Video report by ITV News Correspondent Lucy Watson
Hull is in the grips of a illicit drug epidemic that has already claimed 15 lives this year.
Fentanyl is 100 times stronger than heroin by volume, and as heroin becomes weaker on the street, more and more users are turning to fentanyl to get their high.
And it is Hull's most vulnerable who are at the most at risk.
"There's people out there chasing fentanyl now, because when they first started taking heroin 20-25 years ago, that's when it was strong," Mark, a heroin user of more than a decade, told ITV News.
"Now the heroin in Hull, I don't know about the rest of the country, but it's crap.
"They're not getting any buzz from it."
And it is the ease of acquiring fentanyl that worries those trying to help people under the spell of the deadly drug.
"It is fairly easy to get a hold of - we live in the internet age," addiction counselor Andrew Dettman said.
"And the drug dealers, they get a hold of fentanyl and they grind it down and adulterate other drugs with it, and people are dying."
Danny - a heroin addict who first took fentanyl in March - recently lost his cousin Paddy to a fentanyl overdose, but it has not stopped him from seeking it out.
"I wish it had scared me that much for me to just leave everything alone but I guess, well, it hasn't," he said.
His description of fentanyl's effects give an insight into how strong it actually it.
"I don't remember sliding down, I don't remember hitting the floor," he said.
"I just remember waking up four or five hours later."
Tony Margetts, a substance misuse manager, was one of the first to notice the unusual number of deaths in Hull.
"Some of these people died so quickly that they hadn't got time to pull the syringes out of their arms.
"It is so potent that it is impossible to use safely on human beings, and you can get enough under your fingernail to kill you.
There are organisations like homeless charity Emmaus trying to help, but they are facing an uphill battle, especially as addicts never know exactly what they are taking.