The drug dark web dealers refuse to sell
The dark web has long been a hotspot for illicit substances, with anonymous drug dealers selling everything from marijuana to ice, hidden behind a computer screen.
But there's one substance even online drug users are getting too concerned to dabble with - fentanyl.
Major dark web drug suppliers have begun voluntarily banning the synthetic opioid substance, saying it is too dangerous to trade, The Guardian reports.
According to the National Crime Agency in the UK, dealers on the dark web are "delisting" the drug, classifying it among items that are too high-risk to trade.
Vince O'Brien, one of the agency's leads on drugs, said it appears to have been fuelled by fears that selling a drug linked to fatalities would more likely garner police attention.
"If they've got people selling very high-risk commodities then it's going to increase the risk to them," he told the newspaper. "There are marketplaces that will not accept listings for weapons and explosives - those are the ones that will not accept listings for fentanyl. Clearly, law enforcement would prioritise the supply of weapons, explosives and fentanyl over, for example, class C drugs - and that might well be why they do this.
"There are also drug users on the dark web who say on forums that they don't think it's right that people are selling fentanyl because it is dangerous and kills a lot of people."
WHY FENTANYL IS SO DANGEROUS
Fentanyl is a synthetic drug that was developed by a Belgian chemist in the 1950s.
The drug is commonly prescribed to people with cancer and chronic pain, and it's legally sold in Australia in slow-release patches that cost a few dollars each.
The drug began production as a cheap painkiller, because its synthetic nature means it's not reliant on opium poppy crops and can be mass produced.
But the main trouble with the drug is its extreme potency. It's said to be 30 to 50 times stronger than heroin, and up to 100 times more potent than morphine, and is commonly referred to as a "zombie drug".
"(Fentanyl) has commonly been used in anaesthesia and for more than 30 years. But the dose we administer is tiny, compared to when it is used as an illicit drug," Professor David A Scott, Australian and New Zealand College of Anaesthetists (ANZCA) president, told news.com.au last year.
"Fentanyl has commonly been used in anaesthesia and for more than 30 years. But the dose we administer is tiny, compared to when it is used as an illicit drug," he said.
"Anaesthetists like it because it comes on quickly and wears off after a relatively short period of time.
"The patches can be quite high levels of the drug, but the intention when they are used in palliative care is to dispense a low, slow dose so it provides a constant background level of fentanyl.
"Certainly, the opioids that are out there now are causing more harm than any good. In some cases they're causing death - and that's completely unacceptable.
"People take it for the same reason they take other opioids like heroin - it is a potent narcotic and so you have a euphoric high from using a large dose."
Prof Scott says those in the greatest danger are illicit drug takers taking fentanyl either unknowingly - when it is mixed with the less potent heroin - or in ignorance of how much more powerful it is.
"The high would not be dissimilar to heroin or morphine - they get a euphoric feeling," he said.
"You must remember some people aren't actively choosing to take it - they are buying heroin for the heroin euphoric feeling, and then are overdosing because someone at some point has cut in fentanyl as well.
"And if you get a big dose, especially one mixed illicitly where there's no quality control - it's even more deadly.
"It slows breathing. They fall unconscious. And then it stops the breathing.
"We should be very afraid of fentanyl in its illicit form. Used illegally, it's very potent," he said.
"The gap between getting the high they are after and having their breathing stop is just too, too narrow. The risk of death is just too high."
In Australia, the black market has been thriving over the last few years.
There were just under 2000 drug-related deaths in 2016, but experts fear the epidemic is spreading.
Last April, the Australian Border Force (ABF) deployed officers to cyberspace to combat increased trade in synthetic drugs like fentanyl on the dark web, where a single patch can be resold for up to $100.
The move followed a dire warning from their American counterparts that Australia could be heading for an overdose epidemic.
"Fentanyl has caused a large number of deaths across North America over a number of years and we fear it could result in a similar toll here as a result of its potency," former Australian Border Force boss Roman Quaedvlieg told News Corp at the time.
Fentanyl has sparked a crisis across the United States and Canada in recent years.
More than 72,000 Americans died from overdoses in 2017, up from 20,000 in 2001, according to the Centers for Disease Control and Prevention. The number of deaths related to synthetic opioids including fentanyl doubled in 2016 and is now 30,000.
As in Australia, heroin in the US is now often mixed with fentanyl - often imported from China. It has sent the number of deaths skyrocketing.
To better understand how great a problem fentanyl has become, just look at the recent deal struck by US President Donald Trump and his Chinese counterpart at the G20 summit.
Earlier this year, Mr Trump accused Chinese suppliers of fuelling America's opioid crisis:
It is outrageous that Poisonous Synthetic Heroin Fentanyl comes pouring into the U.S. Postal System from China. We can, and must, END THIS NOW! The Senate should pass the STOP ACT – and firmly STOP this poison from killing our children and destroying our country. No more delay!— Donald J. Trump (@realDonaldTrump) August 20, 2018
Over the weekend, China's leader Xi Jinping agreed to reclassify fentanyl as a "controlled substance" to curb the growing problem in the US.
The White House described it as a "wonderful humanitarian gesture".
The new designation means people in China who sell the drug to the US "will be subject to China's maximum penalty under the law," according to a statement from the White House.