The phenomenon of the placebo effect seems to have been increasing over the past 25 years or so, so much so that it is matching any positive results for the painkillers being tested.
The placebo effect has also been found to be as powerful as antidepressants and antipsychotics in earlier studies.
In the new analysis, researchers at McGill University in Montreal looked at the results from 84 trials into powerful painkillers between 1990 and 2013. Over that time, the increase in the placebo has increased to the point where it has the same pain-killing effects as the drugs themselves.
People given the placebo have reported a 30 per cent decrease in pain from nerve damage, which is the same as that achieved in those taking the painkiller.
Strangely, the increase in the placebo response has been rising over the years, and seems to be centred on the US. Similar studies carried out in Europe or Asia haven't reported similar rises in the placebo effects. The placebo response also appears to be greater in the bigger, and longer-running, trials.
As a result, drug companies are finding it very difficult to demonstrate their painkillers are effective, say the researchers.
(Source: Pain, 2015; 1: doi: 10.1097/j.pain.0000000000000333)