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