Sheila Myers has had headaches for years. A dance teacher, she says that some days they would be so bad she could barely see the children in front of her.

Then three years ago her father died, her family business collapsed and the bank called in their huge mortgage. Under these stresses, her regular headaches became almost daily.

'I had scans looking for a brain tumour, but they found nothing. Medication did nothing I ate paracetamol, Migraleve like Smarties.'

Then a friend gave her a bracelet of five small magnets. 'I haven't had a headache for 18 months. I'm a completely new person. I can't explain it all I can say is that this bracelet has changed my life. If you want to say it's in my head, that's fine, but I won't take the bracelet off,' says Sheila who maintains her lifestyle in every other way is unchanged.

Joan Salmon is equally convinced her magnetic bracelet has banished the pain caused by arthritis and osteoporosis. A former nurse, she's considered the possibility that a change in medication, lifestyle or diet might have done it, but: 'I simply haven't changed anything. I've been on medication for these conditions for some time but I was finding it extremely difficult to walk. Now I walk with great comfort. I don't need painkillers any more.'

According to Jeffery Frankel, director of Magna Jewellery, one of several manufacturers who market products claimed to help a range of conditions including arthritis, back pain, carpel tunnel sundrome, migraine, premenstrual syndrome and depression, these are just two examples of hundreds who write to him about the benefits of his magnetic jewellery.

But ask many doctors and they'll say there just isn't a mechanism by which simple magnets can achieve such things. So is this another case of mind over matter, or does magnetism hold some magic hidden from conventional medicine?

Due to lack of research, proponents of magnet therapy tend to offer up studies done on electromagnetism the effect produced when an electric current is combined with a magnetic field as evidence for the beneficial effects of permanent magnets, arguing that the biological mechanisms must be the same.

The most common hypothesis is that magnets improve circulation, thus rebalancing the body. The theory is that by raising tissue temperature, magnetic fields promote increased blood flow to affected areas. This washes away toxins and boosts anti-inflammatory white blood cells levels, both of which contribute to decreasing pain.

Californian neurologist Dr Ronald Lawrence, president of the North American Academy of Magnetic Therapy goes one step further. He suggests that movement through a magnetic field generates low levels of electric currents within blood cells that enhance blood flow.

In one study he reported finding a 300% increase in blood flow in the fingers when patients wearing a magnet on their wrist moved the joint very slightly. These microcurrents may also be diverting or blocking electrical signals which inform the brain that something in the body is hurting in a similar way to acupuncture.

The problem is that while there is strong scientific evidence that electromagnetism can cause these effects, there is precious little to say that magnetic fields with no electrical input can do the same.

To obtain solid proof, magnets must be compared with a mock device or placebo, but few tests have been done.

In 1986 Japanese researchers reported almost all of 50 subjects given 1,500 gauss magnetic belts to wear showed marked improvement in back pain, compared to 23% with placebo, and in 1997 American researchers reported that wearing a 300-500 gauss magnet for 45 minutes reduced pain in 18 out of 25 patients with post-polio syndrome compared to only five of 25 patients exposed to a placebo.

But while these studies may be scientifically legitimate, they are small and, says Professor Tony Barker, consultant clinical scientist at the Royal Hallamshire hospital department of medical physics and clinical engineering, constitute nothing like enough proof for the claims made.

The inventor of an electromagnetic therapy currently being trialled for use in depression, he is damning of claims made for magnetic jewellery and the similar devices. 'There is no scientific evidence that permanent magnetic fields of the sort of strength that you could get from any of these devices have any noticeable biological effect on the body.

'The main claim made for these things is that they raise body temperature and cause vasodilation (expansion of the blood vessels) which improves circulation.

But if this were true your hand would get pinker and warmer when you held the magnet and it clearly doesn't. The body simply isn't sensitive to such low-level magnetic fields there aren't any mechanisms by which they could be.'

People are being confused, says Barker, by the fact that in recent years, scientists have been discovering that electromagnetism may have useful therapeutic effects.

One application already used worldwide and again often erroneously quoted by simple magnet proponents as proof of their worth involves using a technique called pulse magnetic therapy to heal broken limbs that refuse to knit under plaster.

But while published reports suggest the technique can work in 80% of cases, it's important to note that this process involves electromagnetism, not simple magnets.

Another technique known as magnetic nerve stimulation (TMS) being trialled in conditions such as depression and incontinence uses electromagnetic energy to make nerve cells 'twitch', says Dr Mark George, a psychiatrist and neurologist at the Medical University of South Carolina who is using TMS to treat people with chronic depression.

Two hand-sized batteries are used to produce a very brief 300 millisecond very powerful burst of magnetic energy which when it meets tissue, converts into an electric pulse. In your hand, this could make your thumb wiggle; in one part of the brain it might produce a little flash of colour before the eyes. And when targeted carefully, research suggests this nerve twitch can provoke the release of brain chemicals that can ameliorate deep depression.

Randomised controlled trials in both the US and in Israel using 20 minutes of TMS daily for two weeks have found that on average 50% of patients given TMS improve by 50% on mood rating scales compared to 28% of those given placebo treatment.

'Getting half the people well after two weeks with no side effects is as good as any treatment we have now. It's extremely exciting, though it is still experimental,' says George.

'TMS is similar to ECT in that it goes to the part of the brain that is not functioning and causes a discharge which brings the brain cells back on line, but it is much much more gentle.'

Magnetic stimulation is also being used to treat urinary incontinence. Dr Niall Galloway from Emory University in Atlanta, Georgia, is testing a 'magnetic chair' which strengthens bladder muscles. 'When the fully-clothed patient sits in the chair, the pulses of the magnetic field trick the bladder muscles into rhythmically contracting and relaxing in the very way we would exercise them if we could.'

In controlled tests where patients sat for 20 minutes twice a week for eight weeks, 83% showed improvement in muscle strength and bladder control, he says.

But, says Dr George, the effects of this kind of magnetic stimulation cannot be used to support the use of low-intensity static magnets.

'The magnets we are using are thousands of times more powerful and the exposure thousands of times briefer. We think the biological effect we get is because this brief powerful surge makes the nerve twitch. If the intensity of the field is reduced, even if exposure is longer, this wouldn't happen.'

All this doesn't matter to Sheila and Joan. But whether you follow their example and invest £30 to £60 on such jewellery products will depend on just how much you value evidence-based medicine.

• Magna Jewellery can be contacted on 0181 958 9719 or 0181 421 8848.