Reasearch has shown their benefits can be completely wiped out by manipulating expectations.  Heat was applied to the legs of 22 patients, who were asked to report the level of pain on a scale of one to 100. They were also attached to an intravenous drip so drugs could be administered secretly.The initial average pain rating was 66. Patients were then given a potent painkiller,  without their knowledge and the pain score went down to 55. They were then told they were being given a painkiller and the score went down to 39.

Then, without changing the dose, the patients were then told the painkiller had been withdrawn and to expect pain, and the score went up to 64,  ie with  the drug they were reporting the same level of pain as when they were getting no drugs at all.

Patients had brain scans during the experiment. The expectation of positive treatment was associated with activity in the cingulo-frontal and subcortical brain areas while the negative expectation led to increased activity in the hippocampus and the medial frontal cortex.

The study was conducted on healthy people who were subjected to pain for a short period of time. Professor Irene Tracey, from Oxford university said people with chronic conditions who had unsuccessfully tried many drugs for many years would have built up a much greater negative experience, which could impact on their future healthcare.

Medics tend to focus on physiology and don't take this kind of expectation into account, nor do clinical trials. The study, published in Science Translational Medicine, also identifies the regions of the brain which are affected.

See also http://www.bbc.co.uk/news/health-12480310F