OBJECTIVE: The aim of this study was to assess potential drug-drug interactions (pDDIs) at hospital admission, during hospitalization and at discharge and to evaluate the number of pDDIs created during hospitalization. METHODS: The medication of 851 patients was screened for pDDIs (major and moderate severity) using the screening program Pharmavista. The frequency of pDDIs per patient, per number of drugs and drug pairs was estimated. RESULTS: During hospitalization, the frequency of major and moderate pDDIs per patient was 1.11, which was higher compared to hospital admission (0.59) or to hospital discharge (0.60). The frequency of major and moderate pDDIs per drug prescribed (13.7% vs. 9.1%) or per drug pairs analyzed (4.5% vs. 2.3%) was higher at hospital admission compared to hospital discharge. 47% of all major and moderate pDDIs at discharge were due to a medication change during hospitalization. CONCLUSIONS: Although the number of major and moderate pDDIs per patient did not increase from hospital admission to discharge, it is important to realize that 47% of all major and moderate DDIs at hospital discharge were created during hospitalization. Prescribing drugs with a low risk for pDDIs as well as careful monitoring for adverse drug reactions are important measures to minimize harm associated with DDIs.