Polypharmacy, aging and potential drug-drug interactions in outpatients in Taiwan: a retrospective computerized screening study.

BACKGROUND: Polypharmacy is common in outpatients and has been identified as a major risk factor for drug-drug interactions (DDIs), which are an important cause of adverse drug reactions. There has been a rapid increase in the number of elderly patients worldwide. However, there have been few studies quantifying the impact of both patient age and the number of concomitant drugs prescribed on the probability of potential DDIs per person in general outpatients.

OBJECTIVE: To assess the extent to which polypharmacy and aging are associated with potential DDIs in outpatients at a medical centre in Taiwan.

METHODS: The medications of 81,650 outpatients who visited a medical centre in Taiwan between January 2004 and March 2004 were retrospectively screened for potential DDIs using a computerized drug-interaction program. The main inclusion criteria were a minimum of two drug prescriptions and duration of use of 14 or more days. We also analysed the DDI pattern, which included severity, level of documentation and onset of potential DDIs, and assessed the impact of the number of drugs prescribed and of aging on the prevalence of potential DDIs per person.

RESULTS: The prevalence of potential DDIs was 25.6% (20,902 of 81,650). The mean ± SD age of the 20,902 patients with potential DDIs was 57.5 ± 16.5 years, and 47.6% of these patients were male. The mean ± SD number of prescribed drugs in patients with potential DDIs was 5.8 ± 2.4, and 67.7% of these patients were prescribed more than four drugs. The majority (55.7%) of DDIs were of the C2 pattern (severity: moderate; documentation: probable). The prevalence of potential DDIs increased in a linear mode with increasing age (p < 0.001) and with the number of drugs prescribed (p < 0.001); furthermore, in addition to being independently associated with potential DDIs, these two factors interacted to increase the risk further.

CONCLUSIONS: This study showed that approximately one-quarter of 81,650 outpatients who visited a medical centre in Taiwan over a period of 3 months in 2004 had potential DDIs. We observed independent increases in potential DDIs per person in association with aging and increasing number of prescribed drugs. Furthermore, a significant interaction between these two factors was observed: the effect of aging on the prevalence of potential DDIs increased as the number of prescribed drugs increased. Potential DDIs in outpatients can be reduced by minimizing the number of drugs prescribed following careful consideration of both their benefits and risks, particularly in the aging population.