Miguel was in the middle of a three-day sex and drug binge when he looked at Grindr and saw an advert asking for people to participate in a film about sex and drug binges. “I hooked up with a guy,” he says, “and then we contacted them.” Soon a cameraman was filming him injecting drugs.

And with that Miguel became one of the central storylines of the new documentary feature Chemsex. This is the first film to explore a gay subculture that’s recently found notoriety in the mainstream media after the NHS and the British Medical Journal identified it as a health priority. And this comes almost a decade after chemsex caused some arguments in the gay community itself, inspiring sadness, attrition, blame and confusion.

Chemsex is identified in the film as the habit of engaging in weekend-long parties fuelled by sexually disinhibiting drugs, such as crystal meth, GHB, GBL and mephedrone. These parties involve multiple people and are mostly facilitated online. The testimonies in the film from people involved in the subculture directly link chemsex to alarming rates of HIV infection. In London four new positive diagnoses are currently made daily. There is candid talk on film about “pozzing up”, the practise of knowingly becoming infected with the virus. Meth, meph and G create a potent cocktail enabling extremes of behaviour, which carries significant risks for the sexual and mental health of habitual users.

For anyone unsure about the impact of chemsex on real lives, the tale of Miguel should offer some clarity. “I rapidly agreed to have my face unblurred,” he says. “They filmed me on various comedowns, meltdowns and on one very losing-the-plot crystal meth binge. I think this documentary is a huge step in reaching out to the general public and showing that we’re not hopeless junkies who will die in their own various bodily fluids.”

Chemsex - video review Guardian

As a gay man, it is impossible to watch the film and be unmoved. The cast look and sound like your friends. The documentary uncovers a world we are not used to seeing onscreen but which is hiding in plain sight. Take the drug GBL. It entered public consciousness this year after the toxicology reports on the bodies of four dead young gay men in and around Barking cemetery revealed overdoses of the liquid narcotic. (For the uninitiated, lethal amounts of the drug are difficult to quantify and chemsex is still unknown enough for the Barking constabulary to fail to connect the deaths until the fourth body had been discovered.) Then there is steroid use, for many men the first hurdle jumped over in society’s needle taboo, which is barely acknowledged in the media. And it is a long six years since valid questions raised by boyband star Stephen Gately’s death were effectively suppressed after Daily Mail columnist Jan Moir aired some questions with clumsy disapproval.

Chemsex is a film made for Vice by two straight filmmakers, 34-year-old William Fairman and Max Gogarty, 27. There’s no doubt it digs deep into the subculture. There are ghostly stories of everyday annihilation – social-panic inducing yet also filled with gentle empathy – that are likely to stay with viewers. It speaks of the generation for whom life itself is a movie of their own editing, captured on endless smartphones, torsos tightened in the bathroom mirror. Yet up to now there hasn’t been a reliable film rummaging beneath the undergrowth.

Hidden world: William Fairman and Max Gogarty, the Chemsex diectors, photographed at the Soho Theatre in central London. Photograph: Richard Saker/The Observer

Prior to Chemsex, film and TV auteur Andrew Haigh looked closest to capturing this gay demographic, directing a debut documentary in 2009 about a male prostitute fuelled by ketamine, Greek Pete. Yet the further he has gone into fiction the more Haigh has moved from his starting point. Indeed, anyone who wants to see the exact flipside of Chemsex’s harsh portrait should watch the scene in Haigh’s glossy HBO series Looking, in which the beautiful, affluent gay hero takes a first ecstasy tablet at a moonlit party in a wooded glade before being theatrically rogered against a tree trunk by Russell Tovey.

Instead Chemsex has something of the hard found-footage and testimonial flavour of this year’s other cinematic addiction piece, the harrowing Amy Winehouse documentary Amy. “This is ugly,” says David Stuart, sexual health worker at the pioneering London clinic 56 Dean Street, who provides Chemsex’s voice of reason. “Spend a week in clinic with me and you go home with the dark feeling you get when you’ve watched the film.”

Stuart says that when the filmmakers first visited the clinic to shadow him, “The story, which they thought was about gay men taking drugs and having fun but leading to harm, was flipped upside down. They were silent and they were ashen.”

“It is a horror story,” says Tom Abell from the film’s distributors, Peccadillo Pictures, a company that operates at the vanguard of sophisticated gay storytelling.

‘Intimacy is a skill we learn as children. A lot of gay men didn’t experience that’: David Stuart of 56 Dean Street, London’s sexual health clinic. Photograph: VICE Media

“It wasn’t conceived as one,” says Fairman. “It was conceived as a story about people’s relationships with addiction.”

The film climaxes with real-time footage of Miguel during a heartbreaking, feral episode of crystal meth psychosis, which is deeply troubling to watch. A year into a successful recovery programme in northern France, Miguel has not yet seen it. “Watching this film at this point in my life will make me sink back into the worst period of my life, a series of rock bottoms. The documentary will always be there tomorrow.”

Before Chemsex was screened at London Film Festival in October, there were murmurings of disapproval online.

The main accusation against the filmmakers was sensationalism. Stuart sets out his stall early and calmly in the film – a compelling argument about a crisis that healthcare professionals are not informed enough about to treat effectively. Beneath the Dionysian excess of a clandestine party culture, a real mental health problem is being unearthed.

He talks of the problems men come to 56 Dean Street with, relating back to childhood when they might have been – or felt like – the only gay person in their family, school or church. “Intimacy is a skill we learn as children in the ideal family unit. A lot of gay men we are seeing in clinic didn’t experience that. They were performing all the time, being over-straight, overcautious, keeping the secret secret. That’s the opposite of intimacy.

Dangerous liaisons: users ‘slamming’ at a Chemsex party. Photograph: VICE Media

“Then suddenly they’re all grown up, in a hypersexualised gay world with an app on their phone that helps facilitate very fast sex in a population of people who are more prone to HIV and hepatitis C, and they’re trying to incorporate intimacy into their lives with no frame of reference.”

When gay men reach majority, he says, “They don’t want a drug like heroin that isolates them and helps them dissociate more. Gay men need a drug for a different purpose. They want a different drug: ecstasy, cocaine, all these kind of drugs that were popular with us throughout the 90s. They enable you to empathise. You take ecstasy on a dancefloor and you’re not alone any more. That’s a beautiful, beautiful thing.”

After the high, the comedown. “A side effect was a population of gay men who normalised drug use accepted a drug that helps them enormously. Ten years ago something changed, almost overnight. Three new drugs – boom! – are in the laps of vulnerable gay men needing something.”

Another complaint against the film came from gay men who said: “We don’t take drugs – this is not representing us.” And yet while it’s true that this is a subculture, it is growing. 56 Dean Street has been approached by 33 healthcare organisations across many European cities asking for advice on how to cope with an endemic, and the clinic is seeing up to 100 new patients a month with chemsex addiction issues.

Stuart has also heard gay men questioning what chemsex even is, wondering why its parameters shouldn’t include all stimulants, including alcohol. “There are people who say that human beings – all sexualities, all genders – have been using alcohol in sexual contact forever,” says Stuart. “It impacts the choices they make. It’s involved with unwanted pregnancies, with STIs. Call that chemsex, if you like. Fine.” He doesn’t. “If you use drink or marijuana to enable sex there is an amazing support system to help you. I don’t need to reinvent that wheel.

Love drugs: GHB, which fuels many of the parties. Photograph: Alamy

“When marijuana makes you stay awake for three days and makes you do things sexually that you might not have done otherwise, when it makes you not care about tomorrow or your sexual health at all because it’s so disinhibiting, and when you start using marijuana to have sex with populations who are at very high risk of HIV and hepatitis C, then I will open a clinic for you.”

There is also another concern. That a parent of a young gay man making his way to the big city to find his place in the world may see the film and worry that this is all that lies ahead. That we have returned to the bad old days where parents will be once again predicting for their children a life of loneliness or doom.

Yet the film feels like a powerful testimony to help vulnerable men who see themselves as no more than vessels, of no value, existing for the outer edges of human consumption. The film arrives amid a £200m health cut. Of the many people I hope get to see Chemsex, perhaps Jeremy Hunt will be among the first. “There are a lot of gay men who need help,” says David Stuart.

Tom Abell agrees: “There is always that danger of opening this world up to vulnerable people. But I think it’s a greater danger that they don’t know about it.”

Chemsex is released on 4 December