E-prescription is amongst the most widespread medical electronic support functions. However, several studies reported acceptance and utilisation rates not as high as expected. This paper performs firstly an analysis of the literature on e-prescription characteristics and functionalities especially with respect to their actual usage. Then a specific field study was conducted in an Internal Medicine ward, to investigate human factor issues associated to the introduction of an e-prescription system. Finally, the findings of the field study are framed within the actual implementation of various electronic support outputs resulting from the European Project "Patient safety through intelligent procedures in medication" (PSIP). The results show the importance of a systemic view when designing, implementing and evaluating medical support systems, as the pre-existing structures and tools largely influence the impact of those systems and their effects.