Image copyright North Yorkshire Police Image caption Ross Brennan and Aarron Gledhill were jailed by a judge at York Crown Court

Two university friends who bought and sold the deadly drug fentanyl via the dark web have been jailed.

Ross Brennan, 28, and Aarron Gledhill, 30, made hundreds of thousands of pounds mixing and selling the high-strength painkiller and other drugs.

At least 70 people have died in the UK in the last nine months after taking fentanyl, often when mixed with heroin.

Brennan and Gledhill, both from Huddersfield, pleaded guilty to drug and money laundering offences.

Brennan was jailed for 13 years and eight months while Gledhill was sentenced to three years and nine months by Judge Andrew Stubbs, at York Crown Court.

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Image copyright North Yorkshire Police Image caption The two men ran a "sophisticated online drug dealing business" from Brennan's flat in York and a property in Huddersfield

Image copyright North Yorkshire Police Image caption North Yorkshire Police uncovered the drugs operation after making a routine welfare check

North Yorkshire Police uncovered the operation when a neighbour raised concerns for the welfare of Brennan, who has autism.

Det Supt Steve Thomas said: "What we found was a production laboratory for synthetic drugs and more traditional drugs.

"[We also found] computer equipment that over the weeks and months revealed a sophisticated drug dealing business operating on the dark web under the guise of a business called 'Savage Henry'."

Image copyright North Yorkshire Police Image caption Brennan and Gledhill sold drugs via the dark web site AlphaBay, which has been closed down

Brennan and Gledhill, who met at university in Huddersfield, made up to £1,000 a day operating the illegal online supermarket for two years on a site called AlphaBay, which has since been closed down.

The duo bought, mixed and sold the drugs, including fentanyl crystal meth and cocaine, using a copy of Chemistry for Dummies before posting them on to people across the UK.

They sourced the drugs by post from Vietnam, the USA, Canada, the Netherlands, Belgium, Ukraine, Poland and Mexico.

The force said it could only estimate the number of transactions that took place as they used data-shredding software, but the site had received more than 4,000 reviews.

Their operation was said to be worth between £275,000 and £1.5m, depending on the fluctuating value of the virtual currency Bitcoin they traded in.

Image caption Fentanyl is considered to be 50 times more potent than heroin

What is fentanyl?

Fentanyl - which hit the headlines after it was linked to the death of US singer Prince - is an extremely strong painkiller, prescribed for severe chronic pain, or breakthrough pain which does not respond to regular painkillers.

It is an opioid painkiller, which means it works by mimicking the body's natural painkillers, endorphins, that block pain messages to the brain.

According to America's Drug Enforcement Agency, it is considered to be 50 times more potent than heroin.

The risk of harm is higher if the wrong dose or strength is used.

Typical symptoms of a fentanyl overdose include slow and difficult breathing, nausea and vomiting, dizziness and increased blood pressure.

Det Supt Thomas said: "These are two individuals who clearly didn't give any consideration to the welfare of their customers. They were only interested in the profit.

"These are people who had downloaded a 'Chemistry for Dummies' guide and then sent out potentially dangerous substances, that on any occasion could have resulted in a serious impact on someone's health or even death."

Image caption Det Supt Steve Thomas said Brennan and Gledhill showed no concern for the people they supplied

He said Brennan would exaggerate his autism and present himself as someone who "struggled to cope with normal life and society" but was really a "sophisticated criminal who was trying to pull the wool over a number of agencies' eyes".

Officers had discovered encrypted messages from Brennan in which he "pretended to be more ill than he was".

"They also knew that what they were selling was potentially dangerous, that's the thing that really put a chill through the investigation team," Dept Supt Thomas said.

During their investigation police discovered a Skype message from Brennan to Gledhill in which he said he knew there were "bodies out there on me".

Four people who bought drugs from Brennan and Gledhill have died, police said, but they added they could not prove if their deaths were directly linked to the Savage Henry website.

Brennan, of Great Northern Street, and Gledhill, of Almondbury Bank, pleaded guilty to conspiracy to import class A drugs, conspiracy to sell class A drugs and concealing criminal property.

Brennan also admitted three charges of making indecent images and one of distributing indecent images. He has been put on the sex offenders register for 10 years.