The sobering conclusion came in a review of major studies published in three influential medical journals between 1990 and 2003, including 45 highly publicized studies that initially claimed a drug or other treatment worked.
Subsequent research contradicted results of seven studies — 16 percent — and reported weaker results for seven others, an additional 16 percent.
That means nearly one-third of the original results did not hold up, according to the report in Wednesday's Journal of the American Medical Association.
"Contradicted and potentially exaggerated findings are not uncommon in the most visible and most influential original clinical research," said study author Dr. John Ioannidis, a researcher at the University of Ioannina in Greece.
Experts say the report is a reminder to doctors and patients that they should not put too much stock in a single study and understand that treatments often become obsolete with medical advances.