To compare the clinical efficacy of donepezil between the naive group and the switching group.

The purposes of this study are:

1. to compare the efficacy between switching patients with Alzheimer's disease (AD) from galantamine or rivastigmine to donepezil because they were not responding adequately, and naive patients with AD who initiated therapy with donepezil
2. to help to clinicians in choosing the best treatment

Inclusion Criteria:

1. A diagnosis of probable AD according to the criteria of the NINCDS-ADRDA
2. Korean version Mini-Mental State Examination scores between 10 and 26
3. History of cognitive decline that had been gradual in onset and progressive over at least 6 months
4. A caregiver who could assist the patient with medication, attend the assessment and provide information about the patient.

Exclusion Criteria:

1. they had evidence of any neurodegenerative diseases other than AD (i.e. Parkinson's disease, Huntington's disease)
2. Psychiatric disorder or severe behavioral disturbances that required psychotropic medications
3. Cerebral injuries induced by trauma, hypoxia, and/or ischemia
4. Clinically active cerebrovascular disease; History of seizure disorder
5. Other physical conditions that required acute treatment.