Donepezil is a selective acetylcholinesterase inhibitor that is widely prescribed for Alzheimer's disease (AD). It has been shown to be of benefit in mild, moderate and severe stages of AD, vascular dementia and dementia associated with Parkinson's disease. Donepezil is absorbed slowly, but completely, from the gut, reaching peak plasma levels in 3-4 h and, with daily dosing, steady-state concentration in 15-21 days. Within a relatively narrow range, there is a linear relationship between dose and pharmacodynamic effects, measured as red blood cell acetylcholinesterase inhibition and clinical efficacy. Donepezil is principally excreted unchanged in the urine, but there is also hepatic metabolism; some of its metabolites may be active. Despite being 96% bound to plasma proteins, it has few interactions with other drugs, and the 5-mg dose can be given safely to patients with mild-to-moderate hepatic and renal -disease. Side effects, which are mainly a consequence of its cholinomimetic mechanism of action, are usually mild and transient. Although donepezil was originally developed to inhibit the breakdown of the neurotransmitter acetylcholine as symptomatic therapy for AD, recent studies raise the possibility of other effects this drug has on the pathogenesis of AD.