The present article describes the informatization process of a hospital service, taking as an example the implementation of a prescription writing software. This tool allows doctors to prescribe via computer the pharmaceutical regimen to be prepared and dispensed by the nursing personnel. The ethnographic study in the hospital service describes the introduction process of this tool from the perspective of the daily activities of the nursing staff. The article highlights a number of difficulties the users are facing and describes the solutions they create, in particular the “compensation strategies” they develop for turning the tool operational, reliable and efficient. The article also shows that the compensation strategies the users are creating in situ are partly invisible for the designers of the software. Distant from
the daily activities of the users, the software designers neither perceive the problems involved with the use of the product, nor do they perceive the locally created solutions. They ascribe the performance of the tool to the quality of its conception and possibilities of improvement. This article invites the readers to reflect about what turns an information system efficient and effective and about the lessons learned for the conception of these tools.