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Having a policy governing pharmaceutical marketing, known as "detailing," was associated with a 1.67 percentage point decrease in market share for the average promoted drug. Before the policies, the average promoted drug had a market share of 19.3 percent. Those with tougher policies, including an enforcement component, appeared to have more significant results.

"These weren't terribly onerous restrictions, yet at the same time, they changed prescribing in a way that has really significant cost implications," Larkin said.

Among the centers that did not have statistically significant changes was Stanford University, one of the earliest adopters of restrictive policies. In 2010, ProPublica reported how Stanford was not enforcing its rules limiting the relationships between doctors and drug companies. It has tightened its oversight since. Stanford doctors prescribed fewer promoted drugs but not enough for the result to be significant.

"We were surprised that some of them [teaching hospitals] were not as significant as we expected them to be," said Larkin, without singling out particular hospitals. "You can't just put in a policy. You have to think about it carefully, think about the efforts that really matter and involve the [medical] community."

The study had several limitations. First, it did not find that the policies caused the change in prescribing, only that there was an association between the two. Also, the study was observational, meaning that doctors were not randomly assigned to hospitals with and without policies. And the study took policies at their word, not looking at their implementation or follow through.

Dr. Howard Bauchner, JAMA's editor in chief, said the study helps to crystalize the need to limit pharmaceutical company marketing in teaching hospitals "as a way of ensuring that there's no influence, no inappropriate influence over prescribing."

Bauchner said he isn't bothered that the researchers only found significant results in fewer than half of the teaching hospitals studied. "Nothing is ever 100 percent effective," he said. "To me that's no different than a clinical trial. Not everyone benefits."

An editorial that accompanied the research suggests that alternative approaches to educating doctors about drugs 2014 besides relying on drug company promotion 2014 need to be tested. "It has never been more important for physicians to come together to consider these alternatives, generate evidence about their effectiveness, and move the health care system toward solutions that lower costs for patients and minimize" conflicts of interest, wrote Colette DeJong and Dr. R. Adams Dudley of the University of California, San Francisco.