US researchers say they have evidence of why some people get pain relief from sham treatment.
The scans showed up endorphin activity
They looked at the so-called placebo effect - when a person is successfully treated by a dummy drug just because they believe it works.
Using brain scans the University of Michigan Health System scientists found placebo treatment triggers the brains natural painkillers, called endorphins.
Their work on 14 volunteers appears in the Journal of Neuroscience.
Researchers have already shown that some people given a placebo experience reduced pain sensation and have lower activity in brain regions that process pain as a result.
Dr Jon-Kar Zubieta and his team set out to see precisely what was happening in the brain.
They injected a salt water solution into the jaw muscles of the volunteers to cause pain.
At the same time, the volunteers had their brains scanned by a positron emission tomography (PET) scanner that would show up any endorphin activity.
During one of the scans, the volunteers were told they would also receive a medicine that might relieve the pain. This medicine was actually a dummy drug.
Throughout the experiments the volunteers were asked to score their level of pain and what they were experiencing.
After they received the placebo, nine of the volunteers reported much less pain and were able to tolerate higher doses of the pain-inducing salt water injections.
Their brain scans also showed that they had more endorphin activity after simply being told they were about to get the "medicine".
The most pronounced effects were seen in four parts of the brain known to be involved in processing and responding to pain, namely the left dorsolateral prefrontal cortex, the pregenual rostral right anterior cingulate, the right anterior insular cortex and the left nucleus accumbens.
Furthermore, activity in the dorsolateral prefrontal cortex was associated with the expectation of pain relief.
Activation of the other brain areas was associated with relief of the intensity of pain, how unpleasant it was and how the individuals felt emotionally during the pain.
Dr Zubieta said the findings show that the placebo effect is not purely psychological and has, at least partly, a physical explanation.
"The endorphin system was activated in pain-related areas of the brain, and that activity increased when someone was told they were receiving a medicine to ease their pain."
Dr George Lewith from Southampton University, who has studied the placebo effect and acupuncture, said: "I'm not at all surprised by the findings.
"They are consistent with what we know and have suspected. There is a physical side to the placebo response. You get a physiological change induced by expectancy."
He said that research so far suggested that 80-90% of people who benefit from analgesic drugs would probably get relief from a placebo too.