Patient safety has emerged as one of the most important and widely accepted aims of contemporary health care organizations. Medication errors are common and pose potentially serious threats to the safety of patients. Consequently, initiatives to improve patient safety often target nurses’ work with medications. A plethora of research studies examine medication errors and contribute to dominant formulations that impose stringent rules thought to enhance medication safety. However, little literature has examined the materiality of medication work and few research studies are oriented to the knowledge practices of nurses. This study used an institutional ethnographic approach, grounded in nurses’ everyday experiences, to contribute a different way of knowing about medication work. The study findings revealed a disquieting disjuncture between theoretical accounts of medication work and the actual everyday practices of nurses working with medications. Nurses routinely use their professional judgment and discretion to adapt stringent rules in order to keep patients safe and accomplish medication work effectively. However necessary and sensible it may be, nursing work that does not adhere to standard practice and institutional policy may be perceived as “breaking the rules”. Rule breaking is risky for nurses, as their professional competence is scrutinized and evaluated through regulatory and theoretical frameworks that value adherence to rules as the only way to demonstrate safety. The core argument developed in this analysis is that discretion is an essential element of competent medication practice, and that framing discretionary work within a rule based discourse obscures the safety work that nurses are routinely and regularly engaged in.