Scientists have found that the placebo effect doesn't only lie in the mind, solving the mystery of why some people benefit from remedies that do not contain any active pain-relief ingredients.
The new study has suggested that placebos work, in part, by blocking pain signals in the spinal cord from arriving at the brain in the first place.
When patients expect a treatment to be effective the brain area responsible for pain control is activated, causing the release of natural endorphins.
The endorphins send a cascade of instructions down to the spinal cord to suppress incoming pain signals and patients feel better whether or not the treatment had any direct effect.
The sequence of events in the brain closely mirrors the way opioid drugs, such as morphine, work-strengthening the view that the placebo effect is grounded in physiology.
The new finding gives weight to the argument that many established medical treatments derive part of their effectiveness from the patients' expectation that the drugs will make them better.
The latest studies on antidepressants suggest that at least 75 per cent of the benefit comes from the placebo effect.
Doctors also observe that patients report feeling better only days after being prescribed antidepressants, even though the direct effects take several weeks to kick in.
In the study, the researchers used functional magnetic resonance imaging (MRI) to scan the spinal cords of 15 healthy volunteers, focusing on an area called the dorsal horn, which transmits pain signals coming up through the spinal cord into the pain-related areas in the brain.
During the scan, the volunteers received laser "pinpricks" to their hands.
The volunteers were told that a pain-relief cream had been applied to one of their hands and a control cream to the other.
However, without informing the volunteers, an identical control cream was administered to both hands.
When people believed that they had received the active cream, they reported feeling 25 per cent less pain and showed significantly reduced activity in the spinal cord pathway that processes pain.
"We've shown that psychological factors can influence pain at the earliest stage of the central nervous system, in a similar way to drugs like morphine," Times Online quoted Falk Eippert, of the University Medical Centre Hamburg-Eppendorf, who led the study, as saying.
The study has been published in the journal Science.