This article is from the Alternative Medicine Therapies guide.
Because iridology is not a treatment therapy, but rather a way of detecting underlying signs of developing disease, iridologists will let you know about your overall health as well as any trends you may be experiencing toward illness. In this way, their goals are similar to those of all preventive medicine--to recognize health problems at their earliest stages and to suggest ways to keep disease from developing.
One problem with iridology, especially in the United States, has been a lack of adequate training among most of the individuals purporting to be iridologists. Many of the courses in iridology are little more than a weekend long, culminating in a "certification." And even though some licensed chiropractors and naturopaths may practice iridology, their education, by European standards, is often inadequate.
Moreover, marketing companies promoting supplements sometimes offer crash courses in iridology to their distributors. The goal, of course, is to encourage treatment using the company's products. The risk of overdiagnosis then may become a problem, since the practitioner now has secondary motives to "find" conditions on iris examination that are treatable with the products he's paid to recommend.
Because iridology is frequently performed by inadequately trained amateurs, it has not fared well in evidence-based medical studies. The end result is that there is very little published research on iridology (except in iridology journals), and what exists is inconclusive.
Two mainstream scientific studies testing iridologists' abilities to detect gallstones and kidney infection, published in the British Medical Journal and the Journal of the American Medical Association respectively, concluded that iridology was not helpful for either problem. A recent meta-analysis went further and said that iridology was "not useful and potentially harmful." However, professional iridologists have faulted these studies for attempting to diagnose conditions that simply cannot be revealed by their methods.