It has helped groups as diverse as Vietnam veterans, psychiatric prisoners and sex offenders to deal with personal trauma. It has helped ease the symptoms in specific illnesses, such as asthma and rheumatoid arthritis. It has been shown to boost the immune system and in one study even helped unemployed Texans find new jobs. Most recently, it helped US students to come to terms with 11 September. There are no side-effects and it is available to anyone of any age, pretty much anywhere, over the counter.

If it were a drug, this versatile little treatment would surely have a public profile to match Viagra. Indeed, the lack of a pharmaceutical company to promote it is perhaps part of the reason why its benefits are so little known. It's cheaper than any drug - the cost of a pen and paper. Because the miracle treatment is simply what I am doing right now: writing.

Until now, most research into the therapeutic benefits of putting words on to the page has been carried out in the US, much of it at the University of Texas. But now the Arts Council of England is to fund a project at King's College, London, which researcher Gillie Bolton hopes will give greater prominence to therapeutic writing. Not that she herself needs to read any research to be convinced of its benefits.

'I suffered some very traumatic experiences, and writing saved my sanity,' she says simply. 'In my early thirties, I was in a bad psychological state, but didn't really know why. My husband suggested that I write my autobiography. I did, conjuring up a lovely, glorious story. Then I came up with something far more chaotic but closer to the truth. Then I refined it again, this time into poetry.' For Bolton, this rewriting is a vital part of the process. 'It's not just the first cathartic outpouring that matters, it's the redrafting. I came to understand what had happened to me only through doing this.'

So why didn't she just seek regular therapy? 'I couldn't trust a therapist the way I could a piece of paper. Paper's always there to reread or rewrite. Once you've said something you can't unsay it, but with a page of writing you can. You don't ever have to share it. You can burn it if you want.' For this reason, proponents of therapeutic writing argue that while the process may bring up difficult issues, the writer is always in control of them, which is not always true of talking therapies.

Bolton is now a research fellow in medical humanities at the University of Sheffield using therapeutic writing in reflective practice seminars with medical and health professionals. 'We use it to explore work issues, relationships with colleagues and the like,' she says. 'A lot of people say, after writing something, they could never have told me without writing it first. I sometimes ask doctors to write in the voice of a patient. They find this illuminating. The value of doing this is hard to overstate, from their future patients' point of view and their own.'

Texas-based psychotherapist Miriam Kuznets offers similar therapy to groups of clients. 'I suggest a prompt - for example, imagine a working day one year from now or describe a home that was important to you when growing up - and then I let people write for 15 to 20 minutes. They can then share what they've written or not. Writing lets you choose your words, so it works well with people who are less able to verbalise their feelings or who are sceptical about talk therapy. It's more concrete than just talking, and you can do it on your own, anywhere.'

Kathleen Adams, founder of the Centre for Journal Therapy in Colorado, puts this beautifully in her book Journal to the Self : 'For nearly 30 years I've had the same therapist. I've called on my therapist at 3am, on my wedding day, on a cold and lonely Christmas, on a Bora Bora beach, and in the dentist's reception room. I can tell this therapist absolutely anything.

'My therapist listens silently to my most sinister darkness, my most bizarre fantasy, my most cherished dream. And I can scream, whimper, thrash, rage, exult, foam, celebrate. I can be funny, snide, introspective, accusatory, sarcastic, helpless, brilliant, sentimental, profound, caustic, inspirational, opinionated or vulgar. My therapist accepts all of this without comment, judgment, or reprisal.' Adams writes in cheap spiral-bound notebooks - she calls it the '79 cent therapy'.

Although there is a gulf of difference between the two, therapeutic writing can also unlock creative writing. Whitbread and Orange prize-shortlisted novelist Jill Dawson has kept a journal since she was nine. 'It has helped me personally and also made me a better writer,' she says, 'because going over and over something eventually makes it clearer. A dream you don't understand may make sense two years later. Obviously, it undergoes radical transformation before it becomes writing that you would want published, but it is a part of the process. You can find feelings by writing in this raw way that you can then explore using different events in a story.'

But you don't have to be able to write like Adams or Dawson to benefit. 'I was working in a hospice,' says Bolton, 'and one virtually illiterate man in a great deal of pain with cancer just wrote a list of names. They were the children he'd previously disowned. He decided to see them and told me afterwards how happy he was that he'd done so. He was clearly suffering a lot less. There was genuine symptom reduction. You don't need to be literate at all - you can do something similar with a tape recorder.'

Indeed, if you fancy trying therapeutic writing for yourself, Bolton recommends chucking the rules of writing out of the window. Begin with what she calls a 'mind dump'. 'Just write for six minutes, whatever comes into your head, don't edit, don't worry about grammar, spelling or style and don't stop writing,' she says. 'After that, you can focus on a theme. Choose something concrete, not abstract. Childhood memories are very useful. For example, imagine that you are holding an object that was once important to you. Describe it and then write what follows.'

It is in exercises such as these that therapeutic writing differs }from the traditional journal. While entries of the 'Woke up late, train broke down, photocopier out of order' type are better than nothing, Miriam Kuznets says it's worth striving for more. 'The problem with a lot of journals is that they are superficial. You need a direction and a focus to get the most out of the process.' Obviously, this is where a therapist or a group can help, but they're not essential. There are many more exercises like the mind dump and the others mentioned in the books listed below.

Anyway, in my experience, when you're actually ill, everything is useful. Noting symptoms, treatment problems and other concerns can be helpful in a number of ways. First, it feels like you're taking control, overcoming the sense of powerlessness that illness can bring. Second, armed with decent notes you'll be able to give your doctor or complementary therapist more accurate information and to ask them the questions you really want answered.

Third, listing practical questions naturally raises the more difficult, existential ones, and before you know it, you're into some serious stuff about mortality, relationships and what's important in life. Just the sort of diversion you need when you're on a diet of ice poles and drinking yogurts, as I was after radiotherapy on the throat left me pretty much unable to swallow.

I've written about that experience and other health problems both before it and since in books and articles, and it has always helped. But, as was brought home to me while researching this piece, it doesn't just work for the big things. Writing for nobody's eyes, but your own can also soothe away those little irritations that combine to drag down your day. I contacted a well-respected author who had written a book about his own health problems to ask how the process had helped him. He turned me down because I hadn't read the book. Fair enough, but the call left me feeling very small, stupid and stressed. I wrote him an email - part apology, part justification, part exposition of my feelings. It wasn't very long and I didn't send it, but I felt a whole lot better afterwards. Five minutes. It was that easy.

· The Therapeutic Potential of Creative Writing by Gillie Bolton (£15.95, Jessica Kingsley Publishers); Journal to the Self by Kathleen Adams (£9.14, Warner Books); The New Diary by Tristine Rainer (£13.99, Jeremy P Tarcher); Fred & Edie by Jill Dawson (£6.99, Hodder & Stoughton)