There was an outcry in September when we learned that children in Scotland were being given a homoeopathic "MMR vaccine", a product that offered no protection against the serious dangers posed by measles, mumps and, for pregnant women, rubella. This had echoes of the discovery a few years ago by Sense About Science, Simon Singh and Newsnight that some pharmacists were offering homoeopathic pills for protection against malaria to people travelling to Central Africa. Such practices may be disturbing, but they occur because we tend to think there is no harm in indulging the clamour to maintain the alternative health market.

Reading the 11 October issue of the New Statesman, I was shocked by an advertisement in the accompanying supplement, "Social Care: Who Pays?", referring to me and my work. Rarely had I seen an advert so inaccurate and borderline libellous in a respected publi­cation. The advert, which appeared to breach the British Code of Advertising, was by a lobby group called Homeopathy: Medicine for the 21st Century (H:MC21). It contained unjustified attacks on myself and colleagues, including statements that gave a dangerously false impression of homoeopathy's therapeutic value.

As the advert questioned my own competence, I should address this first. I started my medical career in a homoeopathic hospital, where I was trained in homoeopathy for several months. Many years later, it became my job to apply science to this field and I felt I had a duty to keep an open mind - open but not uncritical.

A critical mind would notice that the two basic principles of homoeopathy fly in the face of science, logic and common sense. The first assumption is that "like cures like". For instance, if onions make my eyes and nose water, homoeopathic remedies derived from onions can be used to treat my patients' hay fever, which sometimes causes runny eyes and noses. The second assumption proposes that diluting remedies homoeopathically makes them not less but more potent, even if the final preparation no longer contains a single molecule of any active substance. These theories are not based on anything that remotely resembles fact. Like does not cure like, and endlessly diluting remedies certainly does not render them stronger, but weaker. But is there some entirely new energy to be discovered that we do not yet comprehend? Not understanding homoeopathy does not necessarily mean that it is useless.

The best way to find out is to determine whether homoeopathic remedies behave differently from placebos when patients use them. In other words, we need clinical trials.

Data gap

About 150 such studies (mostly conducted by homoeopaths) and well over a dozen syntheses of this research are available. Their results are sobering: the totality of the most reliable evidence fails to show that homoeopathic remedies work better than placebos. So, after about 200 years of research, there is no good data to convince non-homoeopaths that homoeopa­thic remedies are any different from pure sugar pills. Pro-homoeopathic lobby groups such as the one that placed the advertisement therefore have to employ propaganda to try to convince consumers who may not know better. This is perhaps understandable, but surely not right.

What of patients' experience, some might ask. Thousands of people across the world swear by homoeopathy. Are they all deluded? Clearly not. People undoubtedly do get better after seeing a homoeopath. There are many observational studies to show that this is true. Homoeopaths therefore keep telling us that their treatments work, regardless of the implausibility of homoeopathy's principles and the largely negative trial evidence.

When we rationally analyse this apparent contradiction of evidence versus experience, it quickly dissolves into thin air. The empathic encounter with a homoeopath is just one of many factors that provide ample explanation for the observation that patients can improve even when they receive placebos. A case in point is Bristol Homoeopathic Hospital's 2005 study, cited in the offending advert. The 6,500 chronically ill patients might have im­proved because of the concomitant use of conventional treatments, or because of the attention they experienced, or because of their own expectation to improve, or because the disease process had come to an end. In fact, they might have improved not because of, but despite, the homoeopathic remedies they were given.

Still, some people ask what is wrong with using placebos as long as they help patients feel better. The answer is that it prevents clinicians telling the truth to patients. Being honest would defeat any placebo effect: if I tell my patient, "Take this remedy; it contains nothing and the trial data shows nothing," she is unlikely to experience a placebo response. Hence, homoeopaths, knowingly or unknowingly, deprive patients of informed consent. This paternalistic approach is recognised as unethical. Also, placebo effects are unreliable and normally short-lived; they happen occasionally but often do not. Even if placebo responses are generated, they are usually small - certainly too small to compete with effective therapies.

Twin-track effect

Endorsing homoeopathic placebos would mean that people might use them for serious, treatable conditions. In such circumstances, homoeopathy can even cause (and has caused) the death of patients. Furthermore, if we allow the homoeopathic industry to sell placebos, we must do the same for "Big Pharma". Imagine a world where pharmaceutical companies could sell us placebos for all sorts of conditions just because some patients experience benefits through a placebo response.

Crucially, and paradoxically, we don't need placebos to generate placebo effects. If I, for instance, prescribe an antihistamine for a patient suffering from hay fever, with empathy, time and understanding, that patient benefits from a placebo effect as well as the pharmacological action of the antihistamine. If, by contrast, I prescribe a homoeopathic remedy, I deprive her of the latter, crucial benefit. It is difficult to argue, as most homoeopaths try to, that this approach would be in the interest of my patient.

What follows is straightforward: there is no good evidence that homoeopathy does more good than harm. This is not just my conclusion after 17 years of researching the subject, but a fact based on the best available evidence, which is supported by virtually all experts who are not homoeopaths. The recent decision by the coalition government to continue homoeopathy on the NHS is thus puzzling, to say the least.

The advertisement that prompted this article is misleading about the work of experts which has conclusively shown that homoeopathy can have no place in evidence-based medicine. It is an insult to our intelligence.

Edzard Ernst is professor of complementary medicine at the Peninsula Medical School, University of Exeter, and co-author, with Simon Singh, of "Trick or Treatment? Alternative Medicine on Trial" (Corgi, £8.99)

Here comes the non-science

Homoeopathy was developed in 1796 by the German physician Samuel Hahnemann. He based his treatments on the twin ideas that "like cures like" and "less is more". The latter notion was implemented by taking a substance and diluting it over and over again, so that the final product generally contains not a single molecule of the original active ingredient.

Homoeopaths accept that most of their remedies are devoid of pharmacologically active principles, but they argue that the pills contain a "memory" of the original ingredient. The memory is supposedly imprinted in the diluting agent, which is used to moisten sugar pills.

Although homoeopathy defies the laws of physics, chemistry, biology and therapeutics, there have been numerous attempts to test its impact on patients through clinical trials. In 2005, Aijing Shang and seven colleagues from the University of Berne published an analysis of the best trials in the Lancet.

Their findings confirmed many other such published assessments. Commenting on the paper, they wrote: "This finding is compatible with the notion that the clinical effects of homoeopathy are placebo effects." An accompanying editorial entitled "The end of homoeopathy" said: "Doctors need to be bold and honest with their patients about homoeopathy's lack of benefit."