Frequency and nature of drug-drug interactions in a Swiss primary and secondary acute care hospital

Drug-drug interactions (DDI) are considered a risk factor in medication safety and computerised alerting tools are increasingly promoted and implemented in order to detect and minimise DDI. As only little is known about the frequency and nature of DDI in hospitalised patients in Switzerland as well as about the usefulness of current alerting systems, this analysis based on a computerised medication record in a typical regional hospital setting was performed.All inpatients with at least one drug prescription between 2006 and 2010 were included. A total of 1,654,987 prescriptions were analysed with regard to the maximal seriousness level of DDI between each added prescription versus the existing prescription and with regard to all underlying DDI.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.Drug-drug interactions were very frequent and were very stable over the years studied, involving on average 27% of all prescriptions and 44% in internal medicine. Only a very limited amount of drugs were responsible for the vast majority of DDI, especially when the most severe categories of DDI were considered. Most of the severe DDI alerts could be automatically handled, if for example laboratory values could be taken into account. The DDI database should ideally be supplemented by information enabling more sophisticated computerised support in order to deliver more reasonable results from DDI checks.