Spice, the so-called "zombie drug" linked to a growing public health issue in both the US and UK, is unsurprisingly available to purchase on a number of dark web marketplaces. However, in an unusual turn, even vendors there seem reluctant to sell it to potential customers.

The dangerous drug, a synthetic cannabinoid, recently hit the headlines after an increasing number of users on the streets of Manchester, UK, have been likened to a scene from The Walking Dead after being pictured wandering blankly through the city like zombies.

After locating the illegal substance on two separate underground marketplaces, which will remain anonymous, it was found that even prolific vendors – one a former "top 1%" seller on the notorious Silk Road – were warning potential buyers about its strength.

"[Spice] is extremely strong and unless you are an experienced user of this type of product I would strongly urge you not to order this, it will be too strong for you to enjoy and [is] certainly not a drug you would want to take up as a new user," one description read.

When contacted by IBTimes UK, the person selling the substance said: "It is very harmful physically and mentally, and is generally only used by people that are so unhappy with their life they want to be completely out of reality, but can't afford or have access to things like heroin."

The vendor added: "You should stay away, and it's not even a good high."

According to some experts, legislative changes in the UK against this former legal high have only pushed its trade underground. One dark web vendor claimed the drug was available to post every day, from Monday to Friday. Parcels would be sent via Royal Mail first class.

As previously reported, experts believe Spice became popular with drug addicts because it is viewed as a cheaper – and easier to take – alternative to opiates. On the dark web, prices range depending on the amount sought, however, the cost goes up to more than £2,520 for 1KG.

On a second dark web marketplace, a separate vendor said you "have to be careful" but still tried to push a sale, saying "my stuff is nothing like that" when asked about its effects. "The stuff you [are] hearing about is people who have no idea how to make it," the person claimed.

Julie Boyle, a support worker at homeless charity Lifeshare, previously told The Times that Spice may have more of an impact on users than heroin. "This thing at the moment that's freezing people like statues [...] it's a new strain that's been around for about 10 days," she said.

Around the same time, Wales Online reported that a man who was "snarling and growling" assaulted a number of police officers after taking the drug. Once he was brought into custody in Cardiff he started to bang his head on the walls of his cell, the newspaper said.

Danny Kushlick, a spokesperson at the Transform Drug Policy Foundation, a think tank campaigning for the legal regulation of drugs, told IBTimes UK: "If you do think you're developing a problem with using it, seek advice from your local drugs project."

Kushlick added: "The synthetic cannabinoid spice is now inhabiting a market space inadvertently created by banning the sale of cannabis. Prior to last year's Psychoactive Substances Act, Spice was sold legally, courtesy of a legal loophole, but officially not for human consumption.

"It is a much more powerful drug than cannabis that is being used especially problematically by a vulnerable group of socially excluded young men."

In total, 58 fatalities linked to legal highs were recorded in British prisons from 2013 to 2016, the Prisons Ombudsman has said.