing a "test strip kit" for this purpose.

Information on which pharmacies have signed up to supply Naloxone for free in NSW are here. If you're in WA, more information on the program can be found here, and if you're in SA, head here.