Data Availability Statement: The data of the current survey study are hosted by the Royal Dutch Medical Association. Their policy is not to make the current data publicly available, for two reasons. First, participants did not provide informed consent for data-sharing. Second, the Royal Dutch Medical Association considers data on substance use (disorders) among physicians as sensitive information, that cannot be shared. However, the data used for the current study are available for researchers who meet the criteria for access to confidential data, after approval from the internal Conclusions About one-third of physicians reported experience with a presumption of substance use in a colleague. Whilst most physicians intend to take action upon such a presumption, two-thirds actually do act upon a presumption. To bridge this intention-behavior gap continued medical education on signs and symptoms of SUD and instructions on how to enter a supportive dialogue with a colleague about personal issues, may enhance physicians' knowledge, confidence, and ethical responsibility to act upon a presumption of substance use or other concerns in a colleague.