We've had Prozac and Viagra, now there's Xenical, a new anti-fat wonder drug to make us slim, healthy and happy. Rada Petrovic asks if this latest, much-hyped magic bullet will do us more harm than good
A new drug for controlling weight will be available here on prescription by the end of September. As with Prozac and Viagra it's hard not to attribute the tag "revolutionary" to it. But in the same way as these two drugs have energised previously lacklustre areas of pharmacology - depression and impotency - Xenical is set to transform the treatment of excessive weight.

Drugs for obesity have not progressed much since amphetamine-based pills were launched in the Fifties. Even the two most recent, enthusiastically received drugs - Ponderax and Adifax - were another spin on the same tune, acting in old-school style on the central nervous system to suppress appetite. Both these drugs were withdrawn in the UK last Autumn due to fear of associated heart and lung problems.

The difference with Hoffmann-La Roche's Xenical is that it's the first effective weight-loss drug that doesn't act on the brain. Instead, it works locally in the intestine to block the absorption of fat by 30 per cent. As the drug is not absorbed into the body, but passes out with the fat to which it binds, side-effects and long-term health risks seem negligible.

In trials, obese patients followed a moderately reduced calorie diet, shaving 600 calories off their maintenance quota and restricting fat content to 30 per cent - the level advocated by the World Health Organisation as ideal. Gastro-intestinal side- effects such as oily stools and abdominal cramps became less frequent as the trial progressed, suggesting that with time the bowel becomes more tolerant to the drug.

Taking in too much fat, however, dramatically magnifies these side-effects to the point of severe diarrhoea. Whether intentional or not, Xenical exerts a sort of aversion therapy which neatly deters users slipping from their controlled-fat regime. "Patients learn that there are consequences to eating more," said Dr Jules Hirsch of Rockefeller University. Other side-effects are a reduction in the absorption of fat-soluble vitamins A, D, E and K, and a possible increased susceptibility to breast cancer. Fear of cancer has stalled Xenical's US launch, but evidence has been dismissed as inconclusive by the European licensing board.

The big question, of course, is how much can you expect to lose? In the first year of the trial, Xenical patients lost an average 10.3 kg, roughly 10 per cent of their body weight, as opposed to a diet-and-placebo group who lost on average 6.1kg. A 10 per cent reduction in body weight is enough significantly to reduce the risk factors associated with obesity - the ominously termed co-morbid diseases such as hypertension, heart disease, diabetes and some cancers. The unpalatable medical fact is that every extra pound you gain between the ages of 30 and 62 increases your chances of dying prematurely of one of these conditions by one to two per cent.

These diseases are becoming more prevalent as the world gets fatter. You probably know that one third of Americans are classed as clinically obese, and may shrug it off as payback for that transatlantic zeal for giant-sized Cokes, burgers and French fries. But 13.8 per cent of British men and 17.3 per cent of women are officially obese - weighing 20 per cent or more above their ideal. These figures are predicted to reach US levels in just one generation. Even developing countries are noting an increase in obesity as they get sucked into the cushioned ways of Westernisation. Estimates have pitched the cost of obesity-related diseases at three to seven per cent of total healthcare costs - enough to prompt the WHO to declare that obesity is "a timebomb waiting to explode", and our Government to set drastic targets for reduction by 2005. Rather unglamorously, it seems that fat, not Aids, is going to be the blot on the shiny new landscape of the 21st century.

Xenical has landed in this charged medical arena, and is already being hyped as a magic bullet. The prestigious medical journal, The Lancet, described it as "a milestone in the effort to treat obesity effectively". Another medical magazine, Pulse, took a rather more popularist, and prescient, line by speculating that it will be as hotly in demand by non-obese women as Viagra is by non-impotent men.

The fact is that most of us are engaged in a lifelong battle to control our weight, and, at first sight, a drug like Xenical seems to provide the answer. Surveys have shown that virtually all women want to weigh 7-l0lbs less than their weight. Men are catching up in the obsessive stakes, too. We all want to be thin and stay thin, and in chasing that goal we contribute millions of pounds each year to the slimming industry. A big slice of that revenue goes to the pharmaceutical companies. Analysts at brokers Lehman Brothers forecast that Xenical will pull in annual sales of $lbn in the US alone by 2007.

In theory, there is no watertight logical reason why Xenical can't be used effectively in moderately overweight or even normal-weight people. As Lyndel Costain of the British Dietetic Association pointed out: "In a normal-weight person it shouldn't do anything different. It can't recognise if it's acting on an obese or a normal bowel." According to Lyndel, on 1,800 calories a day a normal-weight woman would save 200 calories and lose about half a pound a week. Although I hate to say this, that's just less than your average chocolate bar. Strangely, there have been no studies of its effect on people who are not obese. You would have thought that Roche, like the rest of us, would have been curious.

Paradoxically, Roche's New Zealand ad campaign has been vociferously criticised for targeting the general public despite being a prescription- only drug. TV ads featured "attractive, but very overweight New Zealanders who want to take control of their lives and lose weight". There has been pressure on GPs from non-obese people to prescribe the drug, and a proposal to review drug advertising standards by the health minister.

Despite accusations of disingenuousness, Roche are emphatic that Xenical is licensed specifically for obese people - those with a body mass index of 30 and over. (To calculate your BMI divide your weight in kilos by your height in metres squared. Anything under 25 is fine: above that you're on a sliding scale to potentially life-threatening fatness.) They also stress that Xenical is meant to be used in conjunction with their Medical Action Plan - a weight-management programme containing diet and exercise guidelines which will be bundled with the drug. "We see it as the cornerstone of a long-term programme," said Jenny Milne of Roche. "You need to make changes in your lifestyle in combination with Xenical. It's not designed to be used on its own."

However, Susan Jebb, a consultant scientist who worked on the Medical Action Plan, seemed to divert from this straight and narrow PR line: "You don't absolutely need Xenical. If people follow the plan they will lose weight. But if they take Xenical as well they'll lose more weight." Which begs the question: why take it in the first place, if you can achieve your goal over a longer period of time without the added chemical fallout? "The deepest irony about Xenical is that it needs to be used in conjunction with a diet," says Shelley Bovey, author of The Forbidden Body; Why Being Fat Is Not a Sin, and a leading spokesperson on the issue of obesity. "On a conventional diet you'd normally expect to lose more than 10 per cent of your body weight in a year. Ten per cent sounds fine and is a very sensible way to lose weight, but you don't need a drug to do that."

Xenical was, however, effective at staving off the bete noire of any weight-reduction programme: regain. Anyone who has lost weight through dieting will know that you're only as successful as your last weigh-in, and in 95 per cent of cases that means failure. Success depends, too, on long-term monitoring which Roche hope will come from GPs. This is both an optimistic and naive assumption given the usual strapped-for-time schedules which most GPs work to. Roche are planning to launch a helpline, although this will reiterate the product information in its leaflet rather than give the motivational boost that users would be seeking.

Patients may do better in terms of follow-up if they have been prescribed the pill by a private practice or slimming clinic. Ironically, it's also in these clinics, outside the tighter rein of the NHS, that the drug may be misprescribed. Legislation allows slimming clinics to distribute anti- obesity drugs, but some of these clinics aren't overly concerned about restricting supply to the target group. "We can't control Xenical's distribution via our wholesalers," concedes Jenny Milne. "But we'd like to make sure that it's prescribed to the right patient with the right support."

The reality is that Xenical will soon be in the public domain and accessible to anyone who can pay for it and has the determination to track it down. Like Viagra, it's probable that Xenical will find its way on to the black market. Alarm bells are already going off; the Eating Disorders Association is drafting a press release warning that Xenical could pose a danger to anorexics who can't afford to reduce fats and fat-soluble vitamins in their diet.

The fundamental question anyone who is contemplating Xenical should ask themselves is how long they are prepared to take it, and what will happen when they stop. After all, Xenical may train you to curb your desire for doughnuts, but it won't reveal why you desperately want to eat three in one sitting. Indeed, the real danger is that patients will shift their psychological dependency from food to the drug itself. "If people use this drug," says Shelley Bovey, "they will come to see it as a crutch and will be terrified of letting go of it, especially if they have a history of failed diets, which most women do. Xenical will be seen as the magic element."

The reason why new-generation drugs like Prozac, Viagra and now Xenical are so emotive is because they touch on our deepest neuroses - unhappiness, sexual inadequacy and lack of allure - and make us feel empowered against them.

The catch-22 comes if, or rather when, you have to readjust to life without them. For Prozac users this means, instead of springing, dragging yourself out of bed each morning. For Viagra users it's a return to the frustrations of middle-aged sex. For Xenical-users, it's the dread of returning to your old, fat self, together with all the indignities, social disapprobation and medical scorn you used to suffer. The bitter pill we all need to swallow is that these drugs, once taken, are for life. That is, at least for the life we've grown accustomed to living.