Functional gastrointestinal disorders (FGIDs) are common and from the most recent global epidemiology study, an estimated 40% of the world population suffer from the condition. FGIDs cause significant morbidity to patients, despite not influencing mortality. IBS is among the most important functional gastrointestinal disorder with an estimated 3.8 to 9.2 % of the general population worldwide were affected by this disorder.

Rifaximin (gut specific antibiotic) and low FODMAP diet (dietary based therapy) were proven to be effective in treating irritable bowel syndrome (IBS), however there was no head-to-head study comparing both treatments. This study will help doctors to understand the efficacy of different IBS/SIBO treatments. With the evaluation of factors that can predict treatment response, doctor could potentially treat IBS and SIBO more effectively in future.

The purpose of the study is to compare the clinical symptoms and psychological improvement in patients with irritable bowel syndrome (IBS) after treatment with Rifaximin versus treatment with low FODMAP diet. The factors that is associated with treatment response will also be evaluated. In IBS patients with small intestinal bacterial overgrowth (SIBO), eradication rate of SIBO will be evaluated.

Inclusion Criteria:

1. Patients diagnosed with Irritable bowel syndrome using Rome III diagnostic criteria with Abdominal Pain/ Discomfort Intensity of weekly average of worst abdominal pain/ discomfort in past 24 hours score of ≥ 3.0 on a 0 to 10 point scale AND Stool Consistency of at least 2 days per week with at least one stool that has a consistency of Type 6 or Type 7 BSS or <3 CSBM per week
2. Subject who can provide written informed consent and willingness to comply with the requirement of the protocol
3. Able to communicate in English, Malay, or Mandarin languages

Exclusion Criteria:

1. Patients with known hypersensitivity or contraindication to Rifaximin
2. Pregnant / breastfeeding women
3. Patients who are on probiotics for the past 1 month
4. Presence of family history of GI malignancy or alarm features suggested malignancy - e.g. Unintentional weight loss (≥ 10% of body weight in recent 6 months), GI bleeding
5. History of gastrointestinal (GI) malignancy
6. Patients with any hepatobiliary or pancreatic diseases
7. Patients with severe depression, anxiety, or other psychological disorder
8. Patients with any terminal disease
9. Other conditions determined by the investigator to be inappropriate for this clinical study