Over the last 25 years, the efficacy of placebos has been climbing relative to the real drugs with which they are compared. Placebos have become more effective in the United States particularly, whereas the effect has been much less pronounced elsewhere. The reasons why the placebo effect might be strengthening are largely unknown.
Researchers at McGill University have attempted to explain the strengthening of the placebo effect, examining data from 84 trials of drugs to treat neuropathic pain. The researchers published their findings online ahead of print in August in the journal Pain. The article appeared in print in December.
Although the McGill researchers were unable to demonstrate exactly what is causing the strengthening of the placebo effect in the U.S., which reached 30% reduction in pain in 2013, they pointed to two United States-specific associated trends.
Firstly, clinical trials are becoming longer; the average clinical trial was conducted over 4 weeks in 1990, but 12 weeks was the norm in 2013. Secondly, clinical trials are becoming larger; whereas 50 patients were included on average in 1990, an average of 700 were included in 2013.
“The data suggest that longer and larger trials are associated with bigger placebo responses,” said senior author of the Pain paper Jeffrey Mogil in a press release. “This, in turn, tends to result in the failure of those trials — since it makes it harder for pharmaceutical companies to prove that the drug being tested is more effective than treatment with a placebo.”
The study authors offer other hypotheses to explain the ascendance of the placebo effect in the U.S. as well, including a potential role for direct-to-consumer drug advertising, which is all but unheard of in the rest of the world. Any situation that makes patients more confident in a drug’s abilities might exacerbate the placebo effect.