Also, prominent doctors in given areas can influence the choice of drugs other doctors make, Shah said. And drug-company marketing may play a role, he said.
"At this point we don't have good insight into these differences," he said.
This problem isn't unique to Avandia, Shah said. "This is not uncommon with a lot of drugs," he said. "This is a good case example."
The report was published in the Nov. 17 edition of the New England Journal of Medicine.
The study also found that the American Diabetes Association's January 2009 consensus statement advising against prescribing Avandia appeared to have had a "negligible influence" on trends in its use.
The study authors think the FDA could do a better job of alerting all doctors about warning labels. "The FDA could provide a tool for doctors and patients to show the risks and benefits of going on the drug," Shah said.
As for Avandia, in September the FDA introduced further restrictions on use of the drug. The agency is requiring Avandia's maker, GlaxoSmithKline, to develop a program that will limit access to the drug to patients for whom other treatments have not worked.
Also, doctors will have to state and document a patient's eligibility to use Avandia. They will also have to tell patients about the cardiovascular safety risks associated with Avandia, and patients will have to acknowledge that they understand those risks.
Commenting on the new study, Dr. Luigi Meneghini, professor and director of clinical operations in the division of endocrinology, diabetes and metabolism at the University of Miami Miller School of Medicine, said "there was still some confusion about the negative effects of Avandia."
"Physicians tend to be skeptical and not change their habits unless there is solid evidence, and with Avandia the evidence was not as solid as one wou
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