It's been branded "the biggest health problem" currently facing prisons in Britain, and guards say they are still struggling to stop it.
The flooding of jails with the psychoactive drug "spice" – also nicknamed the "suicide drug" – has been blamed for contributing to record levels of violence, self-harm and suicide behind bars.
And now rare footage, featured in an upcoming documentary, shows how a liquid version of this synthetic cannabis has made it even easier to smuggle.
Channel 4's The Secret Life of Prisons takes a grim look at the brutality of British jails and the black market that has fuelled disorder and violence.
Featuring a woman named "Paula", she boasts of how lax prison security has allowed her to build a thriving business smuggling contraband like spice into prisons inside chocolate bars, trainers and even soaked into children's drawings.
Taking orders from inmates via text message, she says she makes up to 10 visits to prisons a week.
She is filmed preparing "spray-on spice" – a new liquid form of a lethal psychoactive drug that is said to be nearly impossible to detect by prison guards.
"This is the liquid spice and the normal kid's drawings that would be sent in everyday into any prison," she says while spraying the drug onto a A4 crayon drawing. "When it's dry it doesn't smell. If it's X-rayed nothing comes up this sheet worth about £50.
"It's new to the market. Just fold it up, put it in your envelope and send it with your normal mail. When they receive it the other end all they'll do is break off pieces, roll it up and put it in with their tobacco [and] smoke it."
She adds: "This is the main thing at the minute, everyone just wants this and it's worth more than gold. The people obviously like it in the jails because it doesn't show up on a drug test."
First sold as a legal high, spice is now banned by the government but can still be bought online.
The drug, along with other new psychoactive substances, has been linked to almost 60 deaths of inmates over the past three years and is blamed for causing numerous cases of psychosis. Even prison guards have complained of falling ill after accidentally inhaling the fumes.
One disturbing video clip in the documentary, filmed by a prisoner, shows the devastating impact the drug has on Simeon, a 24-year-old inmate.
He is seen being egged on by friends as he inhales spice inside a prison wing. But things soon turn ugly, with the clip showing him collapse in agony after suffering a heart attack.
"My heart stopped," he tells the programme. "That's when they pronounced me dead. I still went back after I've died. I went back and had a drug the next day."
Spice has also been blamed for causing terrifying mental health problems and of fuelling a rise in self-harm.
Simeon spoke of seeing a friend "chop" his penis off with a razor blade after going psychotic on the drug while in prison.
Another recent incident, in HMP Peterborough, saw a prisoner batter his cellmate to death with a television after accidentally smoking a cigarette spiked with the drug.
A recent report, published by ex-offenders' organisation User Voice in June, found a third of prisoners surveyed in nine jails had used spice in the previous month. It concluded the growing popularity of the drug had contributed to an increase in violence, bullying, mental health problems, and even death.
Despite successive governments vowing to clamp down on illegal substances behind bars, inmates continue to say they "flood" jails – with one former prison governor describing spice as "the biggest health problem facing prisons".
And one recent inspection of one prison, HMP Bedford, where 230 prisoners recently rioted, said it was easier to get drugs like spice than basic items like bed sheets and clothes.
Steve Gillan, general secretary of the Prison Officers' Association (POA), said the government "still doesn't have a hold on the problem", blaming cuts to prison officer numbers over the past five years.
"The bottom line is there aren't enough staff," he told IBTimes UK. "Our prisons are awash with spice and mobile phones – there's always been drugs in prisons but not to the extent we're seeing now.
"There's lots that can be done about it ... but we really need feet on the ground. We use to have a correspondence department to search prisoners' mail, for example, but now it's just dished out.
"[The prison service] has now trained up sniffer dogs, but only after it depleted dog units. Some prisons still don't have sniffer dogs. And every batch of spice is different. Your detection is only as good as how up to date you are."
Spice, like other contraband, has also contributed to the problem of vulnerable addicts landing themselves in debt to violent prison gangs who go on to beat up and humiliate prisoners in exchange for the drug.
Clips see inmates beaten bloody by "bored" prisoners.
"Basically, some kiddies, a group of kiddies come up to you and say, mate are you willing to take a punch for a gram of spice?" Simeon says. "The kid will have a black eye a bad broken nose and everything."
Peter Clarke, the chief inspector of prisons, warned earlier this year of the "devastating effect" that synthetic cannabis substances were having on some prisons.
"Prison staff have told me that the effect on individuals and prisons as a whole is unlike anything they have seen before," he told the Guardian in May. "Their presence in prisons has given rise to debt, bullying and violence. They are destabilising some prisons, making it difficult for normal prison life to continue.
"Both at local and national level there needs to be clear strategies to deal with the supply of these drugs into prisons, and to care for those who suffer from their effects. At the moment the situation appears to be getting worse, not better."
The Ministry of Justice announced last week a £1.3bn investment plan in new prisons over the next five years, and plans for 2,500 extra officers, drug tests for prisoners and more autonomy for governors.
Justice Secretary Liz Truss, who presented a white paper announcing the reforms last week, also said "cutting edge" technology will be introduced to block the use of illegal mobile phones, while 300 sniffer dogs have been trained to detect new psychoactive drugs.
She said: "These extra officers and new safety measures will help us crack down on the toxic cocktail of drugs, drones and mobile phones that are flooding our prisons, imperilling the safety of staff and offenders and thwarting reform."
What is 'Spice'?
- "Spice", or "Black Mamba", is the name given to a synthetic form of cannabis – an umbrella term for chemicals that mimic the effects of THC (the active ingredient in cannabis).
- It is commonly looks like a herb but is also sold in a liquid form.
- While claiming to mimic the effects of cannabis, the actual impact on users can be unpredictable and, in some cases, severe or cause death.
- The drug, along with a range of other new psychoactive substances, or so-called "legal highs", was made illegal in the UK in May 2016.