Using a new formulation of rifaximin, a non-absorbable antibiotic, to test if it can affect microbes in the gut of patients with dementia favorably.

Hypothesis: Rifaximin SSD therapy is safe and well tolerated in patients with AD and VaD with beneficial changes in systemic inflammation and systemic biomarkers of dementia due to improvement in microbiota function compared to placebo-related changes in a single-blind, placebo-controlled Phase 1b/2a trial .

Overall Objective: In a single blind placebo-controlled trial in patients with Alzheimer's or vascular dementia, to determine the effect of rifaximin SSD compared to placebo on gut microbial structure and function, cognitive and daily function, and caregiver burden.

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

Not IRB approved to do so