EHR-integrated secure messaging is increasingly used for clinical communication, yet relatively little is known about how it is used and who its primary users are. In this study, we characterized secure messaging users and their messaging behaviors within a large health system. We found that secure messaging was widely used by a diverse range of healthcare professionals, with many users interacting with over 20 messages per day. The short message response times and high messaging volume observed highlight the interruptive nature of secure messaging, raising questions about its potentially harmful effects on clinician workflow, cognition, and errors.