Every so often a magic bullet comes along which promises to transform our health in a single pill, and so we grab it with both hands and ask all the hard questions later.

Today's magic bullet is melatonin, the so called "miracle" hormone that is supposed to stop you not only from suffering jet lag or insomnia but also from getting cancer or even from getting old. America is in the grip of melatonin fever, and Britain has recently caught the bug, too. After a newspaper recently ran an article about the supposed anti-aging properties of melatonin, the Nutri Centre, London's largest alternative pharmacy, received 1800 phone calls in a single morning.

This reaction is not dissimilar to what happened with many miracle substances of the past. A few sincere individuals believe they've discovered an instant, miracle cure to most complaints an all-purpose cure without any side effects and suddenly perfectly healthy people begin taking it before we really understand how it works or what else it does to us.

The problem starts with the term "natural". As soon as we hear it, we suspend the ordinary disbelief with which we greet most drugs. What could be the problem with something just like what our bodies make? If it's natural, it must be good for us. If it's natural, it doesn't have to be subjected to hard science or our scepticism.

But as always, the story is a bit more complicated. Melatonin might help or harm you, depending how you take it and when. It may hold promise for patients with terminal cancer. But if you're already healthy, you might be worse off taking melatonin for jet lag or night shifts, and it could affect your fertility .

It's wise to recall that medical history is littered with the casualties of a number of so called natural products. After all, most drugs are derived from something in nature. Scientists usually extract only the active principle of a plant, leaving behind all the other elements, which often act as a check against any toxic effects of the active substance, and then administer it in massive doses. Time after time, this method has disastrous consequences.

More fundamental is the problem of believing in magic bullets. It's a comforting thought, the idea that we can divest ourselves of all responsibility for our health and instead just take a pill, which will make any of our bad lifestyle habits magically disappear.

It's what many people demand of medicine expecting doctors to patch up an unhealthy life.

The truth is, there is no magic bullet. There is only making the less glamorous decision to eat well, exercise well and live a happy and productive life.

The irony is that this might turn out to be the hardest pill of all to swallow.

!ALynne McTaggart