(Top photo: Screenshot from 'Trainspotting')

The "WORST TOILET IN SCOTLAND", into which Trainspotting protagonist Renton dives deep to retrieve the opium suppositories he's just shit out, involved a bit of artistic license. But the bookies through which Ewan McGregor walks to get there was very real, located as it was in a shopping mall on Edinburgh's Muirhouse housing estate.

Muirhouse, one of the city's most deprived estates, and Leith, near the city's docks, provided a backdrop to the cult film's iconic scenes, soliloquies and sounds. The former was where the author of the original book, Irvine Welsh, grew up, and both areas were home to high levels of heroin addiction and HIV in the late 1980s, when the film is set.

Two decades after the first film was released, in 1996, a Trainspotting sequel is about to hit our screens. The reunited actors have been filming back in Muirhouse, where the estate's ugly post-war high rises have been replaced by less daunting low rises, and the "eyesore" shopping mall is about to get a multi-million pound revamp. Although, in a community where many are dependent on food banks, residents doubt the window dressing will change much.

In the film, Renton, in his late twenties, ends up "choosing life" over heroin, going cold turkey after the death of his friend Tommy from HIV, escaping to London and legging it from his downbeat mates with a load of cash.

But what of the real "junkies" on the Muirhouse estate, in Leith, Edinburgh, Glasgow, Liverpool, Manchester and the rest of the UK, who became known as the "Trainspotting generation" of heroin users? Those who peopled a nationwide epidemic fuelled by two aligning factors: sky high unemployment and a flood of high quality brown smokable heroin from Iran and Pakistan. What happened to them?

Many have been making the news – usually in the "news in brief" sections of local papers – over the last few years as drug-related deaths in the UK have soared to record numbers. They are part of an ageing cohort of old school heroin users who started injecting in the 1980s and 90s and are now keeling over and dying, from overdoses and ill health.

(Photo: by Kitron Neuschatz)

That's the wider story. When it comes to the real crew Trainspotting was based on, we know in great detail what happened because the doctor running the clinic they have been attending for decades is still there. And he went looking for those who moved away.

Roy Robertson, a Professor of Addiction Medicine at Edinburgh University and a GP in Muirhouse since 1980, helped conduct a study that chased up around 800 heroin users who had received treatment at his Muirhouse clinic since 1980, to find out what had become of them. Many of them – people who were probably injecting themselves at the very moment cinema goers were seeing Renton, Sick Boy, Tommy do exactly that on the big screen – were now in their fifties.

The study found that one in four had died, almost entirely for reasons linked to long-term drug use. One in three were still injecting heroin and two-thirds were on methadone scripts. Less than one in five had got off drugs. Of the survivors, most were living unremarkable lives in Edinburgh and further afield, either on benefits or in low paid jobs.

"Unfortunately, for the real life Rentons and Sick Boys alive today, most are still using opiate drugs, living in social housing, on benefits, probably with hepatitis C and possibly with liver or lung disease," Robertson told me.

One heroin user who grew up in Muirhouse and Leith was tracked down to a council flat in Newcastle. He told Robertson he "hated" Trainspotting because "it was me; I was that character". Now in his late fifties, he told Robertson: "It was the best two years of my life, it was fantastic fun. I spent the whole time in Leith completely spaced out. It was crazy – there was lots and lots of heroin. No one cared – HIV had not been invented!" Soon after, "it all came clattering down on him" when he found out he had HIV and diabetes.

"He's older than he should be. He's on antivirals, but he's still alive. He's married with kids and he's working for a courier agency," said Roberston. "A lot of the Trainspotting generation look back at this era like others might look back at their first year of university. It was exciting, lively, it was the good times. They look back at the 1980s as being, 'Wow, that was good – anything was possible.' Now it's all ground to a halt, it's terrible for them."

Back in the early 1980s, the first kids in Muirhouse to start turning up at Robertson's clinic with heroin problems had no idea what drug they were taking: "I had teenagers who'd all left school together coming through the door jaundiced and with needle wounds asking for help. When I asked them what drug they were taking they said they were on 'smack'. I asked them if they meant heroin and they said, 'No! smack.'

"Looking back it was a very scary time. The cavalry hadn't arrived in terms of treatment services, and no one really liked drug users – no one was willing to take them seriously. We had kids dying from AIDS; there was a plague-like feel to it," said Robertson. "Even looking through the windows of my clinic now I can see a house where three members of a family died, one after the other, in the back room, from AIDS."

Robertson met Welsh – who became addicted to heroin at the same time as the fictional Trainspotting gang, in his late twenties – during an interview the pair did for Radio Scotland for the book's 1993 launch. Despite being accused of glamourising drugs – most famously by Republican presidential nominee Bob Dole, who said the film promoted "the romance of heroin" – Robertson says the novel painted an accurate picture of the heroin scene and did "a huge service in getting the story out there when politicians were not bothered".

For Robertson – who, by the time the film came out, had been treating heroin users on a daily basis for 16 years at his Muirhouse clinic – the movie adaptation marked a major shift in the public's perception of the country's growing drug problem. "People realised drugs was not just about old hippies and actors and celebrities – this was happening to lots of young street kids. And we owe Irvine Welsh a great debt for that."

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WATCH: 'Swansea Love Story', our documentary about young heroin users in Wales.

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Despite being about a bunch of heroin users, Trainspotting struck a chord across Britain. Its 1996 release came at a pivotal time in British drug culture. Ecstasy and cocaine use were on the rise, alongside a cranked up rave scene. As Renton's teenage girlfriend Diane tells him near the end of the film: "The world is changing, music is changing, even drugs are changing. You can't stay in here all day dreaming about heroin and Ziggy Pop."

That said, in the mid-90s heroin was experiencing a second wind, spreading out of the major cities into pastures new, and later had a brief revival on the British music scene, spearheaded by Pete Doherty. Yet, by 2000, the heroin epidemic had reached its zenith. After a peak of around 400,000 people in treatment by the end of the 1990s, numbers of new heroin users began to fall. Now, there are around 300,000 people in treatment, most of whom are middle aged.

So why did young people turn their back on heroin? Harry Shapiro, director of the charity DrugWise, says the number of users fell because of a huge investment in drug treatment, an economic upturn and the fact that, like Ebola in west Africa and crack cocaine in the US, all epidemics have their natural lifespans.

As drug researcher Dr Russell Newcombe further explains, heroin epidemics are limited by what he calls a "susceptible population" – those who, when exposed to the drug, will start using and probably become regular users. Once this population is saturated, he says, the number of people susceptible to heroin use reduces, and therefore new users drops off.

Then there was what Michael Linnell, of UK DrugWatch, describes as the "scarecrow effect". He says the main reason heroin use declined was "the perception of heroin users as 'dirty' and it being a 'dirty' drug". As heroin users become older and more visibly sick and desperate, they stood as a warning to young people of the perils of the drug.

So if the Trainspotting gang were to land in today's Edinburgh drug scene, would they still be using heroin?

"Instead of 'choose heroin' they might have chosen Spice?" says Gary Sutton, head of drug services at Release. "It's a very different social context now compared to the 80s and 90s. The Worst Toilet in Scotland will have changed to be well-appointed, tiled and have zero drug tolerance notices framed on the wall. It will be full of students complaining they can't get a wifi connection, while the drugs might be ketamine, Spice and nootropics."

Dr Newcombe says that based on current drug use trends in deprived parts of Edinburgh, they would still be likely to use heroin, albeit alongside a cocktail of other drugs such as methadone; novel psychoactive benzodiazepines, such as etizolam; synthetic opioids such as tramadol; and various prescription sedatives, such as diazepam, zopiclone and gabapentin – all washed down with cheap booze and a bag of Spice.

A still from 'Trainspotting 2'

On the eve of Trainspotting 2, should we be on the lookout for signs that heroin will make a comeback in Britain, as it has done in North America?

The US and Canada model is unique. The route of its return to popularity there was via a prescription opioid epidemic, which in turn was fuelled by an out of control health system in which doctors were paid by Big Pharma to prescribe opioids while patients were hoodwinked by blanket advertising into thinking these pills would end their pain. This scenario is unlikely to happen under the NHS.

However, Linnell says there are whispers of a new generation of young heroin users popping up: users who have gone through the mephedrone and synthetic cannabinoid route to end up using heroin. Dr Newcombe has heard from one Liverpool heroin selling outfit that they still have many young customers, but that many are smoking heroin occasionally rather than injecting on a daily basis, which means they can exist off the radar.

Sutton doubts young people have the same nihilistic streak as they did in the post-punk 80s and 90s, but adds that heroin will always loom in the background.

"The film was set in a time where heroin offered a way out of the grim realities of post-Thatcher Britain," says Sutton. "Heroin is quite glamorous – it's extreme and it says what punk says: I don't care. At the time we were aware that the old work opportunities did not exist. People questioned what was the point of slaving to be like your parents, whose values you despised. The 'choose heroin' monologue at the start of Trainspotting was absolutely perfect.

"But it's hard to run a smack habit these days: there's no squats, benefits are shit, they make you look for shit jobs that only exist because they are so shit no one wants them. Yet, will the most seductive drug in the history of humankind continue to seduce vulnerable and unhappy people in the future? Of course it will."

And the evidence that heroin is still locked into deprived communities is there to see in Muirhouse. Robertson tells me that drug deaths are rising fast in Edinburgh, but not all are of the Trainspotting generation. In the last two months he's had three people from his clinic who have died from heroin. "One was a 28-year-old woman who died of a heroin overdose at her home. The other was 37. I was speaking to her two weeks ago about her health and was not expecting her to die – she was found with two empty 400ml bottles of methadone by her."

Robertson says massive cuts to drug services in Scotland is indicative of a government that appears to be walking away from the entrenched problem of heroin addiction in areas such as Muirhouse.

"I can't imagine Trainspotting 2 is going to be like that," he added. "But it's very depressing now. It feels like the 1980s again."