When is it right for a government to mass-medicate the public? It's hard to imagine a scenario. If we faced the spread of a new and lethal plague, most people would probably accept draconian intervention. But it would have to be serious.

Today, however, we're told by the Government's Alan Johnson that he intends to pursue a policy of mass medication of the British public. Not to prevent smallpox or the bubonic plague, but to tackle tooth decay. Well, tooth decay is bad news, but it's hardly the stuff of nightmares. However, fluoride, the medicine he's chosen, may well be.

We don't know if fluoride works. In the United States, where 65 per cent of people are routinely subjected to the chemical, the worst tooth decay occurs in poor neighbourhoods of the largest cities, the vast majority of which have been fluoridated for decades. When fluoridation was halted in parts of Finland, East Germany, Cuba and Canada, tooth decay actually decreased.

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One of the reasons for this is that fluoride is believed to work best when applied directly, for example to the tooth. Drinking fluoride to prevent cavities is like swallowing bandages to cure a broken arm. Another reason is that a policy of mass medication through the water supply assumes that we are all the same age, size and weight, and therefore require the same dose.

What we do know is that fluoride is toxic – so toxic, in fact, that in 1984, the makers of Colgate, Procter & Gamble, reportedly admitted that a small tube of their toothpaste "theoretically at least contains enough fluoride to kill a small child".

Fluoride has been linked to cot death, eczema and Alzheimer's. It has been shown, at low doses, to cause genetic damage. And it has been linked by doctors from the National Cancer Institute and the National Health Federation to cancer.

Because fluoride causes collagen, an essential structural component in skin, muscle, ligaments and bone, to disintegrate, big question-marks are being raised over its possible contribution to arthritis, a problem that has increased by 63 per cent since 1997, and which now affects 70 million Americans.

Other reports are appearing that link the accumulation of fluoride in bones to an increase in hip fractures among the elderly. The Journal of the American Medical Association reported recently that "with increasing dose of fluoride in the drinking water, the hip fracture ratio increases," a view echoed by The Lancet, The Annals of Epidemiology and other science journals.

Further studies have linked fluoride use to hyperthyroidism (underactive thyroid glands), one of the most widespread medical problems in the US, affecting more than 20 million people and leading to fatigue, weight gain, depression and heart disease. That's scarcely surprising, given that fluoride used to be prescribed by European doctors to depress thyroid activity.

Alan Johnson's principal concern is for the poorest in society. But studies as far back as the Fifties, by the American Dental Association and the Canadian National Research Council, have shown that people with poor diets are more susceptible to the health risks of long-term ingestion of fluoride.

What is extraordinary is that fluoride was ever considered for mass medication. It has always been contentious. Indeed, the first ever lawsuits against the US 's atomic bomb programme, the Manhattan Project, concerned fluoride, not radiation. What's more, the first health tests for fluoride were designed to establish how much industry could afford to release into the environment without damaging human health.

In the summer of 1943, a group of New Jersey farmers reported that something was "burning up" their peach trees, maiming their horses and cattle and killing their chickens. The source of these ills was a nearby DuPont corporation factory that was producing millions of pounds of fluoride for use in the Manhattan Project. Immediately after the war, the farmers filed suit against DuPont. At the time, the Manhattan Project's chief of fluoride toxicology studies, Professor Harold C Hodge, asked his superiors if there "would be any use in making attempts to counteract the local fear of fluoride through lectures on fluoride toxicology and perhaps the usefulness of fluoride in tooth health?"

The most widely cited study into the benefits of water fluoridation was conducted in New Zealand between 1954 and 1970, and it is used by fluoridation advocates to this day. But the study failed to meet the most basic criteria for scientific objectivity, not least because the decline in tooth decay that the community in question experienced was also seen in other non-fluoridated communities in the region. The then Mayor of Auckland, Sir Dove-Myer Robinson, described the so-called Hastings Experiment as a "swindle".

At best, the jury is out. Perhaps there are reams of recent studies that lay these fears to rest. If so, Johnson needs to share them with us. Mass medication is a big deal, and there should be proper debate before it is applied. Is fluoride safe? Does it even work? More importantly, is it right for a government to impose a controversial medicine on the entire population to deal with a non-life-threatening complaint?

When the Government first mooted this idea, Hazel Blears and Elliot Morley, who were then ministers at the Health and Environment departments respectively, suggested with breathtaking arrogance that "those who remain adamantly opposed would be able to use water filters that remove fluoride or buy bottled drinking water". Alan Johnson, the current Health Secretary, needs to demonstrate a far greater respect for British people than that.

Zac Goldsmith is the Conservative Party parliamentary candidate for Richmond Park