It's not all in the mind — the so-called placebo effect is real and reaches right down to the spine, German scientists said on Thursday.
The finding may help in the hunt for better ways to tackle pain and other disorders.
Using modern imaging technology the researchers found that simply believing a pain treatment is effective actually dampens pain signalling in a region of the spinal cord called the dorsal horn, suggesting a powerful biological mechanism is at work.
"It is deeply rooted in very, very early areas of the central nervous system. That definitely speaks for a strong effect," lead researcher Falk Eippert of the University Medical Center Hamburg-Eppendorf told Reuters.
Eippert and colleagues used functional magnetic resonance imaging, or fMRI, to study changes in spinal cord activity.
They applied painful heat to the arms of 15 healthy men and compared the spinal cord responses when they thought they had been treated with either an anaesthetic cream or a placebo.
Both creams, in fact, were inactive but the fMRI scans showed nerve activity was reduced significantly when subjects believed they were getting the anaesthetic.
The ability of sham medicines with no active ingredient to produce real clinical benefits has long perplexed doctors and frustrated drugmakers.
Patients are typically given either an experimental drug or a dummy in clinical trials and the fact that those on placebo often get better, too, makes it hard to determine whether a new drug is working.
The placebo effect is particularly strong when treating central nervous system conditions, like depression and pain.
Traditionally, experts have viewed the effect as psychological, but the new German research is the latest in a growing body of evidence that there is an important physical component.
Just what turns down pain signalling in the spine when a placebo is given is unclear, although Eippert suspects a range of chemicals including natural opioids, noradrenaline and serotonin may be involved.
Writing in the journal Science, Eippert and colleagues said their work "opens up new avenues for assessing the efficacy and possible site of action of new treatments for various forms of pain, including chronic pain".
The word placebo comes from the Latin for "I shall please".