6 October 2015

Placebo response growing over time - but only in America

by Will Parker

A new study finds that rising placebo responses may play a part in the increasingly high failure rate for clinical trials of drugs, but the authors of the study say that the increase in placebo responses occurred only in trials conducted in the United States.

Accepted for publication in the journal Pain, the study analyzed the results of 84 clinical trials of pain drugs conducted around the world from 1990 to 2013. Over that period, the pain inhibition experienced by patients in the placebo group increased steadily, reaching an average 30 percent decrease in pain levels by 2013. Similar increases in placebo response have previously been observed in studies of clinical trials of antidepressants and antipsychotic drugs. Those studies, however, didn't pinpoint the U.S. as the source of the trend.

The authors, from McGill University, examined reported features of the clinical trials to determine what factors might be responsible for the changes over time. They found that in the U.S., but not elsewhere, trials are becoming longer (from an average of four-weeks long in 1990 to 12 weeks in 2013) and larger (from an average of fewer than 50 patients in 1990 to an average of more than 700 patients in 2013).

"The data suggest that longer and larger trials are associated with bigger placebo responses," said Jeffrey Mogil, senior author of the new paper. "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."

"It remains to be determined why the United States is an outlier with respect to its clinical trials," added co-author Alexander Tuttle. He and his co-authors note, however, some potentially important differences between the U.S. and other countries. These include the existence of direct-to-consumer drug advertising in the U.S. (New Zealand is the only other country in the world that allows this), the greater spread of for-profit "contract research organizations" in the U.S., and perhaps greater exposure to the placebo concept in popular media in the U.S.

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Source: McGill University