Anthony Newbury is a dentist, but listening to him you'd be forgiven for thinking he was some kind of faith healer. "One chap came twice from Ireland, and he said: 'You'll never believe this, but the clinical depression I've suffered for 40 years disappeared after the first visit,'" Newbury recalls. "He told me that unfortunately it hadn't happened soon enough to save his first marriage - but still, that's a nice thing to be able to do for someone."

The wonder treatment in question is the removal of mercury amalgam fillings. Stoking a debate that has raged since the 70s, a small but growing band of dentists blame widespread mercury poisoning for a terrifying range of conditions, ranging from gum disease, migraine, headaches, depression, chronic fatigue, eczema and asthma to rheumatism, Alzheimer's, Parkinson's, kidney disease and multiple sclerosis. Either they have exposed a cost-saving conspiracy between the dental establishment and the NHS that is costing people their good health and, in some cases, their lives - or they are lending credibility to a classic case of medical scaremongering that helps line the pockets of greedy dentists.

Their argument is intuitively convincing. Mercury is the second most toxic metal on the planet after plutonium, and all dentists now agree that it is not, as previously believed, safely "locked in" when mixed with other metals in amalgam. Instead, it constantly releases toxic vapours, particularly during chewing, and has been banned for use in dentistry in Sweden. The Department of Health's committee on toxicity, while ruling the dangers unproved, warns that pregnant women should avoid the placement or removal of mercury fillings. Says Newbury: "Everybody knows mercury is toxic, and if they brought it out now for use in fillings they'd be laughed out of the country."

Jack Levenson, founder of the British Society for Mercury-Free Dentistry and author of a book on the subject, says any other problem on the same scale would have been labelled an epidemic long ago. "The British Dental Association's figures say 3% of the population are particularly sensitive to mercury, and when you have an outbreak of flu, 0.4% counts as an epidemic," he argues. "There is nothing in medicine that compares to the health benefits of having mercury fillings removed."

In extreme cases, patients have spoken of serious symptoms vanishing within days of having their fillings removed. Levenson says the same thing happened to him when sweaty palms brought a halt to a potential professional career in tennis: having his fillings out appeared to eliminate the problem.

One leading antiamalgamist, who prefers not to be named, smells a conspiracy. "The new materials would increase the NHS dental bill sixfold. Then there's all the litigation from patients, and the pressure on health insurance companies. There's strong pressure to keep the lid on it."

Mercury amalgam has been used by dentists for 150 years, though the effects of mercury poisoning were well-known before that: the use of mercury chloride in the preparation of felt hats - apart from providing the inspiration for a character from Alice in Wonderland - was blamed for the widespread incidence of St Vitus's Dance among hatmakers. For those campaigning to end its use, amalgam's long history explains why it was able to avoid the safety tests it would face if introduced today. "We've been smoking for 300 years," says Anthony Newbury, by way of comparison, "but only recently have we been told of the dangers, and cigarettes still haven't been banned."

For more sceptical dentists, though, mercury's longevity makes exactly the opposite point. It has been used in billions of fillings - currently 15m a year - and the only evidence of harm is to be found in patients' anecdotal evidence or scientific studies whose methods have since been thrown into question.

"It's utter nonsense," says Professor Barry Eley of King's College, London. "These fillings release small quantities of mercury, but well below the levels associated with toxicity. There is absolutely no scientific evidence that mercury released from amalgam is associated with the dan gers claimed." The health department's warning to pregnant women is, he reckons, a case of "taking the maximum safety posture. Undoubtedly an awful lot of money is being made by dentists saying that you need all your mercury fillings replaced. Obviously, that can't be done on the health service, because there is no clinical justification."

Other evidence makes the antiamalgamists' case look even less solid. Mercury was banned in Sweden - according to public pronouncements, at least - for environmental reasons; the BDA's former claim that 3% of the population are sensitive to mercury refers to reactions to all mercury in the environment and does not imply that 3% of people will necessarily react badly to amalgam fillings. Many of the tests for mercury levels used in studies, which frequently date back to the 70s, have since been accused of skewing results, while doubts have been raised over the "electrodiagnostic" tests used by some dentists today to measure levels of mercury in patients' mouths. Recent research by scientists at the University of Heidelberg found that many who complain of side-effects from mercury fillings may be unwittingly using them as a scapegoat for other disorders.

The worst excesses of the anti- amalgam movement are exemplified by one Hal Huggins, an American dentist who published some of the earliest claims of the perils of amalgam. Huggins operated a lucrative consultation by mail and recommended a range of special dietary supplements - not unlike the charcoal tablets and vitamins recommended by antiamalgamists in Britain today - until his licence was revoked for malpractice in 1996. The presiding judge ruled that Huggins had diagnosed mercury toxicity in every patient who had visited him, including several who didn't even have mercury fillings.

The number of complaints made against British dentists for unscrupulously recommending amalgam replacement is, sadly, unknown: complaints made to the General Dental Council are confidential until they reach the level of the council's professional conduct committee. A spokesman could not recall any ever having done so. "The GDC are doing a very poor job of protecting the public on this matter," says one experienced dentist. "They catch the very severe cheats, but they don't bother with the day-to-day things."

Despite the entrenched nature of the debate, there is a surprising amount of agreement over whether you should automatically have all your mercury fillings replaced, and the answer is: don't. Antiamalgamists, though they would argue that unexplained health problems may well be due to mercury in your mouth, agree that removal of fillings exposes the body to an intense dose of toxins, even when properly carried out using specialised suction equipment and dental dams. Besides being more expensive, the composite material used instead has its own dangers too, since it releases peroxides and free radicals into the body. The decision remains a personal and financial one - but if your dentist doesn't at least inform of the existence of powerful evidence suggesting that the risk from mercury amalgam may be much smaller than some claim, find another dentist.

Sick to the back teeth
Like most other paid-up members of the 21st century, I suffer from a variety of niggling, non-life-threatening health problems, which I have always ascribed to stress, pesticides, traffic fumes, sick building syndrome, hangovers or too much junk food. One thing I never gave a second thought to was my 12 amalgam fillings, which I started to accumulate as a chocolate-loving 10-year-old, writes Claire Wallerstein.

An appointment with holistic dentist Dr Jack Levenson, however, has left me with the suspicion that there is a mini-toxic waste dump inside my head; that I, along with millions of others, could be suffering from long-term, low-level mercury poisoning.

Dr Levenson, president of the British Society for Mercury-Free Dentistry, has removed the fillings of more than 6,000 patients over the past 20 years, and claims that nearly all found their health improved. "I believe that many conditions fashionably attributed to viruses could in fact be caused by mercury toxicity," he says.

On my visit to his surgery, my first task is to fill in a questionnaire. I tick away at symptom after symptom, but even I, a professional hypochondriac, can see that many are fairly minor - nightmares, shyness, bad breath, bedwetting, dandruff - and could have many potential causes.

One point that interests Levenson is my allergy to costume jewellery - often made with other substances found in amalgam fillings, such as nickel. "If you're allergic to it on your skin, what's it going to be like inside your head?" he asks.

This had never occurred to me before. Nor had I ever realised the numbness and tingling I often feel in my limbs could be a classic symptom of mercury poisoning.

Next, Levenson measures the electric current in each of my fillings. The highest - in the one that makes me cringe most whenever I bite a Kit-Kat wrapper - reads 3.3 microamps. "A few microamps may not sound a lot, but remember that the electrical current of the brain is measured in nanoamps - thousandths of a micro-amp," he says. "The roof of the mouth is effectively the floor of the brain, and electrical interference may be implicated in problems such as migraine, epilepsy, Alzheimer's, Parkinson's and motor neurone disease. In many patients there is a measurable change in brain waves after the removal of fillings."

Next, I am hooked up to a "vega meter" which measures the energy flowing through an acupressure point on my thumb when I am exposed to different materials. My energy levels plunge in reaction to the amalgam, but not white plastic.

By now I feel sure that having a head full of mercury probably isn't a good idea. But what horrifies me more is the cost - around £1,500 - of having my fillings removed. The problem is that you can't just get the fillings ripped out, as mercury vapour is released when they are ground up for removal.

Dr Levenson says extraction should be done in a well-ventilated surgery, and that the patient should have a rubber dam and extraction pump in their mouth to avoid inhalation of particles. They should also wear clothes covering as much of the skin as possible, and breathe air from a nasal tube.

During the operation, Levenson sometimes uses intravenous vitamin C as a detoxifying agent. He also prescribes a whole range of vitamins, homeopathic remedies, massages and saunas afterwards to help remove the reservoir of mercury that has built up in the body over the years.

So what can be done for people like me with a mouth lined with mercury, but not enough money to have it removed? Apparently the best thing is to limit your exposure by avoiding unnecessary chewing (which increases the production of mercury vapour), which means cutting out snacks between meals, and avoiding salty, sour and hot foods. Refined carbohydrates and sugars are also a no-no, as they help oral bacteria to produce lactic acid, which corrodes fillings.

It's also sensible to cut down on mercury intake from other sources, such as contact lens solutions, and to try to avoid your fillings when brushing your teeth. Supplements of vitamin C, sulphur, selenium, seaweed and charcoal may help to remove mercury from the body.

• The Brompton Dental Surgery, 221 Old Brompton Rd, London W1.