In September, the New York Times revealed that one of the world’s top cancer specialists had quietly received millions of dollars from drug and other companies. And what’s more, he’d failed to disclose the payments in many of his high-profile medical journal articles.

A few months before that, a study found that many experts who write medical textbooks received company payments, but again, their readers didn’t know about it.

This week, our Australian study of the doctors who write influential clinical guidelines has uncovered a similar problem. At least one in five of them have had a financial relationship with a drug company that was not disclosed in the guidelines.

This is a huge concern because guidelines can have big impacts on the sort of care doctors provide, and on which drugs they prescribe. And receiving benefits like free lunches or consulting fees from companies may cause bias.