As part of electronic prescription systems medical alerts are a powerful decision support for physicians. They can warn physicians in case of drug interactions or other adverse events. However, a large part of medical alerts is not taken into account. The aim of the present thesis is to investigate the impact of medical alerts on physicians. We address the question with three different studies carried out at the University Hospitals of Geneva. The first study evaluates physicians' satisfaction with the deployed system. The second study evaluates the work context to learn about the use of medical alerts in a real work situation. Finally, we evaluate in an experimental study physicians' interaction with an adapted prototype of a prescription system. Among other results we found that physicians only consider alerts in unfamiliar medical situations. The prototype where alerts are centralised in a table seem to be better adapted to this behaviour.