Nocebo effect may prevent pain relievers working

If a patient believes a pain-relieving drug will have little effect before taking it, the drug may not work at all, a study has found.

Scientists looked at how a patient's expectations can alter the effectiveness of the pain relief they receive.

Most doctors are familiar with the placebo effect; patients report feeling better after being given a pill which they think will do them good, despite the "drug" being nothing other than a sugar pill.

However, scientists say the opposite of this is the nocebo effect; a patient's doubts can override the effects of a potent pain relieving drug.

"People with chronic pain will often have seen many doctors and tried many drugs that haven’t worked for them," said study leader Professor Irene Tracey of the Centre for Functional Magnetic Resonance Imaging of the Brain at Oxford University.

"They come to see the clinician with all this negative experience, not expecting to receive anything that will work for them. Doctors have almost got to work on that first before any drug will have an effect on their pain."

In the study, the scientists applied different levels of heat to the legs of 22 volunteers who ranked the level of pain they experienced on a scale of one to 100.

The volunteers were also attached to an intravenous drip so that a potent painkilling drug - remifentanil - could be given.

When the drug was given without the volunteers knowing about it, the average initial pain rating of 66 went down to 55.

The volunteers continued to receive the opioid at the same dose, but were then told that the drug was about to be given.

Although the drug dose was exactly the same as before, the average pain ratings dropped further to 39.

Once again, without making any changes to the drug dose, the volunteers were led to believe that the drug had been stopped and were warned that there may be an increase in pain.

The volunteers pain intensity increased to 64.

Despite being given a potent opioid drug, the level of pain reported by the volunteers was as high as reported at the start of the experiment, before any pain relief was given.

The findings showed that the volunteers really did experience different levels of pain when their expectations were changed, although the pain relief remained the same.

MRI scans of the volunteers brains matched their reports of pain. The scans showed that different parts of the brain’s pain networks responded according to the volunteers’ expectations of pain.

Although the study was carried out on a small group of healthy volunteers, Professor Tracey said it was important that doctors should not underestimate the effect a patient's prior expectations can have on their treatment.

The study is published in Science Translational Medicine.

This article was published on Thu 17 February 2011