OBJECTIVES: To assess the prevalence rate of concomitant drug use and the potential for drug interactions in Alzheimer's Disease (AD) patients prescribed donepezil. METHODS: Outpatient pharmacy claim data from the Protocare Sciences Proprietary Managed Care database during the period from October 1, 1998 to December 31, 1998 was reviewed. Donepezil-treated patients were defined as patients having at least one outpatient pharmacy claim for donepezil. Patients at risk of potential drug-drug interaction were identified as those patients who were prescribed at least one drug that has the potential to interact with donepezil based on published literature including the package insert. AD patients with the above criteria were classified as being at risk for adverse reactions. RESULTS: Among AD patients using donepezil, 41% were prescribed at least one medication with the potential to interact with donepezil. CONCLUSIONS: Based on this study, forty-one out of every 100 patients who use donepezil are likely to experience drug-drug interactions. The risk that this confers warrants physician attention and policy considerations at several levels of organization. To avoid adverse clinical effects and the economic consequences of drug interactions, donepezil should be prescribed with caution, and alternative medications that do not have a potential for interaction should be considered.