Doctors who attended medical schools that limited gifts to students from pharmaceutical companies—sponsored lunches, for example—may be less susceptible to drug marketing, a study published last week in the BMJ found.

Researchers from Yale University looked at the prescribing practices of doctors who had attended one of 14 schools that were early adopters of such policies. They looked at how often the physicians prescribed Vyvanse or Invega, two heavily marketed drugs used to treat attention deficit hyperactivity disorder and schizophrenia, over older, similar drugs.

Compared with doctors who attended the same schools before the gift ban was in place and peers from other schools, the physicians were less likely to prescribe the two marketed psychotropic drugs.

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Increasingly, medical schools are sharpening policies prohibiting drug marketing on campus as researchers add to the evidence that it can influence physician attitudes and behavior over the long term.

The University of Massachusetts Medical School, for example, enacted a policy in 2008 prohibiting students from accepting gifts from drug and device makers. Harvard Medical School also bans pharmaceutical representatives from visiting campus, and companies cannot sponsor student events.

The Yale study found that the counter-effect on prescribing patterns was greater among students who attended schools with strict policies.

The study did not find a significant effect for a third drug, an antidepressant marketed as Pristiq. It is not clear why, but lead author Marissa King, associate professor of organizational behavior at the Yale University School of Management, said the drug is less commonly prescribed than the other two drugs studied.

The researchers evaluated the effects of gift restrictions only put in place as of 2004. They noted that much has changed since then, with more schools adopting policies or tightening their rules.

King said the study does not evaluate whether such policies could slow the acceptance by young doctors of potentially important new drugs. But, she said, “my gut instinct is that if it is actually a radical breakthrough, physicians are going to adopt it anyway.”

The American Medical Student Association grades schools on their conflict of interest policies. On the 2011-2012 scorecard, the most recent one posted on the group’s website, UMass and Boston University School of Medicine received a B. Harvard and Tufts University School of Medicine earned top marks.