THE belief that a medicine will relieve pain can prompt the brain to release natural painkilling chemicals, according to research that may explain the placebo effect.

Scientists in the United States have shown that the brain makes a distinct chemical response when patients are given a treatment they expect to work, shedding light on how therapies that have no active ingredients can nonetheless have medical benefits.

Many established medical treatments, particularly alternative and complementary therapies, are thought to rely partially or even entirely on the placebo effect. It is estimated, for example, that only about 20 per cent of the benefits of SSRI antidepressants come from the drugs, with the placebo effect accounting for the remainder.

The latest research, by a team at the University of Michigan, has revealed that when patients are unknowingly given a placebo to fight pain their brains release painkilling chemicals known as endorphins.

In the study, published in the Journal of Neuroscience, 14 healthy male volunteers agreed to receive pain-inducing saltwater injections in their jaw muscles while were undergoing positron emission tomography (PET) scans to examine endorphin activity in the brain.

During the scan, volunteers were told they were to receive a regular dose of a pain-relieving drug. In fact, this was a placebo.

They were asked to rate the pain they were experiencing at 15-second intervals, which was then correlated with medical data from the scan.

The data and post-scan interviews showed that participants reported significantly less pain when they received the placebo than when they received the jaw injection alone.

Jon-Kar Zubieta, a research leader, said: “This study deals another blow to the idea that the placebo effect is a purely psychological phenomenon.”

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