Incidence and contributors to potential drug-drug interactions in hospitalized patients.

Drug-drug interactions (DDIs) are drug combinations that result in pharmacological or clinical responses that differ from solitary administration. Previous studies of DDIs have been limited to particular drugs or particular patient populations. The authors performed a retrospective cohort study of all adults admitted to a teaching hospital between 1999 and 2005. All medications administered to patients were identified and compared with a standard reference of important DDIs. The authors measured the potential DDI incidence density as the percentage of time in the hospital during which patients were exposed to at least 1 DDI and used multivariate Poisson regression to determine its determinants. A total of 19.3% of 140 349 hospitalizations had at least 1 potential DDI. The potential DDI incidence density was 18.8%. Factors having the greatest influence on potential DDI incidence density included increased patient age (adjusted rate ratio patient >75 years vs <30 years, 2.25; 95% CI, 2.15-2.35), increased number of drug orders (adjusted rate ratio, 2.27 [2.23-2.30] for logarithm), and patient service (adjusted rate ratio, 1.49 [1.46-1.52] for surgical vs medical service). Potential DDIs were present during one fifth of hospitalization time.