Once it was cocaine, speed or heroin, but now the fashion is for legal pills, washed down by spirits. Last week's news that actor Heath Ledger, right, died from an overdose of prescription tablets shed light on a startling new trend - misuse of over-the-counter pills now kills more Americans than illegal drugs. Elizabeth Day in New York

Alex is a man who prides himself on sticking to routine. He likes to start the day with a large cappuccino from Starbucks and to end it with a handful of anti-depressants washed down with vodka. 'It's my treat after coming home from work,' he says. 'I guess it just chills me out a little.'

In many ways Alex, 31, is a typical well-heeled young New Yorker. He works in finance, holidays in the Hamptons and enjoys partying at the sort of exclusive nightclubs where having your name on the guest list is a prerequisite to entry. He also likes to get high on prescription drugs.

Tonight he is celebrating a friend's birthday at Marquee, one of the city's hippest nightspots. The main bar, lined with leather banquettes, is cast in a shadowy half-light. In the upstairs lavatory there is a small framed sign on the back of the door reminding guests the use of illegal drugs will not be permitted.

But Alex would not consider himself a drug abuser. For him, those small white Xanax tablets on his bathroom shelves are simply a recreational accompaniment to the $15 Grey Goose vodka martini he has just been served. And, what's more, they're entirely legal.

Over the past five years the United States has seen a ferocious increase in prescription drug abuse. According to the 2006 National Survey on Drug Use and Health, 49.8 million Americans over the age of 12 have reported non-medical use of illicit drugs in their lifetime, 20.3 per cent of the population. Among teenagers aged 12-17, prescription drugs are second only to marijuana in popularity, and in the past 15 years there has been a 140 per cent increase in painkiller abuse. It is the fastest-growing type of drug abuse in the US. Even more worryingly, prescription drugs have made it on to the party scene as a legal, seemingly safe, way to recreate an illicit high.

Until last month this was a largely silent epidemic. But the death of Heath Ledger, a regular at Marquee and other nightclubs, thrust it into the spotlight. The 28-year-old actor died from 'acute intoxication' caused by an accidental overdose of anti-anxiety medication and prescription painkillers.

'Americans love to get pills for everything that ails them,' says Dr Howard Markel, a professor of paediatrics and psychiatry at the University of Michigan. 'The misuse of those drugs has become one of the major health problems of our time.' The UK has less of a prescription culture than the US, although many experts believe the advent of internet pharmacies means it is only a matter of time. In the US, where pharmaceuticals are advertised on prime-time television, pill-popping has become normalised, a socially acceptable means of alleviating stress, sleeplessness or anxiety.

The most commonly abused prescription medications fall into three categories: opiate-based painkillers (OxyContin and Percocet); central nervous system depressants prescribed for anxiety and sleep disorders (Valium and Xanax); and stimulants, used to treat attention deficit disorders (Ritalin and Adderall).

Within these categories, the pharmaceutical industry has provided a full set of substitutes for just about every illegal narcotic available. Methylphenidate, the active chemical in Ritalin, targets the brain's pleasure-producing centres in the same way as cocaine. Antidepressants can act as serotonin-boosting 'uppers'. A few years ago OxyContin, an extremely powerful painkiller developed for cancer patients, became known as 'hillbilly heroin' after an epidemic of abuse took root in poor rural communities.

Such mishandled drugs now kill 20,000 a year, nearly twice as many as 10 years ago.

Dependence on legal drugs is not a new problem - during the American Civil War morphine abuse was labelled 'the soldiers' disease' - but the practice of prescribing drugs has metamorphosed from a medical treatment of last resort to a way of life. 'The problem has been greatly worsened by the internet, and that affects all countries - including Britain,' says Susan Foster, of the National Centre on Addiction and Substance Abuse at Columbia University, New York. 'As long as you have a credit card, anyone can log on and have potentially lethal drugs delivered to their door. You don't even need a prescription. You have what's called an "online consultation" where you are asked how old you are, how bad your pain is.'

The substances most commonly traded over the internet are tranquilisers such as diazepam and stimulants like Ritalin. However, the most dangerous are the opiates, which include codeine and morphine.

The painkiller fentanyl can act like heroin and traffickers get hold of supplies by forging stolen prescriptions, breaking into pharmacies and stealing stocks or buying the drugs from patients who have been prescribed it. Another opiate painkiller, buprenorphine - prescribed for heroin addicts trying to kick their habit - is peddled in countries as diverse as India, Iran, Finland and France. From 2001-05, the global consumption of buprenorphine more than tripled to 1.5 billion daily doses.

Doctors are woefully ignorant of the dangers; a 2005 study by Casa found that 43.3 per cent of them did not ask about drug abuse when taking histories. Even if they do, the seasoned drug abuser will go from one doctor to the next until they get the quantities they want - a practice known as 'doctor-shopping'.

This was Jeana Hutsell's experience. A petite 35-year-old from Canton, Ohio, with cropped peroxide blonde hair and square-framed glasses, Hutsell became hooked on Percocet, an opiate-based painkiller, when she was prescribed it 12 years ago after an operation for Crohn's disease. 'I went to the doctor with abdominal cramps and he began writing me copious prescriptions,' she says. Within a year, her habit had escalated to 60 pills a day and she was sewing emergency stashes into the lining of her handbag. 'I felt they gave me personality. They made me chattier, friendlier.'

Hutsell began forging prescriptions, sometimes walking into hospital casualty departments over the weekend and saying she had run out. 'I felt justified and safe because my doctor was giving them to me. I wasn't getting them on the streets - I was going to a pharmacy.'

Whereas illegal street narcotics - heroin or crack cocaine - are more likely to be used by the poorer socio-economic classes, prescription drugs have become the preserve of the rich. In the privatised American healthcare industry, these pills do not come cheaply: an antidepressant like Wellbutrin can cost from $1,000 to $2,400 a year.

Wealthy individuals also enjoy the luxury of paying private physicians - known as 'script doctors' - to provide them with prescriptions. And often, because the drugs are viewed as performance-enhancers, they will be taken by those at the higher end of the social strata: by the college students and Wall Street traders. In the 1980s cocaine was the glamour yuppie drug. Now, the line of white powder is being overtaken by the little white capsule.

Phoenix House is a tall, grey stone building on the Upper West Side, a former 19th-century hotel with mosaic-tiled floors in the hall. The genteel appearance belies its gritty purpose: Phoenix House is a rehabilitation centre for drug and alcohol abusers, treating 6,000 people a day. In recent years, such centres have seen a substantial increase in prescription drug admissions - some counsellors say that they account for 90 per cent of new patients.

Professor David Deitch, the chief clinical officer, does not want to use the word 'epidemic', but he concedes that 'the genie is out of the bottle'. 'You see prescription drug abuse in the same circles that you saw cocaine abuse - the high-performing executive class. They might have a big day, so they take some something to get to sleep. Then they'll take another pill the next morning to enhance their performance. Then they'll go out and use all kinds of drugs at a party, and then to recover from the party the next morning they'll take a different pill. It's pervasive.'

Celebrities who have admitted their own struggles with prescription medication include Elizabeth Taylor, the talkshow host Rush Limbaugh, and Cindy McCain, the wife of the Republican presidential candidate John McCain. More recently, there have been rumours that Britney Spears has been self-medicating. The impact has percolated down to impressionable adolescents. One of the most popular forms of recreation among high-school students is the 'pharm party'. Teenagers raid their parents' medicine cabinets, then pool their resources. 'You throw your drugs into a bowl in the middle of the room, then people pick pills out and chase them with alcohol,' says Susan Foster. 'We've seen these internet recipe sites where you go online to find out how to mix drugs for a certain effect. You can trade drugs online - in fact, at one college the students reported that they had a prescription drug trade forum on the university website.'

Markel tells the story of one of his patients, a 16-year-old student called Mary, who liked to down a few tablets of OxyContin with a single shot of vodka. She called the combination 'the sorority girl's diet cocktail' because it gave a stronger kick of inebriation with fewer calories than alcohol alone.

'There's a cachet to this sort of drug abuse, encouraged by the Paris Hiltons and the Lindsay Lohans going into rehab, so it becomes a really cool druggy, party culture,' Markel says. 'Now teenagers don't want to smoke and drink, they want to take a pill because it's so easy to get and some of them can really make you feel good.'

But it is easy to overdose on prescription drugs, partly because your consciousness is impaired and it can be difficult to remember how many you've taken, and partly because mixing medication without specialised knowledge can produce fatally toxic results. And however legal these drugs might be, their misuse carries the same consequences as illegal narcotics: the familiar, dispiriting tale of the addict losing their family, friends, job, home and, sometimes, their life. After two years of Percocet addiction, Jeana Hutsell took stock of the wreckage her life had become: 'I was homeless, I didn't have a car, my family didn't like me. I realised that I was the cause of all my problems. That was the turning point.'

Others are not so lucky. Randy Colvin, an abuser of Valium, Xanax and Percocet, died of a drug overdose on his 35th birthday. 'We tried to save him and we lost,' says his older brother Rod. 'For 15 years we tried to get him into treatment and each time he would be in denial, he would be furious with us. My mother and I even tried to get a court order so that he could be sectioned. We did everything we possibly could. Addiction is a family disease. His death was very painful. '

For Heath Ledger's parents, the grieving process is still in the rawest stages. Their son cemented his fame for reasons that were nothing to do with his talent. Instead he is for ever associated with a seedy death on the floor of a Manhattan apartment, just one more victim of the pill-popping epidemic that has become America's secret illness.

Danger Zone

Painkillers

Popular brands include Nurofen and Solpadeine, which can prove addictive.

Anti-anxiety drugs

Tranquillisers have been involved in numerous recent fatal overdoses.

Sleeping tablets

Users can become dependent in just two weeks. Ledger was taking Restoril.

Anti-depressants

Prozac withdrawal symptoms are common and can be physically painful.

· The following correction was printed in the Observer's For the record column, Sunday February 17 2008. Above, we identified Xanax as an anti-depressant. Xanax is prescribed for generalised anxiety disorder, anxiety associated with depression and panic disorder, but it is not used for the treatment of depression.