Prurient peepshow value is thankfully thin on the ground in this harrowing documentary about a gay subculture

Sex and psychoactive drugs unleash neurochemicals in the brain that can take us out of ourselves, make us feel better than ourselves, or even fuse us – in temporary but ecstatic union – with our fellow human beings. Being drawn to experiment with both at once, like sparking two electrical currents to produce an explosion of euphoria, is a growing temptation. The new documentary Chemsex understands it, rather more gravely, as an enormous risk.

This film focuses specifically on one extreme subculture on the London gay scene. Drug-fuelled sex parties aren’t a new phenomenon. But mobile-phone apps such as Grindr have brought them much closer to the average gay man’s reach: if you wanted to go looking for a crystal meth orgy these days, you could find it with just a few swipes of an iPhone.

For a lot of vulnerable men on the scene – like the city boy interviewed here who lost his job and turned to prostitution to pay for the habit – chemsex promises a sense of escape so overwhelming that sobriety feels like hell.

Directors William Fairman and Max Gogarty take a serious-minded approach to the psychological scars this behaviour both masks and worsens. The fear of HIV that inhibited the sex lives of so many men in the 1980s and 1990s is far from vanquished, but the oblivion of chemsex can offer the illusion that it is.

Phone apps such as Grindr have brought sex parties within the average gay man’s reach

Because the virus is now medically considered more treatable than diabetes, many promiscuous gay men would prefer to know they have HIV than worry about getting it. Some even seek it out. This is a reckless strain on national health resources, of course, but should probably be looked at as a form of mental illness – a self-immolation in the most dangerous underground forms of sexual self-expression.

Prurient peepshow value in this frequently harrowing film is thankfully thin on the ground. The music, by contemporary composer/producer Danny L Harle, is dark, driving, sombre and mournful. One of the main interviewees is the substance abuse expert David Stuart, an advisor at London’s Soho sexual health clinic on Dean Street, who has many strong ideas for therapy and treatment.

It’s a subject that could lend itself to salacious tabloid scaremongering – and in wanting to put out such a cautionary message, the filmmakers might give that superficial false impression. But their concern, chiefly speaking, is for the lives already affected, not for a mainstream determined to steer well clear.

The phenomenon of chemsex – which the British Medical Journal has called to be classed as a public health priority – is viewed here as a dangerous, ever-deepening chasm that can trap those with unconventional sex lives. Building bridges out of there won’t happen in the dark.