This study will explore the different factors associated with drug response to acetylcholinesterase (AChE) inhibitor (donepezil) and NMDA receptor antagonist (memantine) in patients with Alzheimer's Disease.

These patients will be grouped according to the medications prescribed by their attending physician at baseline, 3rd month, and 6th month of follow up:

1. Alzheimer's Disease patients given AChE inhibitor monotherapy
2. Alzheimer's Disease patients given combination therapy of AChE inhibitor and NMDA receptor antagonist

They will be observed for treatment response for up to 6 months.

Inclusion Criteria:

• newly diagnosed with mild or moderate dementia using the Montreal Cognitive Assessment and Clinical Dementia Rating (CDR) performed by a licensed psychometrician
• clinically diagnosed by an expert adult neurologist as having probable AD using the National Institute of Neurological and Communicative Disorders and Stroke and Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria
• treatment naive for any acetylcholinesterase inhibitors or memantine OR those who have not taken any acetylcholinesterase inhibitors or memantine in the last three months for any reasons except for adverse drug reaction
• age 65 years old
• residing in the National Capital Region
• able to read and understand written and spoken English and Filipino

Exclusion Criteria:

• with structural or vascular causes of dementia other than subcortical lacunes (2 or less) as seen in plain CT scan
• dementia diagnosis other than AD as determined by an expert adult neurologist
• with untreated depression or related psychiatric disorders in the last 6 months
• use of systemic antibotics in the previous three months prior to providing fecal specimens
• use of corticosteroids, immune stimulating medications, and immunosuppressive agents within the past 2 weeks or those who regularly need them for immune-related disorders
• use of proton-pump inhibitors, H2-receptor antagonists, H2-receptor antagonists, tricyclic antidepressants, narcotics, anticholinergic medications, laxatives or anti-diarrheal in the past 4 weeks
• large doses of commercial probiotics consumed (greater than or equal to 108 cfu or organisms per day)
• major dietary change during previous month (defined as eliminating or significantly increasing a major food group)
• major GI tract surgery in the past 5 years, with the exception of cholecystectomy and appendectomy
• major bowel resection at any time
• active uncontrolled GI disorders or diseases, including inflammatory bowel disease, indeterminate colitis, irritable bowel syndrome, persistent infectious gastroenteritis, colitis or gastritis, persistent or chronic diarrhea of unknown etiology, recurrent Clostridium difficile infection, untreated Helicobacter pylori infection, chronic constipation

All elderly patients from (1) University of the Philippines - Philippine General Hospital; (2) The Medical City; and (3) Cardinal Santos Medical Center that meet the eligibility criteria