A 79-year-old woman with Alzheimer's disease was admitted due to acute cholinergic symptoms induced by overdose (45 mg) of donepezil (DPZ). Physical examination showed bradycardia, sinus arrhythmia, vomiting and respiratory insufficiency. She was treated with atropine intravenously and her cholinergic symptoms subsided in the next evening. After 5 days she was discharged from the hospital and DPZ was withholded for about 4 weeks. The plasma concentration of DPZ was 54.6 ng/ml on admission and gradually decreased to the normal limits in about 90 hours. The calculative half-life of DPZ was about 55 hours. Since the half-life of DPZ is long and differs between individuals, hospitalization and treatment with atropine is recommended for cholinergic side effects induced by overdose of DPZ. It is important to prevent dosing errors in cooperation with medical providers, patients and families.