The tolerability and safety of donepezil HCI in patients with mild to moderate Alzheimer's disease (AD) were examined in an integrated analysis of phase II/III placebo-controlled trials. Patients with mild to moderately severe AD (n=1,920) were randomised to receive donepezil (n=1,291) or placebo (n=629). Adverse events, physical examinations and clinical laboratory tests were assessed. A high completion rate (79%) was achieved in these trials. Of the 1,291 patients receiving donepezil only, 142 (11%) withdrew because of an adverse event compared with 43 of the 629 (7%) placebo patients. The most common adverse events included nausea, diarrhoea, headache, insomnia, dizziness, rhinitis, vomiting, asthenia/fatigue and anorexia. Donepezil had no clinically significant effect on any laboratory evaluations and was not associated with hepatotoxicity. These results demonstrate that donepezil is well tolerated and has a favourable safety profile at clinically effective, once-daily doses of 5 mg and 10 mg.