The tl;dr: If a medical study seems too good to be true, it probably is. Eryn Brown in the Los Angeles Times writes about a statistical analysis of nearly 230,000 trials compiled from a variety of disciplines, published today in the Journal of the American Medical Association. The analysis by Stanford's Dr. John Ioannidis and a team of fellow researchers looked at study results claiming a "very large effect," and found that those claims seldom ended up being true when other research teams tried to repeat the same results.
One such example: the cancer drug Avastin. Clinical trials suggested the drug might double the time breast cancer patients could live with their disease without getting worse. But follow-up studies found no improvements in progression-free survival, overall survival or patients' quality of life. As a result, the U.S. Food and Drug Administration in 2011 withdrew its approval to use the drug to treat breast cancer, though it is still approved to treat several other types of cancer.
With early glowing reports, Ioannidis said, "one should be cautious and wait for a better trial."