Rifaximin Versus No Intervention for the Treatment of IgG/IgA (Non-IgM) Monoclonal Gammopathy of Undetermined Significance

Inclusion Criteria:

* Age ≥ 18 years
* Ability to understand and willingness to sign a written informed consent document. Legally authorized representatives may sign and give informed consent on behalf of potential study participants
* Clinical diagnosis of IgG/IgA (non-IgM) monoclonal gammopathy of undetermined significance (MGUS) based on International Myeloma Working Group (IMWG)-2014 criteria (Rajkumar et al, Lancet Oncology, 2014). It is recognized that not all patients with clinical diagnosis of MGUS undergo bone marrow biopsy. Bone marrow biopsy confirmation will not be required if not clinically indicated per treating provider
* Agree to use adequate contraception

  * For women of child-bearing potential: prior to study entry and for the duration of study participation
  * For men: prior to study entry, for the duration of study participation, and one month after completion of rifaximin administration (for men)
* No antibiotic use in the preceding 2 weeks

Exclusion Criteria:

* Participants who are receiving other investigational agents
* Pregnant women
* Known hypersensitivity to rifaximin
* Another concurrent malignancy requiring active therapy