Implementation of CPOE systems in Healthcare Institutions has proven efficient in reducing medication errors but it also induces hidden side-effects on Doctor-Nurse cooperation. Wepropose a usability engineering approach to this problem. An extensive activity analysis of the medication ordering and administration process was performed in several departments of 3 different hospitals. Two of these hospitals are still usingpaper-based orders, while the 3^rd one is in the roll-outphase of medicationfunctions of its CPOE system. Weperformed a usability assessment of this CPOE system. The usability assessment uncovered usability problemsfor the entry of medication administration time scheduling by the physician and revealed that the information can be ambiguousfor the nurse. The comparison of cooperation models in both situation shows that users tend to adopt a distributed decision making paradigm in the paper-based situation, while the CPOE system supports a centralized decision making process. This analysis can support recommendation for the re-engineering of the Human-Computer Interface.