There have been astonishing scientific advances in the last 25 years, exemplified by the creation of the Internet and the mapping of the human genome, but in spite of this - or, more likely, because of it - millions of westerners now seek consolation from mumbo-jumbo merchants and snake-oil vendors. That is the premise of my new book. I would have liked to call it Extraordinary Popular Delusions and the Madness of Crowds but the great Victorian journalist Charles Mackay bagged the title as long ago as 1852, so I have settled for How Mumbo-Jumbo Conquered the World: A Short History of Modern Delusions. Mackay described his masterpiece as "more of a miscellany of delusions than a history - a chapter only in the great and awful book of human folly, which yet remains to be written." Most of his examples come from the pre-Enlightenment - an age of witch-hunts and holy relics, alchemy and geomancy. But we've grown out of that now, haven't we? Apparently not. After two centuries of gradual ascendancy, Enlightenment values have come under fierce assault from a grotesquely incongruous coalition of radical mystics and conservative flat-earthers. The space vacated by notions of history and progress has been colonised by gurus and gibberish.

The swelling popularity of quack potions and treatments in recent years is a small but telling manifestation of the retreat from reason and scientific method. According to a 1998 survey by the Journal of the American Medical Association, the use of homeopathic preparations in the United States more than doubled between 1990 and 1997. In Britain, by the end of the twentieth century the country's 36,000 general practitioners were outnumbered by the 50,000 purveyors of complementary and alternative medicine (CAM). When the British journalist John Diamond disclosed in the mid-1990s that he had cancer, he found himself bombarded with well-meant but batty advice, described in his final book Snake Oil and Other Preoccupations:

"Have you tried squid's cartilage? Establishment doctors scorn it, of course, but my aunt is still alive on squid's cartilage two years after her oncologist gave her only six months (well, yes, since you ask, she is having radiotherapy as well). Or there's this wonderful healer who practises the laying on of feet, with astonishing results. Apparently it's all a question of tuning your holistic (or is it holographic?) energies to the natural frequencies of organic (or is it orgonic?) cosmic vibrations. You've nothing to lose, you might as well try it. It's £500 for a course of treatment but what's money when your life is at stake?"

The alluring adjectives 'complementary' and 'alternative' are essentially euphemisms for 'dud': there is only medicine that works and medicine that doesn't, medicine that has been adequately tested and medicine that hasn't. In his introduction to Diamond's book, Professor Richard Dawkins pointed out that if a healing technique is shown to have curative properties in properly controlled double-blind trials, it ceases to be an alternative: it simply becomes medicine. "Conversely, if a technique devised by the President of the Royal College of Physicians consistently fails in double-blind trials, it will cease to be part of 'orthodox' medicine. Whether it will then become 'alternative' will depend upon whether it is adopted by a sufficiently ambitious quack (there are always sufficiently gullible patients)."

It is a brave politician who risks incurring the wrath of those patients: 83 million Americans spend $27 billion a year on 'alternative' medicine, and most of them are entitled to vote. In 1992 the US government's National Institute of Health was awarded $2 million of public money to establish an Office of Alternative Medicine (later the National Centre for Complementary Alternative Medicine). Within eight years, thanks to strenuous lobbying, Congress had increased the budget to $90 million. A few months before the presidential election of 2000, in an overtly political gesture, Bill Clinton announced the creation of a White House commission to ensure 'that public policy maximises the benefits to Americans of complementary and alternative medicine'.

Even the sober British Medical Journal felt obliged to pander to the new quackery by running a series describing alternative therapies. "Our readers wanted it," the editor explained. "They want to know more about it because their patients are interested in it and because they are wondering whether to refer people. They want to know what works and what doesn't." Yet, as Richard Dawkins noted, the pedlars of complementary medicine are uninterested in proving that their remedies 'work'. Homeopathy has often been subjected to rigorous scientific testing since its 'discovery' by the German physician Samuel Hahnemann at the end of the eighteenth century, and it has always failed - most recently on a BBC Horizon documentary in 2003.

This is scarcely surprising, as Hahnemann's perverse 'law of infinitesimals' (the smaller the dose of a drug, the stronger its effect) means that there is effectively nothing to test. Homeopathic products are diluted in 99 parts water (and/or alcohol) and shaken vigorously; a drop of the resulting solution is then added to more water, and so on until the original substance has been diluted many millions of times. A typical dilution is 30C, that is, one part cure to 100 to the power of 30 parts water: even if it contained only one molecule of the homeopathic ingredient, the amount of water required would be far greater than that in all the oceans of the earth. And, of course, no substance can be diluted beyond the point where one molecule remains without disappearing altogether.

An even more fantastic ratio of 200C is claimed for Oscillococcinum, a product for 'the relief of colds and flu-like symptoms', whose active ingredient is a duck's liver. If one molecule of the liver survived the dilution, it would be mixed in 100 to the power of 200 molecules of water, more than the total number of molecules in the entire universe. Oscillococcinum had sales of $20 million in 1996, and all from a single duck's liver - prompting US News & World Report to describe the hapless bird as 'the $20–million duck'.

In a normal double-blind test, half the patients are given the 'remedy' while the rest take a placebo, but in the case of homeopathy the two doses are identical, since neither has any trace of the crucial ingredient. Hoist by their own law of infinitesimals, homeopaths argue that even if there is no molecule of the original substance in the medicine, the water somehow retains a metaphysical 'memory' of it. This idea has been promoted by the French scientist Jacques Benveniste, who submitted a paper to the journal Nature in 1988 claiming to have experimental proof. (When Nature sent a team of invigilators to the laboratory, he was unable to replicate his findings.) Benveniste has since extended his theory further, saying that not only can water 'remember' highly diluted substances, but that this memory can be taken electromagnetically from the water, stored digitally on a computer, emailed to the other side of the world and 'played back' via a sound card into new water - which instantly acquires the same properties as the original.

Like homeopathy, most alternative therapies are closer to mysticism than to medicine. This may explain their appeal to the British royal family, whose survival depends on another irrational faith - the magic of hereditary monarchy, which was so fiercely debunked by Tom Paine and other Enlightenment pamphleteers. The Queen is said to carry homeopathic remedies with her at all times. Princess Diana was a devotee of reflexology, the belief that pressure applied to magical 'zones' in the hands and feet can heal ailments elsewhere in the body. Prince Charles has been a prominent champion of 'holistic' treatments since 1982, having been persuaded of their effectiveness by that absurd old charlatan Sir Laurens van der Post. In 1988 the prince presided at the official opening of the Hale Clinic in London, which offers a choice of more than forty 'alternative therapies' such as acupuncture, aromatherapy, T'ai-Chi, Chakra balancing, colonic irrigation and bio-energy healing ("the healer acts as a channel allowing the positive energy to pass through the patient with one hand and extracting negative energy with the other hand"). Eight years later he set up the Foundation for Integrated Medicine, and in 2001 lent his support to a proposal for a new London hospital that would 'tap into the power of alternative therapy'.

One would expect nothing else from a man who subjects his flowers and vegetables to regular lectures (how one yearns for a plant that can answer back - or, better still, bite his ankle). But even our 'modernising' government now seems to be succumbing to this anachronistic codswallop. Writing in New Humanist last year, I pointed out that 'alternative remedies' had a champion in Cherie Blair, who is a client of the ayurvedic guru Bharti Vyas and officiated at the opening ceremony for Vyas's 'holistic therapy' centre in London. Lo and behold, in November 2003 the government's alluringly named clinical watchdog NICE (the National Institute for Clinical Excellence) issued new guidelines for the NHS on the management of multiple sclerosis. Tucked away on page 32 was the brief announcement that T'ai-Chi, reflexology and other forms of complementary therapy 'may be helpful for people with MS in terms of their general well-being'.

Note the cautious phraseology, which implies that these could have nothing more than a placebo effect: if you think a treatment is doing you good, then you may well perk up a bit anyway. Still, where's the harm in that? Allow the Multiple Sclerosis Society to explain. "Trying everything can be very expensive and demoralising,' one of its leaflets warns. 'If a therapy does not work for you, or you start to feel worse, you may end up feeling that it's your own fault." Besides, if superstition wishes to trespass on science's territory, then it deserves to be judged by scientific standards.

Complementary medicine continues to fail such a test: NICE admits that it hasn't enough evidence to make firm recommendations. Even so, this is the first time that quack therapies have been recognised at all in official guidance to doctors, and rumour has it that we can expect more of these concessions from the guardians of 'clinical excellence' in 2004. How soon before bottles of genuine snake-oil are available on prescription?

This article is an edited extract from Francis Wheen's new book, published by 4th Estate