Many computerised drug prescription systems have been developed, but they are rarely used in clinical practice; among the reasons are their lack of integration with the functioning of medical institutions and the lack of consideration of general and local clinical practice rules. We present in this paper how OPADE, a computerised drug prescription system does answer this shortcoming by introducing prescription guidelines called Prescribing Principles. We argue that introduction of these Prescribing Principles will not only allow for integration of the computer in medical practice but will also introduce a positive feed back loop in the prescribing process.