New Cochrane Review (2010): Placebo interventions for all clinical conditions

Placebo interventions are often claimed to substantially improve many clinical conditions. However, most reports on effects of placebos
are based on unreliable studies that have not randomised patients to placebo or no treatment.

We studied the effect of placebo treatments by reviewing 202 trials comparing placebo treatment with no treatment covering 60
healthcare problems. In general, placebo treatments produced no major health benefits, although on average they had a modest effect
on outcomes reported by patients, such as pain.

However, the effect on pain varied from large to non-existent, even in well-conducted
trials. Variations in the effect of placebo was partly explained by variations in how trials were conducted, the type of placebo used, and
whether patients were informed that the trial involved placebo.

If you would like to know more about this study, read the Cochrane Review (2010) : Placebo interventions for all clinical conditions