Clinically important potential drug-drug interactions in outpatient settings

Although many potential drug-drug interactions (DDIs) are discussed in the clinical literature, only limited numbers of these are clinically important. Little is known about prescribing of agents involved in these clinically important DDIs at the national level.

This study examined concomitant prescribing of drug combinations that have potential for clinically important DDIs in outpatient settings in the United States.

The 2000-2002 National Ambulatory Medical Care Survey and the outpatient department portion of the 2000-2002 National Hospital Ambulatory Medical Care Survey were used to examine 25 clinically important DDIs that are most likely to harm patients if not detected. Descriptive statistics were used to characterize clinically important DDIs, whereas logistic regression analysis was applied to outpatient visits to examine patients at risk for clinically important DDIs.

An estimated 0.63% (95% confidence interval, 0.51-0.75) of outpatient visits involving 2 or more medications resulted in prescription combinations with potential for clinically important DDIs for an annual visit estimate of 2.26 million, or annual visit rate of 8.12 visits per 1000 persons. Fourteen of the 25 prescription combinations with potential for clinically important DDIs were prescribed in outpatient settings, with warfarin accounting for many of the visits that yielded the prescribing of interacting medications. A majority (82%) of the prescriptions for potentially interacting medications occurred in visits involving 5 or more medications. The study found that patients over 44 years of age, especially elderly, Medicare beneficiaries, and those prescribed multiple medications are at risk for receiving prescription combinations with potential for a clinically important DDI.

Although a small percentage of outpatient visits resulted in prescription combinations with potential for clinically important DDIs, these interactions can have significant implications due to medication-related morbidity and mortality. Pharmacists can play a critical role in managing the medication therapy of patients at risk for clinically important DDIs. Further research is needed to evaluate the economic, clinical, and humanistic outcomes of clinically important DDIs, especially among those at risk.