On average, each inpatient received 16 different drugs including on-demand prescriptions and encountered 5 DDI. A total of 27% of all prescriptions caused DDI. Within the last 12 months, 5% of all DDI were classified in category 1 (contraindicated), 3% in category 2, 53% in category 3, 8% in category 4 and 31% in category 5. The vast majority of DDI were caused by a very limited number of drugs.