I have said it so often that I hesitate to repeat myself: complementary/alternative medicine (Cam) is woefully under-researched. But wait, there is one truly remarkable exception to this rule: surveys. Almost every other day a new Cam survey is published. As a result, thousands of these research projects are available, many from the UK. No other medical field could even begin to compete with Cam when it comes to the mountain of survey data.

In the typical Cam survey, a group of people is asked whether they have recently tried Cam. If yes, why, and which therapy for which condition and with what perceived success? Most surveys tell us very little we don't already know, and many are of pitiful quality and poor generalisability. For instance, if I published tomorrow what people in Exeter think about Cam, would these findings apply to people in London? Would the results still be true in two years' time? Most probably not.

Being "blessed" with this plethora of dubious information, we now seem to understand perfectly well who uses Cam and why - the question has been asked a thousand times. But amid these piles of remarkably irrelevant data, amazingly, there is a yawning gap. Nobody seems to wonder about why most people (in the UK, about 80% of the general population) don't use Cam. Well, not quite nobody - two research groups recently did ask that question and found a range of intriguing answers.

Most people felt there was not enough information. Were they joking? Our bookshops are full with Cam books and 41 million websites currently praise the virtues of Cam. What these people probably mean is that there is not enough reliable information. I couldn't agree more, and would even go one step further: what many organisations offer as information is scandalously untrustworthy. Information, it seems, is often misused for promotion. This is, of course, a problem with all health-related information, but in Cam the problem is monstrous.

Other people answered that they don't use Cam because they found it too expensive. Again I agree. A healing session for £100, as it is currently priced in some places, is hardly a bargain. Some people had not tried Cam because they were satisfied with conventional healthcare. Good for them. With all this hype about the latest alternatives, we tend to forget that modern "scientific" medicine is damn good and helps so many who are seriously ill.

People also said that they believed Cam is not effective. According to the best available evidence, they may have a point. For a small minority of complementary therapies, we are fairly sure that they work for certain conditions. But for the vast majority, we simply do not have enough reliable evidence. And with some forms of Cam we are pretty certain that they have no effects beyond placebo.

Some study participants replied that their doctor advised against Cam. This implies that some patients were courageous enough to discuss these matters with their GP. The fact that he then recommends against Cam does not necessarily mean he isnarrow-minded. It could also be that, for that particular patient or condition, there is no effective alternative.

Other people gave religious or moral reasons for not using Cam. This may seem odd, but it can make sense. Some therapies, such as yoga, meditation or healing, can transport patients into a bewildering world of alien religions or philosophies. What may well be an attraction to some could be a deterrent to others.

The last reason for not using Cam made me smile; people find it too embarrassing. Many complementary therapies involve a lot of touch. British GPs seem to touch their patients less and less, so many patients are no longer used to this sort of thing. To me, good medicine must involve a great deal of touch - how else could I, for instance, conduct a thorough investigation of a patient? But perhaps people are embarrassed about other characteristics of Cam. Complementary therapists tend to dive into the intimacy of their patients' feelings and spirituality. And some treatments are nonsensical, bizarre or self-indulgent. I can sympathise with a degree of embarrassment.

Understanding the reasons why some people do and some don't use Cam has its limits. Occasionally (very occasionally, I would say), it provides us with interesting insights. But to what avail? Sociologists who conduct this research often claim that it "informs policy". I have my doubts. There are thousands of Cam surveys, yet I have not yet seen a single policy that depended on this type of data. If you ask me, one good clinical trial takes us miles further than most of these surveys put together.

· Edzard Ernst is professor of complementary medicine at the Peninsula medical school at the universities of Exeter and Plymouth.