• Severe Impairment Battery (SIB) and Mini-Mental State Examination (MMSE) to assess the cognitive function. [ Time Frame: Every 3 months (+/- 5 days). ] [ Designated as safety issue: No ]
• Alzheimer's Disease Cooperative Study - Activities of Daily Living (ADCS-ADL) to measure the functional capabilities of patient. [ Time Frame: Every 3 months (+/- 5 days). ] [ Designated as safety issue: No ]
• Quality of Life in Alzheimer's Disease (QoL-AD) and EuroQol-5 Dimensions (EQ-5D) for quality of life, Treatment Outcome Scale (TOS), and Goal Attainment Scale (GAtS). [ Time Frame: Every 3 months (+/- 5 days). ] [ Designated as safety issue: No ]

The purpose of this study is to evaluate the safety and efficacy of long-term administration of 23 mg donepezil sustained release (SR) in patients with moderate to severe Alzheimer's disease. Patients who complete study E2020-G000-326 with no ongoing serious adverse events (SAEs) and no serious adverse drug reactions will be eligible to enter the open-label extension study.

Inclusion Criteria for Patients:

1. Written informed consent from the patient (if possible) or from the patient's legal guardian or other representative at the time of the Baseline visit, prior to beginning any assessments or activities.
2. Completion of study E2020-G000-326 without ongoing SAEs or history of serious adverse drug reactions during study E2020-G000-326.
3. Patients must enroll in the present study within 3 days of completion of study E2020-G000-326
4. Health: physically healthy and ambulatory or ambulatory-aided (ie, walker or cane); corrected vision and hearing sufficient for compliance with testing procedures.
5. Co-morbid medical conditions must be well-controlled as determined by the investigator.
6. Patients undergoing treatment with selective serotonin reuptake inhibitors (SSRIs) may be included provided that doses are within the approved dose range as specified in the Physician's Desk Reference or local equivalent
7. Concomitant Medications: Patients undergoing treatment with the following medications may be enrolled in the study provided the following conditions are met: chronic daily benzodiazepine use if doses are stable within an approved dose range; bronchodilator medications for treatment of chronic obstructive pulmonary disease (COPD) as long as drug is administered via metered dose inhaler within approved dose range; memantine if taken at doses of 20 mg/day or less, provided that the dose is stable.
8. The patient must have a relative/caregiver who supervises the regular taking of the drug at the correct dose and is alert for possible side effects, unless the patient's legal guardian takes on this task.

Inclusion Criteria for Caregivers. Written informed consent will be obtained from the designated caregiver for participation in study assessments. The caregiver must be sufficiently familiar with the patient (as determined by the investigator) to provide accurate data. Specifically, the caregiver must have sufficient contact with the patient to provide accurate reports of the patient's functioning, must be able to observe for possible adverse events, and must be able to accompany the patient to all visits. It is preferable that the caregiver be the same as in study E2020-G000-326. If no replacement caregiver is available who meets the caregiver inclusion/exclusion criteria, the patient must be discontinued from the study.

Exclusion Criteria for Patients:

1. No caregiver available to meet the inclusion criteria for caregivers.
2. Patients with any active or clinically-significant conditions affecting absorption, distribution, or metabolism of the study medication (e.g., inflammatory bowel disease, gastric or duodenal ulcers, hepatic disease, or severe lactose intolerance).
3. Known plan for elective surgery during the study period that would require general anesthesia and administration of neuromuscular blocking agents, such as succinylcholine, to induce paralysis/muscle relaxation. Minor surgery, such as colonoscopy or cataract surgery, will be permitted as long as it does not require the use of these paralytic agents.
4. Patients who are unwilling or unable to fulfill the requirements of the study.
5. Use of any prohibited prior or concomitant medications.
6. Any condition that would make the patient, in the opinion of the investigator, unsuitable for the study.
7. Patients taking concomitant antidepressant medication known to have significant anticholinergic effects, such as tricyclic antidepressants prescribed at doses recommended for the treatment of major depression.
8. Patients who cannot swallow or who have difficulty swallowing whole tablets.
9. Patients taking any alternative medication such as vitamins and/or herbal products or alternative medical techniques such as acupuncture or acupressure specifically for the treatment of Alzheimer's disease.

Exclusion Criteria for Caregivers.

1. Caregivers who are unwilling or unable to give informed consent or otherwise to fulfill the requirements of the study.
2. Any condition that would make the caregiver, in the opinion of the investigator, unsuitable for the study.