In a first floor room above a gridlocked London street, 20 strangers shuffle on to mats and cushions. There's an advertising executive, a personnel manager, a student and a pensioner. A gong sounds softly and a session of sitting meditation begins. This is one of more than 1,000 mindfulness courses proliferating across the UK as more and more people struggling with anxiety, depression and stress turn towards a practice adapted from a 2,400-year-old Buddhist tradition.

Enthusiasm is booming for such mindfulness-based cognitive therapy (MBCT) courses, which an Oxford University study has found can reduce relapses into depression by 44%. It is, say the researchers, as effective as taking antidepressants. It involves sitting still, focusing on your breath, noticing when your attention drifts and bringing it back to your breath – and it is surprisingly challenging.

Lifestyle magazines brim with mindfulness features and the global advertising giant JWT listed mindful living as one of its 10 trends to shape the world in 2014 as consumers develop "a quasi-Zen desire to experience everything in a more present, conscious way".

But psychiatrists have now sounded a warning that as well as bringing benefits, mindfulness meditation can have troubling side-effects. Evidence is also emerging of underqualified teachers presenting themselves as mindfulness experts, including through the NHS.

The concern comes not from critics of mindfulness but from supporters, such as Dr Florian Ruths, consultant psychiatrist at the Maudsley hospital in south London. He has launched an investigation into adverse reactions to MBCT, which have included rare cases of "depersonalisation", where people feel like they are watching themselves in a film.

"There is a lot of enthusiasm for mindfulness-based therapies and they are very powerful interventions," Ruths said. "But they can also have side-effects. Mindfulness is delivered to potentially vulnerable people with mental illness, including depression and anxiety, so it needs to be taught by people who know the basics about those illnesses, and when to refer people for specialist help."

His inquiry follows the "dark night" project at Brown University in the US, which has catalogued how some Buddhist meditators have been assailed by traumatic memories. Problems recorded by Professor Willoughby Britton, the lead psychiatrist, include "cognitive, perceptual and sensory aberrations", changes in their sense of self and impairment in social relationships. One Buddhist monk, Shinzen Young, has described the "dark night" phenomenon as an "irreversible insight into emptiness" and "enlightenment's evil twin".

Mindfulness experts say such extreme adverse reactions are rare and are most likely to follow prolonged periods of meditation, such as weeks on a silent retreat. But the studies represent a new strain of critical thinking about mindfulness meditation amid an avalanche of hype.

MBCT is commonly taught in groups in an eight-week programme and courses sell out fast. Ed Halliwell, who teaches in London and West Sussex, said some of his courses fill up within 48 hours of their being announced.

"You can sometimes get the impression from the enthusiasm that is being shown about it helping with depression and anxiety that mindfulness is a magic pill you can apply without effort," he said. "You start watching your breath and all your problems are solved. It is not like that at all. You are working with the heart of your experiences, learning to turn towards them, and that is difficult and can be uncomfortable."

Mindfulness is spreading fast into village halls, schools and hospitals and even the offices of banks and internet giants such as Google. The online meditation app Headspace now has 523,000 users in the UK, a threefold increase in 12 months. But mounting public interest means more teachers are urgently needed and concern is growing about the adequacy of training. Several sources have told the Guardian that some NHS trusts are asking health professionals to teach mindfulness after only having completed a basic eight-week beginners' course.

"It is worrying," said Rebecca Crane, director of the Centre for Mindfulness Research and Practice in Bangor, which has trained 2,500 teachers in the past five years. "People come along to our week-long teacher training retreat and then are put under pressure to get teaching very quickly."

Exeter University has launched an inquiry into how 43 NHS trusts across the UK are meeting the ballooning demand for MBCT.

Marie Johansson, clinical lead at Oxford University's mindfulness centre, stressed the need for proper training of at least a year until health professionals can teach meditation, partly because on rare occasions it can throw up "extremely distressing experiences".

"Taking the course is quite challenging," she said. "You need to be reasonably stable and well. Noticing what is going on in your mind and body may be completely new and you may discover that there are patterns of thinking and acting and behaving that no longer serve you well. There might be patterns that interfere with living a healthy life and seeing those patterns can bring up lots of reactions and it can be too much to deal with. Unless it is handled well, the person could close down, go away with an increase in self-criticism and feeling they have failed."

Finding the right teacher is often difficult for people approaching mindfulness for the first time. Leading mindfulness teaching organisations, including the universities of Oxford, Bangor and Exeter, are now considering establishing a register of course leaders who meet good practice guidelines. They expect mindfulness teachers to train for at least a year and to remain under supervision. Some Buddhists have opposed the idea, arguing it is unreasonable to regulate a practice rooted in a religion.

Lokhadi, a mindfulness meditation teacher in London for the past nine years, has regular experience of some of the difficulties mindfulness meditation can throw up.

"While mindfulness meditation doesn't change people's experience, things can feel worse before they feel better," she said. "As awareness increases, your sensitivity to experiences increases. If someone is feeling vulnerable or is not well supported, it can be quite daunting. It can bring up grief and all kinds of emotions, which need to be capably held by an experienced and suitably trained teacher.

"When choosing a course you need to have a sense of the training of the teacher, whether they are supervised and whether they themselves practise meditation. Most reputable teacher training courses require a minimum of two years' meditation practice and ensure that teachers meet other important criteria."