|An example of fatal dosages of heroin and fentanyl|
Best known as the drug that killed pop star Prince, fentanyl is 50 times more potent than heroin and up to 100 times more powerful than morphine.
While it has been used in hospitals for decades as pain relief medication, an illegal version of the drug has caused so many deaths in the US in recent years that it has been described as a "weapon of mass destruction".
But it is also becoming a widespread problem on the streets of Britain, claiming the lives of drug users unaware of its potency.
Fentanyl can be absorbed through skin contact
What is fentanyl?
The opioid was first made in 1960 by Belgian doctor Paul Janssen and introduced in hospitals as an intravenous anaesthetic.
In the mid-1990s it became available as a patch for patients being given palliative care, before a powdered version of the pain reliever began to be pushed as a low-cost recreational drug.
Users experience a state of euphoria and relaxation, factors which make it highly addictive.
|Fentanyl can be absorbed through skin contact|
Last November, 18-year-old Briton Robert Fraser died after unintentionally overdosing on the drug.
Robert's mother Michelle told Sky News: "It shouldn't be on the streets, this sort of stuff.
"These days there is too much and its too easily accessible for teenagers especially as we have mobile phones and the internet.
"It's kids giving it to kids a lot of the time - they don't know what they are giving."
Just a few grains of the white powder can kill a user.
|Just a few grains of the white powder can kill a user|
According to the US government's Drug Enforcement Agency (DEA), fentanyl is being mass-produced in illegal labs in China before being sold wholesale to Mexican cartels.
It is then pushed on to streets in America through existing opiate trafficking networks.
Supplies of the drug can be stretched much further than heroin - using cutting agents a kilo can be turned into 25kg.
It is for this reason that fentanyl is regularly used to bulk out supplies of heroin, a method which has proved fatal for heroin addicts unaware they were taking fatal dosages of another drug.
As one addict in the US told Sky News, fentanyl is "heaven for the dealers but hell for the users".
In April, the National Crime Agency said the drug was believed to have been behind a number of deaths in Yorkshire, Humber and Cleveland.
It came as West Midlands Police raided an illicit drugs laboratory suspected of producing the drug.
The NCA warned the laboratory's fentanyl could have been present in drugs across the country.
What happens if you accidentally ingest it?
The potency of fentanyl has caused serious problems for US police attempting to crack down on the supply of the drug.
Last year, the DEA released a video in which two New Jersey detectives described how they fell seriously ill after inhaling the substance while closing an evidence bag.
Eric Price said he lost colour and thought he was dying as his body started "shutting down".
His partner, Dan Kallen, said the drug made him severely disorientated and gave him "the most bizarre feeling that I never want to feel again".
The officers were not the last to suffer the consequences of coming into contact with the highly potent drug.
In May, an officer from East Liverpool in Ohio collapsed and was taken to hospital after handling the opioid.
Chris Green had worn gloves when searching the car of a suspected drug dealer, but later returned to his police station with white powder on his shirt.
He instinctively brushed the substance off with his bare hand and, within a few minutes, collapsed. Mr Green was revived with four doses of opioid antidote Narcan.
Just a fortnight later, another police officer in Pennsylvania required hospital treatment after accidentally inhaling the substance in identical circumstances.