REMED stands for a "MEDication Errors Review". The REMED method aims at implementing and monitoring actions to improve patient medical care. This approach is part of the continuing healthcare quality and safety improvement and professional practice evaluation. REMED uses methodology and tools. Thus we use this methodology in the context of a drug errorwhich occurred in the dermatology department of our hospital. After this event description, related to a sensitive drug product, Instanyl (transmucosal fentanyl), the drug error is described, then the different causes are analyzed. These last are numerous and occur at any stage of patient medical care. They must allow us to show actions in order to improve patients' safety.