Half a million people in England use hypnotherapy every year. A quick search of the internet shows it is offered for a wide range of conditions: smoking, obesity, tinnitus, phobias, allergies, anxiety - even breast enlargement. But what is hypnotherapy, and what do we really know about its effectiveness?

In this powerful, personal journey Kathy Sykes travels across the world. She follows patients who are trying the therapy for themselves; sees hypnotherapists at work and meets scientists looking inside our brains to try and unravel what might be going on during hypnosis.

Kathy tries hypnotherapy herself to tell us what it felt like for her. She looks at what scientists make of trance. And in Italy, she peeps into the minds of people as they are given suggestions both under, and without, hypnosis. She discovers that suggestions really can affect how we perceive the world.

But does any of this translate into health benefits? Is there really the evidence to back up the widespread belief that hypnotherapy can help you quit smoking? And does it help you lose weight?

After examining the evidence, Kathy decides to explore two areas where hypnotherapy is being used by medical practitioners who believe it can offer something particularly valuable. At the South Manchester Hospital she meets Professor Peter Whorwell who is pioneering the use of hypnotherapy for people suffering from the most extreme form of Irritable Bowel Syndrome; in Scotland she meets Mandy whose dentist is about pull out her front teeth and drill metal implants into her jaw using hypnosis but without a local anaesthetic.

Kathy then travels to Montreal to meet Professor Pierre Rainville, a world expert on pain and follows his latest experiment using brain scans to explore the basis of hypnotic pain relief.