The aim of this thesis was to address some aspects of drug related problems with special regard to drug-drug interactions. In paper I we aimed to describe the scenario and frequency of drug-related problems (DRPs) in in-patients and to determine whether a pharmacotherapeutic advisory intervention aiming at reducing DRPs could affect rates of re-hospitalisation and / or death within 6 months. A total of 299 DRPs among 71% (106/150) of the patients were found, who had not previously been identified in the usual care. Thirty-five per cent (106/299) of DRPs in 39% (58/150) of the patients were judged to be of such importance that advice was given to the physician in charge. The proportion of re-hospitalisation and death in the intervention group was 49% (73/150) compared to 46% (69/150) in the control group (Risk ratio: 1.06, 95% confidence interval: 0.84 to 1.32, P=0.64). In conclusion, drug-related problems were common. The impact of drug-related problems on hard endpoints such as re-hospitalisation and death may however be overestimated. It is of importance to clarify if and in what way drug-related problems are preventable. The purpose in paper II was to evaluate the clinical relevance of the Janus Web application in screening for potential drug-drug interactions. One hundred and fifty DDIs, regarding 58 different interaction pairs, were classified as significant. 126 interactions that were significant by definition did not result in advice. A look at the alerts which featured most frequently in such combinations illustrates the nature of this discrepancy. With the aim to develope a drug-drug interaction software with the goal of achieving and maintaining a general use on a routine basis, it is of great importance that the alerts are clinically relevant. Equally important may be to present the warnings in an adequate way to give the prescribing physician a balanced picture of the problem and thereby avoid "alert fatigue". In paper III we evaluated if steady-state plasma levels of risperidone or the corresponding ...