Rifaximin SSD in Dementia Trial

Inclusion Criteria: * Probable Alzheimer's Disease (AD) or Vascular Dementia (VaD) mild or moderate based on Clinical Dementia Rating Scale. * Males and Females Age ≥ 65 years * Community living with availability of caregiver to accompany participant to study visits and to participate in the study. * Able to consent or legal guardian who can consent (with participant assent). * Legally authorized representative (LAR) and caregiver for the study is the same individual. * Fluency (both participant and caregiver) in written and spoken English to participate in study visits. Exclusion Criteria: * Dementia not due to AD or VaD * Clinically significant agitation or aggression (requiring treatment with antipsychotic medication) * Delusions and/or hallucinations * Severe psychopathology including major depression * Unstable, severe, or poorly controlled medical conditions evident from physical examination or clinical history * Visual and/or hearing disorder that prevents completion of neuropsychologic evaluations. * Diarrhea * Hypersensitivity to rifaximin, components of rifaximin, * and any rifamycin antimicrobial agent * Antibiotic use in the prior 6 months * Taking medications that interact with Rifaximin. P-glycoprotein (P-gp) inhibitor treatment is permitted as long as the use of P-gp inhibitors is discussed with the investigator. * History of alcohol and/or drug abuse * Participation in another investigational drug trial in the last 30 days