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.