The placebo effect may have no scientific basis, according to a study published in this week's New England Journal of Medicine. Doctors have long known that about 35 percent of all patients given a placebo will get better, and they had assumed it was because the patients believed the dummy medication would help them. Many people have taken the idea a step further, believing that we can "think ourselves well" to some extent.

To find out, researchers from the University of Copenhagen and the Nordic Cochrane Center, Rigshopitalet, in Denmark looked at the statistical results of 727 trials, focusing on 114 that tested pharmacological, physical and psychological placebos involving 7,500 patients. They reviewed a broad range of different studies, excluding any that had not tested the effectiveness of both a placebo and no treatment at all. Then they examined how test subjects receiving placebos fared in comparison to groups that had received no treatment.

It turned out that the results were similar. "We found little evidence in general that placebos had powerful clinical effects," the authors write, suggesting that "outside the setting of clinical trials, there is no justification for the use of placebos." They also found, however, that placebos had possible small benefits for studies with subjective outcomes and the treatment of pain.