The complex and dynamic features of neonatal intensive care units (NICUs) have made it necessary to think beyond traditional safety management approaches. The Functional Resonance Analysis Method (FRAM) was thus developed to explore how functional variability affects the overall system. This study performs the FRAM on the drug administration process in a NICU to understand performance variability as conditions change, as well as to understand how variability in functions influences the system in terms of both success and failure. A mixed methods approach was used, including observations, interviews and workshops. From the data obtained, we identified 21 foreground and 16 background functions and developed 58 scenarios in relation to the effects of potential variability on the system. This study shows that the FRAM can be used to determine how to respond to changing conditions, to anticipate how variability might lead to system success or failure, to understand how to monitor it and to learn from all these.