Not long ago, for reasons I still don't understand, I began to feel unfocused and lethargic. I try to eat properly, exercise regularly, sleep peacefully, and generally adhere to the standard conventions of fitness. It didn't seem to be working. My doctor found nothing wrong and my blood tests were fine. Still, I felt strange, as if I were lacking in energy – or in something. So I did what millions of Americans do every day. I sought salvation in vitamins.

First, though, I had to figure out what variety of salvation to seek. There are many thousands of pills, potions, powders, gels, elixirs, and other packaged promises of improved vitality for sale within just a few blocks of my home. I walked to the closest vitamin store, a place called The Health Nuts, and told the proprietor I was feeling sluggish. To counteract my deficit of energy, he recommended a supplement of glutamine, which is one of the few amino acids that passes the blood-brain barrier. When people are under stress – physical or psychological – they begin to draw down on their stores of glutamine. A leaflet attached to the bottle described the amino acid as a magical aid for mental acuity. ("It is helpful with focus, concentration, memory, intellectual performance, alertness, attentiveness, improving mood, and eliminating brain fog and cloudiness.") I dropped it into my basket.

The store also had a garlic section – not actual garlic, but various pills with names like Kyolic and Garlicin, GarliMax and Garlique, all of which claimed to possess the healing properties of garlic, which for centuries has been thought to help ward off the common cold, clear up respiratory infections, and soothe sore throats. Garlic, its advocates claim, is also effective in treating heart problems, lowering cholesterol, and keeping arteries free of blood clots. I grabbed a bottle and moved on to the main supplement section, where multivitamins in every conceivable size, shape, dosage, strength, and formulation were lined up in rows. (There were vitamins for vegans, and for people allergic to gluten, for those who don't need iron and those who do; and there were specific pills for every age group, from the foetus right through to the "wellderly".)

Antioxidants were next to them, all seemingly fuelled by the "natural" power of prickly pear, goji, and açaí, the intensely popular Brazilian berry that supposedly offers benefits such as rejuvenation, skin toning and weight loss, not to mention prevention of various illnesses like heart disease. There was also something called "Blue Granate", a combination of blueberries and pomegranates, both of which "possess wondrous health properties", as the bottle put it. "A synergistic blend of powerful and potent phytonutrient antioxidants." Into the basket it went.

Almost everything advertised itself as an antioxidant. Oxidation is a natural result of metabolic processes that can cause harmful chain reactions and significant cellular damage. Those broken cells in turn release unstable molecules called free radicals, which are thought to be the cause of many chronic diseases. Set loose, free radicals can turn into scavengers, ransacking essential proteins and DNA by grabbing their electrons for spare parts. Antioxidants prevent those reactions, but standing there, it was impossible to know how, or if, they worked. The collection of pills was so enormous, the choice so vast, and the information so overwhelming, that while I may not have been depressed when I arrived at The Health Nuts, spending half an hour there did the trick.

I went home and consulted the internet, which was even more intimidating: there are millions of pages devoted to vitamins and dietary supplements. Fortunately, my eye was drawn immediately to the Vitamin Advisor, a free recommendation service created by Dr Andrew Weil, whose domed head and bearded countenance are so profoundly soothing that with a mere glance at his picture I felt my blood pressure begin to drop.

Dr Weil is America's most famous and influential practitioner of complementary medicine – he prefers to call it integrative – which seeks to combine the best elements of conventional treatment with the increasingly popular armamentarium of alternatives, everything from supplements to colonic irrigation, spiritual healing, and homeopathy. The public's hunger for novel remedies has transformed the integrative approach into one of the more potent commercial and social forces in American society. Nearly every major medical school and hospital in the country now has a department of integrative or complementary medicine. While the movement has grown immensely since he opened his Centre for Integrative Medicine in Arizona in 1994, Weil remains at its heart. Educated at Harvard University, both as an undergraduate and at its medical school, Weil embraces herbal therapies, New Age mysticism, and "spontaneous healing", which is the title of one of his books. But he also understands science and at times even seems to approve of it.

Weil offers sound advice in his many books – calling refined foods, excess starches, corn sweeteners, and trans fats dangerous, for example, and noting that exercise and a proper diet are far more beneficial even than the vitamins and supplements he recommends. His influence is immense, and in a country embarking on an urgent debate about its health care system, that influence has never been felt more powerfully. Weil is in great demand as a public speaker, testifies before Congress, and has twice appeared on the cover of Time magazine. For advocating the many health benefits of mushrooms, Weil is a hero to mycologists the world over. (He is one of the rare Americans to have had a mushroom named after him, Psilocybe weilii.)

His Vitamin Advisor website assured me that, after answering a few brief questions, I would receive "a personalised comprehensive list of supplements based on my lifestyle, diet, medications, and health concerns" – all at no cost, without obligation, and prepared specially to meet my "unique nutritional needs". In addition, if I so chose, I could order the "premium quality, evidence-based" supplements in the proper doses that would "exactly match the recommendations from Dr Weil". The supplements would be "custom packed in a convenient dispenser box and shipped directly to [me] each month". The only thing Dr Weil doesn't do for you is swallow the pills.

I filled out the form. Two minutes after I pressed "submit", Dr Weil responded, recommending a large number of dietary supplements to address my "specific health concerns". In all, the Vitamin Advisor recommended a daily roster of 12 pills, including an antioxidant and multivitamin, each of which is "recommended automatically for everyone as the basic foundation for insurance against nutritional gaps in the diet". My new plan would cost $1,836 a year (plus shipping and tax). Still, what is worth more than our health?

Before sending off a cheque, however, I collected some of the information on nutrition and dietary health offered by the National Institutes of Health, the Harvard School of Public Health, and the Memorial Sloan-Kettering Cancer Centre. It turned out that my pills fell essentially into three categories: some, like cordyceps and triphala, seemed to do no harm but have never been shown in any major, placebo-controlled study to do any particular good; others, like St John's Wort, may possibly do some good in some cases for some people, but can also easily interfere with and negate the effects of a large number of prescribed medicines, particularly the protease inhibitors taken by many people with Aids.

In 2009, researchers from the Women's Health Initiative, working at dozens of major medical centres under the direction of the National Heart, Lung, and Blood Institute, concluded a 15-year study that focused on strategies for preventing heart disease, various cancers, and bone fractures in post-menopausal women. After following 161,808 women for eight years, the team found no evidence of any benefit from multivitamin use in any of 10 conditions they examined. There were no differences in the rate of breast or colon cancer, heart attack, stroke, or blood clots. Most important, perhaps, vitamins did nothing to lower the death rate. Another recent study, this time involving 11,000 people, produced similar results. In 2008, yet another major trial, of men, had shown that the risk for developing advanced prostate cancer, and of dying from it, was in some cases actually twice as high for people who took a daily multivitamin as it was for those who never took them at all. There are hundreds of studies to demonstrate that people who exercise regularly reduce their risk of coronary artery disease by about 40 per cent, as well as their risk of stroke, hypertension, and diabetes, also by significant amounts. Studies of vitamin supplements, however, have never produced any similar outcome.

Antioxidants, often described in the press as possessing wondrous powers, are taken each day by millions. That should stop as soon as possible. While a diet rich in antioxidants has been associated with lower rates of chronic disease, those associations have never been reflected in trials in which people took antioxidants in supplement form. In 2007, for example, the Journal of the American Medical Association published the results of the most exhaustive review yet of research on such supplements. After examining 68 trials that had been conducted during the previous 17 years, researchers found that the 180,000 participants received no benefits whatsoever. In fact, vitamin A and vitamin E, each immensely popular, actually increased the likelihood of death by 5 per cent. Vitamin C and selenium had no significant effect on mortality. "The harmful effects of antioxidant supplements are not confined to vitamin A," said the review's co-author, Christian Gluud, a Danish specialist in gastroenterology and internal medicine and head of the trial unit at the Centre for Clinical Intervention Research at Copenhagen University Hospital. "Our analyses also demonstrate rather convincingly that beta-carotene and vitamin E lead to increased mortality compared to placebo."

It gets worse: folic acid supplements, while of unquestioned value for pregnant women, have been shown to increase the likelihood that men would develop prostate cancer. "Unfortunately, the more you look at the science the more clearly it tells you to walk away," Kelly Brownell said. Brownell, director of the Rudd Centre for Food Policy and Obesity at Yale University, has for years studied the impact of nutrition on human health. "Vitamins in food are essential. And that's the way to get them. In food." In May 2009, researchers from Germany and the United States reported in the Proceedings of the National Academy of Sciences that antioxidants like vitamins C and E actually reduce the benefits of exercise. "Antioxidants in general ... inhibit otherwise positive effects of exercise, dieting and other interventions," said Michael Ristow, a nutritionist at the University of Jena, who led the international team of scientists.

If people like weil relied on the evidence of science, or if their methods had been proved to work, there would be nothing alternative about them. Assessing data and gathering facts are the only useful tools we have to judge whether a treatment succeeds or fails. In the alternate universe of CAM treatment, nobody has to prove what is safe, what works and what doesn't. And that's dangerous because Americans are desperate for doctors who can treat their overall health, not just specific illnesses.

That kind of fervent belief, rather than facts, feeds disciplines like ayurvedic medicine, which argues for the presence of demonic possession in our daily life, and reiki, the Japanese practice of laying on the hands, which is based on the notion that an unseen, life-giving source of energy flows through each of our bodies. Then there is iridology (whose practitioners believe they can divine a person's health status by studying the patterns and colours of his iris), qigong, magnet therapy. None of it works. Acupuncture, while effective in reducing arthritic pain and the impact of nausea, has never been demonstrated to help people quit smoking or lose weight – two of its most popular applications.

Homeopathy, perhaps the best-known alternative therapy, presumes that a disease can be treated by ingesting infinitesimally small dilutions of the substance that caused the disease in the first place. No matter what the level of dilution, homeopaths claim, the original remedy leaves some kind of imprint on the water molecules. Thus, however diluted the solution becomes, it is still imbued with the properties of the remedy. No homeopathic treatment has ever been shown to work in a large, randomised, placebo-controlled clinical trial, but nothing seems to diminish its popularity. On those rare occasions when data relating to alternative medicine does become available, it is almost invariably frightening: in 2004, for example, a large group of researchers reported in the Journal of the American Medical Association that more than 20 per cent of the ayurvedic medicines the group purchased on the internet contained detectable and dangerous levels of lead, mercury, and arsenic.

Those bottles of folic acid and Blue Granate from The Health Nuts were sitting on my desk. They looked so promising and appeared to offer so much: support for a healthy cardiovascular system, as well as better memory and brain function; they would promote urinary tract, eye, and skin health, boost body detoxification functions, and reduce cellular damage associated with the aging process. There was, however, a tiny asterisk next to each claim. "These statements have not been evaluated by the Food and Drug Administration," each one said. "These products are not intended to treat, diagnose, cure or prevent any disease." I looked at the Garlicyn, the amino acids, and the vitamin C. Same warnings. Same nearly invisible print. The fine print on labels like that is rarely read. Pharmaceuticals are strictly regulated; for supplements there is almost no oversight at all.

But belief outranks effectiveness. In 2003, a study that compared the efficacy of echinacea to a placebo in treating colds received considerable attention. Researchers followed more than 400 children over a four-month period, and found not only that a placebo worked just as well, but that children treated with echinacea were significantly more likely to develop a rash than those who took nothing at all.

Subsequent studies have been even more damning. In 2005, researchers from the Virginia School of Medicine reported in the New England Journal of Medicine that echinacea had no clinical impact, whether taken as a prophylactic or after exposure to a virus. Nor did it lessen the duration or intensity of any symptom. In addition, the American College of Pediatricians has urged parents to avoid echinacea mixtures for children who are less than a year old. The response? According to the latest data released by the US government in 2008, echinacea remains the most heavily used supplement in the childhood arsenal.

Almost 40 per cent of American adults made use of some form of alternative medical therapy in 2007, according to the most recent National Health Statistics reports. They spent $23.7 billion on dietary supplements alone. (Britons spent £396 million on vitamins and supplements last year, report Mintel.) It has become one of America's biggest growth industries. There were approximately 4,000 supplements on the market in 1994, when the industry was deregulated by Congress. Today, the exact number is almost impossible to gauge, but most experts say there are at least 75,000 labels and 30,000 products. Those numbers don't include foods with added dietary ingredients like fortified cereals and energy drinks. In fact, the relationship between food, drugs, and supplements began to blur in the 1970s as connections between diet, food, and medicine became more fully understood. The government started telling Americans to alter their diets if they wanted to have long and healthy lives; still, it was against the law to suggest that there was a relationship between the ingredients in a commercial food and the treatment or prevention of a disease.

Then, in 1984, came the Original Sin. The National Cancer Institute lent its unparalleled credibility to the Kellogg Company when together they launched a campaign in which All- Bran cereal was used to illustrate how a low-fat, high-fibre diet might reduce the risk of certain types of cancer. All-Bran was the first food permitted to carry a statement that was interpreted widely as "Eating this product will help prevent cancer". That led to the era of product labels, and completely changed the way Americans think about, not only foods, but dietary supplements and ultimately about their health. Food was no longer simply food; it was a way to get healthy. It would require Dickens' narrative skills and Kafka's insight into bureaucratic absurdity to decipher the meaning of most products for sale in American health food stores today. In the world of alternative medicine, words have become unmoored from their meanings. As long as a company doesn't blatantly lie or claim to cure a specific disease such as cancer, diabetes, or Aids, it can assert – without providing evidence of any kind – that a product is designed to support a healthy heart, or that it protects cells from damage or improves the function of a compromised immune system.

Such claims can appear on any food, no matter how unhealthy. You cannot advertise a product as a supplement that "reduces" cholesterol, but you can certainly mention that it "maintains healthy cholesterol levels". It would be illegal to state that echinacea cures anything, since of course it has been shown to cure nothing. But it's perfectly acceptable to say that echinacea is "an excellent herb for infections of all kinds", although no such thing has been proven to be true.

Even claims that are true are often irrelevant. Vitamin A, for example, is essential for good vision – as supplements will tell you. Insufficient consumption of vitamin A causes hundreds of thousands of cases of blindness around the world each year, but not in the United States; here people don't have vision problems arising from a lack of vitamin A. And since too much vitamin A can cause birth defects and osteoporosis, for example, its potential to harm American consumers is far greater than the likelihood that it will do good.

That the mind can affect the chemistry of the human body is not in doubt, and researchers have shown direct relationships between what a patient expects from a drug and its therapeutic results. In one experiment, Fabrizio Benedetti, professor of clinical and applied physiology at the University of Turin Medical School in Italy, demonstrated that a saline solution works just as well as conventional medicine to reduce tremors and muscle stiffness in people with Parkinson's disease.

Benedetti is also a consultant for the Placebo Project at the National Institute of Health and a member of the Mind-Brain-Behaviour Initiative at Harvard University. In the Parkinson's study, he and his team found that neurons in patients' brains responded rapidly to saline. In another experiment, Benedetti has shown that for people who have no idea that a switch has been made, a shot of saline can provide as much pain relief as one of morphine.

It is perhaps the case, then, that taking megadoses of vitamins or craniofacial massage for the flu may seem comforting. At worst, many have argued, such actions are self-inflicted wounds – like the self-inflicted wound of refusing to vaccinate a child. There comes a point, though, when individual actions become part of something bigger. Progress is never guaranteed. It can vanish if reality ceases to make more sense than magic. Denialism is a virus, and viruses are contagious.

This is an edited extract from 'Denialism: How Irrational Thinking Hinders Scientific Progress, Harms the Planet, and Threatens Our Lives' by Michael Specter (Duckworth), published this month

Great British pill poppers

41% Number of people in Britain, in 2008, taking vitamin supplements

15% The proportion of the population of Britain that trusts homeopathic medicine

£40m The current value of the over-the-counter market for homeopathy in the UK

32% The current number of vitamin fanatics (those taking supplements once a day or more) in this country

60% The growth in the homeopathic market across Europe over 10 years (1995-2005) The figure has leapt from €590 million to €930 million

4 The number of homeopathic NHS hospitals that currently operate in the UK

90% of the world's homeopathic products are consumed by France, Germany, The Netherlands, Spain, Belgium, UK and Poland

£3.45 The average price of a packet of cold-and-flu caplets

£7.50 The average price of a bottle of echinacea tincture