Is hynotism like taking a sugar pill in the belief it's medicine? asks DR PETER NAISH

Have you seen how many hypnotherapists there are in Yellow Pages? When people learn of my own interest in hypnosis they often say things like, "Oh, I've got a friend who's trained as a hypnotherapist." They are everywhere! Does this explosion in numbers mean that hypnosis is a cure-all or are people deceiving themselves?

Could it be that being hypnotised is like taking a sugar pill in the belief it's medicine? If so, that would mean therapists are producing nothing more than a placebo effect.

History gives us a good clue as to how we should be thinking. A couple of centuries ago, Mesmer, as in mesmerism, the earlier term for hypnotism, claimed to cure people by manipulating a mysterious magnetic fluid that ran through their bodies. Of course this was completely disproved; the hypnotist has no forces to wield or direct. With nothing tangible being administered from the outside, it followed that any changes in the patient must be self-generated. That is a very good description of placebo action!

We should not be too dismissive of placebos - they can be very effective. From first-hand experience many will attest to the benefit gained from taking SSRI (serotonin-specific re-uptake inhibitor) anti-depressants, yet as we have heard in the news recently, they do their job little better than placebos. At least hypnosis has no unpleasant side-effects.

Hypnosis may go beyond placebo in situations where it is insufficient simply to believe one is recovering - improvement requires a change in the way one perceives the world. A good example is overcoming a phobia, something that hypnosis seems to help considerably.

However, this moves us into the realm of psychotherapy, where hypnosis can be used as a tool, rather than a treatment in itself. Part of its value derives from the way it helps people to visualise experiences almost as if they are real.

So, hypnosis probably can do more than a placebo, but in doing so becomes medicine-like and requires safeguards. People who are trained briefly in hypnosis, but lack qualifications in medicine or psychology should not administer medicine'.

A trial in which I was an expert witness involved a pet-shop owner who became a hypnotherapist. A teenager consulted him for her anxiety about forthcoming exams. She left, convinced that her father had been sexually abusing her - a salutary reminder of the dangers of hypnotic visualisation techniques in ill-trained hands.

Dr Peter Naish is Senior Lecturer in Cognitive Psychology at the Open University and Chair of Council of the British Society of Clinical and Academic Hypnosis. You can find out The Truth about Hypnosis with Peter at Science Oxford on Wednesday, April 23, at 7.30pm. The event is part of Inside the Brain, an exhibition and series of events exploring the human brain. The Science Matters page is co-ordinated by Science Oxford, cultural centre for science in St Clements, Oxford. For more information, visit www.scienceoxford.com.