The post-authorisation safety study (PASS) is a pharmacovigilance activity often required as a post-marketing commitment to establish a safety profile or address specific safety concerns. An imposed PASS must be submitted in common technical document format. Comparability of observational studies to clinical trials is limited by a number of factors related to the differences in design and conduct of these studies. These include selection bias, which is harder to control in the observational setting, and typically a relatively higher extent and quality of data collection in the clinical setting. The PASS also places a strong focus on risk without collecting new formal benefit information. These factors present medical writers with some new (and not so new) challenges.Offsetting the challenges, the PASS creates opportunities to assess the “real world” prescribing of a drug, to compare the real target population with the label population, and, because of the large scale of such trials, to assess safety across multiple sub groups withgreater certainty than possible in a clinical trial.