In this paper we present the introduction of knowledge based functions into clinical routine at Giessen University Hospital. For this purpose a therapy planning module at the medical intensive care unit has been extensively redesigned in order to support the structured documentation of drug prescriptions. After introduction of this new HIS component in January 1996 research has been initiated to establish a basic drug therapy knowledge base. The main components of a knowledge based system have been fully incorporated into the hospital information system WING and are in routine use since December 1996. During a pre-production phase warnings of reminder functions were logged and reviewed by an interdisciplinary team in order to adapt the system to the actual clinical environment. The paper describes experiences during this fine tuning and adaptation process which was necessary to bring a small set of knowledge modules into clinical routine.