In 1995, sales in the United States of behavioural drugs - which include anti-depressants, anti-psychotics, hormone therapies and stimulants - were valued at $6bn. In 2002, they were worth $23bn - an almost fourfold increase in eight years. And in the big five European countries - Germany, France, Britain, Italy and Spain - during the same period, sales of such drugs more than doubled to $7bn. This vast growth in sales of drugs that modulate our behaviour and our sense of self has turned the pharmaceutical business into the world's most profitable since well before the 1990s.

Two developments, subtly but powerfully related, have changed the nature of our relationship with prescribed drugs forever. The first is what David Healy, historian of pharmacology, calls "the medication of risk". Where in the past people became ill and were then prescribed drugs, today they are medicated to prevent illness. This is an extraordinarily profitable enterprise. Last year, the world's top-selling drug was an anti-cholesterol agent called Lipitor, which brought in revenues of just under $8bn, more than any drug in history. Its main competitor, the world's second-biggest selling drug, Zocor, reported sales of more than $6bn in 2002. Although cholesterol medications are intended only for those who have tried and failed to control their condition with diet and exercise, more than 10 per cent of US adults and a rising share of Europeans are taking or have taken them.

The second development, the one that this article addresses, is the vastly increased role for compounds that modulate emotions and behaviour. To anxiety and sleeping pills has been added the commercial colossus of depression, which generated worldwide sales of $16bn in 2002, and therapy areas such as attention deficit/hyperactivity, hormone replacement and sexual dysfunction. Each is worth billions. Products for three elusive goals - weight loss, smoking cessation and memory enhancement - are on the horizon, as are medications for nervous states such as social anxiety disorder, generalised anxiety disorder and, in children, oppositional defiant disorder. Drugs to hit these "targets" (a favourite industry term) will come to market in the next two to four years.

The behavioural drugs industry is being both pushed and pulled forward. Shareholders expect pharmaceutical companies to return an exceptional 17 or 18 cents on each dollar, so companies tend to focus on medicines that people will take for long periods and that treat conditions with elastic diagnoses. Origination of the drugs is now cheaper: start-up biotechnology companies conduct research and then license their developments to big corporations. The real competitive fight among the multinationals is in selling medicines in "blockbuster" quantities - products with sales of more than $1bn a year - before their patent protection expires, usually within a decade of launch.

These corporations play to two audiences: doctors, who are under increasing pressure from governments and insurers to reduce their (relatively expensive) time with patients while budgets for (relatively cheap) drugs increase; and consumers - no longer mere "patients" - who are alerted to their full universe of potential ailments by public education, lifestyle journalism and corporate advertising, and who demand maximum access to such treatments. Even in Europe, the days of the meek and easily gratified National Health Service recipient are long gone. Pharmaceutical companies have responded by offering medicines that increase doctors' patient throughput and go as far as possible towards meeting consumers' demands for complete healthcare - typically the behavioural drugs that already account for between 10 and 50 per cent (as in the case of Eli Lilly, originator of Prozac) of the revenues of big pharmaceutical companies. And the business is growing fast. "This decade," Healy predicts, "is about lifestyle drugs."

Some argue that the terms "lifestyle" or "quality of life" drugs insult those who suffer from pathological conditions such as depression, schizophrenia, obesity or compulsive behaviour, so we will use the term "behavioural drugs". They have three defining characteristics - first, behavioural medications treat diseases and disorders that depend largely on a doctor's judgment. As Dr Jim Kennedy, chair of the prescribing committee of Britain's Royal College of General Practitioners, puts it: "There's no simple blood test for anorgasmia or for attention deficit." Where there are physical symptoms of a behavioural condition - as with obesity - it is also a highly subjective judgment as to whether the condition is pathological or self-inflicted. Second, and perhaps most striking, behavioural drugs in clinical trials always produce a significant effect in placebo. Finally, although behavioural drugs are marketed with the conviction and scientific certainty that modern sales techniques require, their mode of action is poorly understood. In private, industry researchers concede they don't know how most of the drugs work. Many have been discovered by accident - most famously, Viagra was developed to treat chest pain but turned out instead to give men erections.

The behavioural drugs era began with the first anti-psychotic - called Largactil in Europe and Thorazine in America - in 1952. The compound was given to thousands of acutely afflicted patients, particularly schizophrenics, who were locked up in asylums in Europe and north America. It helped many return to normal life. Yet when Geigy (now part of Novartis) came up with the first anti-depressant - later branded Tofranil - it dithered for years about whether to develop it. There was no captive market in institutions, and the industry consensus was that depression was an uncommon ailment unlikely to respond to medication. But industry misread the market. People badly wanted depression drugs, and in 2002, the US ranked five anti-depressants among the nation's 20 best-selling medications.

The story was similar in what is currently the most controversial therapy area in behavioural drugs: attention deficit hyperactivity disorder, or ADHD, in children. Ritalin, the drug popularly associated with this condition, was patented more than 20 years ago when the drugmaker, also now part of Novartis, believed its potential market to be tiny. Jo Ellins, who is researching the history of three behavioural drugs - Ritalin, Prozac and Viagra - says: "Novartis almost didn't promote Ritalin... Ritalin has been much more driven by parents and schools." The boom in Ritalin prescriptions in the US coincided with a period in the early 1990s of expanded class sizes, universal rejection of physical punishment of students and special funding for schools that identified children with disabilities such as ADHD. More than anything, Ellins dates the the Ritalin phenomenon to what she calls a "whispering campaign" among parents about what it achieves: it calms down difficult-to-manage, sometimes impossible-to-manage, children. Andrea Bilbow, who runs the industry-supported National Attention Deficit Disorder Information and Support Service (ADDISS) in Britain, sums up the proposition for parents who feel they have lost control of their usually male offspring: "What would you rather do, beat your children into a pulp or give them medication?"

Stressed parents wanted Ritalin as other people wanted anti-depressants. Novartis had opened up a market - largely (since by that time Ritalin was nearly out of patent) for the benefit of other drug manufacturers. One company to benefit was the UK-based Shire Pharmaceuticals Group, whose extraordinary story is the real one of the 1990s: not that Big Pharma invented the behavioural drugs market, but that having identified the demand, it began to sell its wares in a much more... well, clinical, fashion.

On a sunny Wednesday morning in the Hampshire International Business Park, Angus Russell, Shire's chief financial officer, recalls how his London commuter-belt company revolutionised the manner in which stimulants are marketed to children in the US. Shire was a medical marketing company that in 1997 bought the rights to a failed juvenile obesity drug w that researchers had noticed had a calming effect on children. The compound, which works like Ritalin to enhance concentration, was repositioned as a treatment for ADHD. Whereas Ritalin had to be taken through the day, however, Shire brought in proprietary slow-release technology that meant its product, Adderall, need only be taken before and after school. As a result a child's peers would not know they were taking drugs, and schools need not have substances on their premises that could be abused. "The big selling proposition," says Russell, "was you can avoid the midday, in-school dose." More important, though, was the manner in which Shire sold the product in the US. It gathered together all available data on the 180,000 US-based clinical psychiatrists, paediatricians and general practitioners who had prescribed an attention deficit drug. By using information about prescribing histories that only became computerised and commercially available in the 1990s, it was able to identify a sub-group of 27,000 doctors who had written 80 per cent of ADHD scrips. Within this group tiers of American physicians were selected according to the volume of drugs they prescribed. A sales strategy was mapped out accordingly. "We literally treat it as a pyramid," says Russell. "The first 1,000 physicians probably prescribe 15 per cent of the market. The top 1,000 get 35 visits a year."

During those visits, Shire's 360 sales representatives stick to a tight script about the merits of their product. Busy doctors may spare them only a couple of minutes. "A lot of people say you have to have scientifically trained people," Russell says. "Our head of sales says: 'Just give me sales guys.'" The strategy has been phenomenally successful. Among the top 1,000 ADHD prescribers in America, Shire claims a 40 per cent market share by value, compared with 23 per cent overall. Back in 1994, Shire's venture capital backers decided to take the company public, setting a target valuation of $500m by 2000. That year Shire was worth $8bn and today it ranks as one of the top 100 UK listed companies. About 700,000 children of the three to four million in the US on ADHD drugs are taking Adderall - a product that is not sold in Europe, where the company deems it too early to begin promotion. In 2002, Shire produced net earnings of more than $250m from revenues of only just over $1bn.

Medication of ADHD in the US is growing faster than ever - by 18 per cent in 2003. "We've driven that market," says Russell proudly. Two giants of the behavioural drugs industry, Eli Lilly and Johnson & Johnson, recently launched new ADHD drugs, which suggests they believe the business is far from saturated. Shire is about to launch Methypatch, which uses the same slow-release technology employed in nicotine patches but instead delivers methylphenidate, the off-patent compound behind Ritalin. The Methypatch can be applied for a few hours if a dose of medicine is forgotten, avoiding the need to give a child a pill late in the day that will keep it awake at night (as stimulants do).

The biggest prize on the horizon, however, is adult ADHD. In 2002 Eli Lilly's ADHD drug Strattera was the first to receive approval from the US Food and Drug Administration (FDA) for adult attention deficit. Shire has applied to market Adderall to adults in America in a country it claims has at least eight million potential patients over 18, versus at most four million ADHD-diagnosed children. Moreover, unlike children, adults can present themselves for evaluation - a factor that is likely to accelerate the already stunning growth of the business.

The Shire story illustrates the broader marketing revolution that took place in the pharmaceutical industry in the 1990s. The number of sales representatives in America trebled during the decade to 90,000, with a similar trend in Europe. Jean-Pierre Garnier, chief executive of one of the largest pharmaceutical companies, GlaxoSmithKline, told the Wall Street Journal recently that his sales machine is now so big it can reach the prescribers of four-fifths of all drugs in the US within a week. The industry's conferences have become vast jamborees to which doctors are invited on an all-expenses-paid basis. The European College of Neuropsychopharmacology (ECNP) Congress in Prague last September attracted 3,000 psychiatrists - less than half the size of the lodestar conference, the annual gathering of the American Psychological Association, held each May. In Prague, Nancy Andreasen, editor of the widely read and supposedly independent American Journal of Psychiatry, signed copies of her new Introductory Textbook of Psychiatry on the Eli Lilly stand. For four days there were sponsored lectures about how to cure behavioural conditions with drugs, at which the speakers did not state how much they were being paid to speak or by whom. Posters were displayed by non-industry scientists highlighting the results of breaking research which, the small print usually advised, was sponsored by industry. Game show techniques are creeping in. Rising behavioural drugs star Sanofi-Synthelabo of France invited psychiatrists to undertake the "Solian quiz". They had to answer 10 questions, most of the answers underpinning the use of Solian (an anti-psychotic drug): depending on score, they walked away with a desk lamp or a modish French radio bearing the Sanofi logo.

The doctors this correspondent talked to said they were much too clever to be taken in by this kind of thing. But it would be curious if the industry bankrolled these conferences without shifting product. One of the best allegories for the trends in behavioural drug marketing is the development of sales strategies for treatments for erectile dysfunction. Viagra's first sponsored spokesman was the grandfatherly former US presidential candidate Bob Dole, who suffered prostate cancer, a condition closely associated with impotence. However, as the scale of the potential market for such a product became apparent, Rafael Palmiro, a Texas Rangers baseball star, took over, and the message changed. "They were saying: 'He hits it out of the park,'" says Katharine Greider, author of The Big Fix, about pharmaceutical marketing. Today, the message has moved on again. One of Viagra's two new competitors, Levitra, is running television advertisements where a young couple are fooling around amorously in their yard. The man hurls a football through a suspended tyre with striking accuracy. The punch line follows: "When you're in the zone, it's good."

Big Pharma, though, can't sell us things we do not want. We possess an irrepressible belief that chemical solutions exist to life's problems: this is what makes the business so big. Gerard Le Fur, the long-standing research director of Sanofi, has a vision of where the industry is going and how much bigger it might become. His company, based at art-laden headquarters on Paris's left bank, has a hotly tipped drug in final stage trials that works on the brain's cannabinoid receptors - those excited by marijuana. It may be a harbinger of a brave new world to come.

Fifteen years ago Sanofi was among the first companies to begin artificial cloning of "receptors", the molecules that occur in the brain and other parts of our bodies that affect mood, emotion and behaviour. Receptors are the chemical "targets" that can either be hit to increase their excitation with an "agonist" or blocked, to decrease their activity, with an "antagonist". The research game is to identify a target, shoot it or block it with a new compound, and thereby modulate human behaviour. In the 1980s the big multinationals invested heavily in automated and computerised systems that allow endless testing in the laboratory, without human trials, by hitting a cloned receptor with every imaginable dose and variant of a compound. "High throughput screening" has not generally produced the breakthroughs the industry predicted - because, say some scientists, it is a grossly simplistic approach to something as complex as human behaviour - but it may have worked for Sanofi.

Le Fur decided early on that he wanted to clone the main, cannabinoid-1 receptor that occurs in our brains and guts. Aware of the cannabis "munchies" effect that leads some people to gorge themselves when high on marijuana, he wondered what a blocker would do. "It's well known that marijuana smoking stimulates appetite and we just wanted to have the opposite," he says. Le Fur was also intrigued by the effect of marijuana on schizophrenia, a condition it can exacerbate by causing hallucinations. He thought there might be therapeutic benefit for schizophrenics in blocking the cannabinoid receptor.

With the isolation of a compound now branded as Rimonabant the schizophrenia hunch proved to be wrong. But in animal and early human trials, obese rats lost weight. Not only this, they shed weight in part by losing their interest in fatty, unhealthy foods while maintaining their desire for adequate protein. The drug also appeared to reduce pleasure derived from - and hence appetite for - nicotine and even alcohol. "Through blocking the cannabinoid receptor in animals we got blocking on smoking, obesity and alcohol," says Le Fur. The findings on smoking and weight loss were confirmed by human trials. Final-stage trials with 12,000 participants for smoking cessation and obesity will complete next year.

If the trials end successfully, Sanofi will have a drug for weight loss that may work by making us eat more healthily. It will have a drug for smoking. It could have a drug - trials are at an earlier stage - for alcohol dependency. Above all, Sanofi would have a drug for giving up smoking and losing weight at the same time. It would be a fantastically attractive commercial proposition. Asked to tip a winner among near-term behavioural drug pipelines, Nigel Barnes, a pharmaceuticals analyst at Merrill Lynch, offers simply: "Sanofi".

But the very scale of such potential success underscores why medical ethicists, police and parents are wringing their hands about diversion and abuse of behavioural drugs. We live in a world where Viagra - which even Healy, a fierce critic of the behavioural drugs industry, says was a breakthrough, life-changing medication for genuinely impotent men - is sold in nightclubs. Children steal Ritalin and Adderal to crush up and snort for party fun or as an exam-cramming aid. Anti-depressants, particularly selective serotin reuptake inhibitors (SSRIs) such as Prozac, are taken by people who could do without them. The earliest of a new generation of memory-enhancing drugs for Alzheimers are already being diverted in significant quantities in the US to people seeking a cognitive upgrade. Rimonabant, if approved for sale, will doubtless be used by slightly overweight people who want to shape up and quit smoking for a couple of months. As Le Fur puts it: "I'm a scientist. I work with pathological states. But I can't say that Rimonabant will only be given to obese patients." The internet has aided this diversion of drugs - everything from Viagra to Prozac is available on the web, dispatched in discreet packaging with no questions asked. The trade has proved impossible to regulate. It is unsurprising that there are those, such as the social critic Francis Fukuyama, who point to a new "Age of Aquarius" in which the drugs are supplied not by hippies, but by Big Pharma.

This may not, however, happen soon. The history of side-effects of behavioural drugs has been ugly. It is only our hunger for chemical solutions that allows us, periodically, to forget this. Henry A. McKinnell, chairman and chief executive of Pfizer - by far the biggest pharmaceutical company in the world and maker of Viagra and Zoloft - concedes: "There are no free lunches in medicine." The story of the benzodiazepines - Librium, Valium, Mogadon and Ativan - that were launched principally for anxiety in the 1960s and 1970s has been so appalling, Healy says, that the pharmaceutical companies dared not even talk to doctors and patients about anxiety for most of the 1980s and 1990s. Instead, in marketing terms, we entered the era of depression. "Companies were forced to market products as anti-depressants as there was such a reaction against benzodiazepines," he says. "In the 1980s the companies would have preferred to bring the SSRIs on as anxiolytics [anti-anxiety drugs], but the benzodiazepine fuss made this difficult. The doctors were awfully twitched."

The doctors were "twitched" over a situation where, for example, in the UK in 2003 - the 40th anniversary of the launch of Valium - half a million people were still long-term dependents of benzodiazepines, drugs now reckoned so addictive that official prescribing guidelines say they should not be taken for more than 28 days in succession. Data from coroners' reports compiled by the Home Office show benzodiazapines a much more frequent contributing factor in cases of unnatural death each year than are cocaine, heroin, ecstasy and all other illegal drugs. Prescriptions of benzodiazepines peaked in 1977 in the UK at 30 million, yet in 2002 there were still 12.5m. The story is the same, only the numbers are bigger, in the US. Alprazolam, a benzodiazepine originally marketed by Upjohn (now part of Pfizer) as Xanax, was the 11th most prescribed drug in America last year, ahead of top SSRIs such as Zoloft and Paxil. Alprazolam does not make it on to the US top 20 drug list by revenues because it is off-patent and hence cheap, but the drug - reckoned by many independent researchers to be among the most addictive in its class - is consumed in massive quantities.

Some of the worst horror stories relate to weight-loss medications. The track record is so bad that the FDA now requires an additional two-year review of patient health after the completion of phase three trials of slimming drugs. This will delay the earliest potential release of Rimonabant from 2004 to 2006. Doctors have been prescribing patients stimulants - often amphetamines - for weight loss for decades with unpleasant side effects. But the FDA changed the rules after the US company Wyeth introduced a stimulant and sedative combination in 1996 called Redux. The drug had an aggressive sales launch and, in the less-than-18 months before its withdrawal, managed to do so much cardiovascular damage to so many people that Wyeth has set aside $12bn to settle legal claims.

More recently Switzerland's Roche, originator of benzodiazepines, launched a weight drug that makes fats pass through the gut without being absorbed. Xenical looked like a blockbuster - until people realised that eating too much fat while on the medication (Roche did warn against such use) causes monstrous diarrhoea. Its sales, in a perfect illustration of our impossible desire to have cake and eat it, are falling precipitously. "Don't wear a white dress to the opera after a fatty dinner on this one," opines one London pharmaceutical analyst.

The past two years have brought to light significant new evidence and concerns about the side effects of behavioural drugs. Large-scale public studies in both the UK and the US have shown that women undergoing hormone replacement therapy (HRT) are twice as likely as other women to suffer invasive breast cancer. Concerns are also growing about the vast numbers of children who have been taking ADHD drugs long-term - in the US, in many cases, for as long as 10 years. Long-term studies of side effects have never been conducted in humans and senior policymakers are beginning to wonder if that is not a dangerous thing. "It is a question I ask myself," says Nora Volkow, the much-respected director of America's National Institute on Drug Abuse and a specialist in brain chemistry. "It's unacceptable in my view." Volkow is an ardent believer that ADHD, properly diagnosed, is a serious illness, but she warns that animal studies are flagging possible long-term impacts of medications on the brain. Three such papers were published in the journal Biological Psychiatry in December: "They highlight the need to do research on this subject," she says.

In the US, lawyers are bringing a gradually escalating number of cases against the makers of SSRI anti-depressants with respect to suicides and other allegedly unmentioned risks. In the handful of cases that have reached a jury verdict, the plaintiffs are winning. In the UK in 2002, a Panorama documentary called "Secrets of Seroxat" (the SSRI that is the market leader in Britain, sold as Paxil in the US) elicited a record public response - more than four million people watched the programme, 124,000 visited its website, 65,000 people phoned the BBC afterwards and nearly 1,400 wrote e-mails describing suicides, attempted suicides and acute withdrawal symptoms. Most worrying, even among those who wrote in support of Seroxat - sometimes saying it saved their lives - two-fifths reported serious problems coming off the drug. Jim Kennedy, from the Royal College of General Practitioners, says, "We keep doing this. We come out with a new drug that's a wonder drug. Then 10 years later the repercussions come back and bite us on the arse."

It is striking that benzodiazepines were marketed as a non-addictive, low-side-effect alternative to barbiturates. SSRIs were then marketed as the non-addictive, low-side-effect alternative to benzodiazepines. The number of anti-depressant prescriptions in the UK - most of them SSRIs - has reached 30m a year, the peak level for the anti-anxiety benzodiazepines.

Charles Medawar, author of a book on benzodiazepines and a strident critic of SSRIs, offers a bleak diagnosis: "The whole thing is happening again," he says, claiming that the addiction and side-effect profile of SSRIs will turn out to be worse than that of the benzodiazepines.

"In qualitative terms I would say that the problems are much more severe," he says. "You are trapped on a [SSRI] drug that has nasty side-effects in weight gain and loss of libido. The side-effects are much uglier than they were with benzodiazepines... GlaxoSmithKline just changed the indicated incidence of withdrawal problems for Seroxat from one in 500 to one in four." In December the UK government banned the prescription of most SSRIs for depression in children after finding that manufacturers had failed to publish trial data showing increased risk of suicide; a government review of SSRI side-effects in adults continues.

Americans find the pessimism about drug use as depressing as the conditions the medications seek to cure. "Look at Japan," huffs Pfizer's Henry McKinnell. "They have the lowest anti-depressant use and the highest suicide rate in the [developed] world." In Britain, Professor Louis Appleby, national director for mental health, concurs. He says suicide rates have reduced in the UK in the past 15 years while in countries that have made sudden switches to SSRI drugs, such as Sweden and Hungary, there is incontrovertible evidence that suicides have fallen as a result. "The main classes of drugs in mental health have stood the test of time," he says, "because their benefits have been greater than the problems they cause."

Sales trends suggest that US versus European differences of opinion over behavioural drugs will continue to see Americans consume more than Europeans. But there is one point on which everyone - chief executives, doctors, pundits and consumers - on both sides of the Atlantic is agreed: people would prefer non-pharmaceutical alternatives to cope with their problems if they were available. Britain's National Association for Mental Health (Mind) conducted a survey in 2002 which showed that 98 per cent of people visiting a general practitioner regarding mental health issues were given medication whereas less than 20 per cent asked for it. Most people said the health system was not giving them enough choice while many who sought alternative therapies had to find practitioners themselves and pay for private treatment.

Britain's public health system has begun to recognise this basic truism. A restructuring of primary healthcare has, according to Appleby, been accompanied by a 50 per cent increase in the number of clinical psychologists since the mid 1990s, while half of general practices in the UK have counselling services. In the US, the situation is necessarily different. In general, the private health system delivers the best healthcare in the world for those who have insurance and something closer to third-world cover for those who do not. But for all users, the US system tends to militate against non-drug therapies in primary care. As McKinnell says, Americans get an average seven minutes of their GP's time per visit. "Our system creates an ironic incentive," says Katharine Greider. "You're under pressure from insurers to keep costs down but there's a contradictory pressure to prescribe. Writing a prescription is an office-terminating event. It signals to the patient: 'It's time to go away now'."

The US system is unlikely to undergo fundamental change in the short term. In the 1990s, Bill Clinton tried and failed. But in the US and in Europe there are increasingly vociferous demands for more information about the drugs we consume. The structure of the contemporary pharmaceutical industry means that drug companies develop drugs, organise clinical trials - via physicians they and their agents select - and then submit the results for approval to the FDA (whose decisions tend to be followed by other regulatory agencies). The companies' ownership of trial data - defined as commercially critical information - allows them considerable influence over the market. Mostly this occurs by omission: companies tend not to release trial results that do not support their products unless they are essential for regulatory approval. "The FDA," says Healy, "audits rather like Arthur Andersen audited Enron. They haven't got the resources to go further."

Healy points to the existence of quality of life (QOL) scales, used for internal research by all the big pharmaceutical companies. QOL scales employ more, and more diverse, data than are required by regulators. They aggregate reports from clinicians as to whether a drug remedies a specific problem and whether, on balance, a drug has a positive impact on a person's overall well-being. Critically, they also require data from patients as to whether they believe a medication works and, again, whether it has improved their life overall. Healy says he has identified almost 100 SSRI anti-depressant trials in which QOL scales have been compiled but less than a 10th have been made public. "In the quality of life scales that have been published," he says, "the SSRIs come out behind older drugs."

Few believe the behavioural drugs industry is a conspiracy to defraud the public, but it is a profit-seeking business that has shareholders as well as patients. In this context it is curious that many expect higher standards of pharmaceutical companies than they do of other corporations - it might be more realistic to take the view that as behavioural science brings us new possibilities in mental as well as physical health, personal choice is more important than ever and consumers should be fully informed.