A Study of the Safety and Efficacy of Memantine in Moderate to Severe Alzheimer's Disease

Inclusion Criteria: * Ambulatory patients aged \>/= 50 years * Diagnostic evidence of probable Alzheimer's disease consistent with criteria from the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) and the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) * Confirmatory magnetic resonance imaging (MRI) or computed tomographic (CT) scan within the prior 12 months. * Mini-Mental State Examination (MMSE) scores \>/= 3 and \</= 14 at Screening (Visit 1) and Baseline (Visit 2) * Ongoing daily acetylcholinesterase inhibitor (AChEI) therapy at a stable dose for at least 3 months prior to Screening (Visit 1). It is preferred that patients continue to receive the same AChEI therapy for the duration of the study. Exclusion Criteria: * Patients with a modified Hachinski Ischemia Score greater than 4 at Screening. * Patients who have taken memantine within one month of Screening (Visit 1) * Patients who have a known hypersensitivity to memantine, neramexane, rimantadine, or amantadine. * Patients whose AChEI therapy is likely to be interrupted or discontinued during the course of the study. * Patients who are receiving therapy with more than one AChEI. * Patients with computed tomography (CT) or magnetic resonance imaging (MRI) evidence of hydrocephalus, stroke, a space-occupying lesion, cerebral infection, or any clinically significant central nervous system disease other than Alzheimer's disease. * Patients with a DSM-IV Axis I disorder other than Alzheimer's disease, including amnestic disorders, schizophrenia or schizoaffective disorder, bipolar disorder, current major depressive episode, psychosis, panic, or post-traumatic stress disorder. * Patients who, in the clinician's judgement, are likely to be placed in a nursing home within the next 6 months. * Patients who had evidence of other neurological disorders that included, but were not limited to, stroke, Parkinson's disease, seizure disorder, or head injury with loss of consciousness within the prior 5 years * Patients who had dementia that was complicated by other organic disease * Patients who had dementia complicated by the presence of predominant delusions