Many American doctors receive payments from drug companies, but few patients know about those financial ties, a new study finds.

Researchers surveyed 3,500 adult patients and then checked on their doctors in Open Payments, a government website that reports drug and medical device company payments to physicians.

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Study co-author Michelle Mello said, “Drug companies have long known that even small gifts to physicians can be influential, and research validates the notion that they tend to induce feelings of reciprocity.” Mello is a professor of law and health research and policy at Stanford University in Stanford, Calif.