"The data suggest that longer and larger trials are associated with bigger placebo responses,” said Jeffrey Mogil, a professor of pain studies and senior author of the study. “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 greater the improvement in patients treated with placebo in clinical trials, the more difficult it can be to demonstrate the beneficial effects of pain-relieving medications,”

So maybe, just maybe, the longer, larger trials are actually showing that many of the drugs are not actually really efficacious after all.