The purpose of this review was to focus on hospital admissions caused by a specific type of adverse drug reaction (ADR) that can be assumed to be avoidable in almost all cases: the drug-drug interaction (D-DI). To determine the epidemiology of D-DIs in hospital admissions, a review of the adverse drug reaction literature was undertaken to answer several questions: (a) what is the incidence of hospital admissions attributable to D-DIs?; (b) what percentage of drug-related hospital admissions are attributable to D-DIs?; (c) are there any patterns to the above findings, i.e. are some D-DIs or specific drugs more likely to have been associated with hospital admissions?; and (d) are there certain patient risk factors (e.g. age) that are associated with D-DIs that led to a hospital admission? Nine ADR studies were found that either included a D-DI category as a cause for hospital admissions, or provided sufficient information so that a causal relationship could be inferred. The incidence of hospital admissions due to D-DIs ranged from 0 to 2.8%. The data found in the studies we reviewed, however, were insufficient to allow meaningful quantification of specific drugs as usual causes for D-DI-related admissions, and because of the very small numbers of patients for which a D-DI was believed to be the cause it is not possible to provide a meaningful summary of risk factors specific for D-DI admissions. We cannot conclude that D-DIs are a significant problem. There is a need to view the quantification of D-DIs in relation to the number of medications prescribed by physicians, dispensed by pharmacists and taken by patients.

This article provides a historic overview of drug interaction screening and reviews 19 studies that have sought to measure the frequency of drug interactions. Differences in study designs, methodologies, and definitions contribute to considerable variation in the reported incidence rates, which ranged from 2.2 to 70.3 percent for all potential drug interactions. The percentage of patients actually experiencing symptoms that could be attributed to a drug interaction, however, ranged from 0 to 11.1 percent. The relative importance of drug interactions as a clinical problem remains unclear. Screening programs that do more than simply identify large numbers of patients who receive potentially interacting drug combinations without indicating which subpopulations of these individuals are likely to be harmed by the drugs have not yet been developed.

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