A Phase 3 interventional study of Rifaximin (drug) and Placebo in Bloating, Rifaximin and Functional Bowel Disorder, sponsored by Mahidol University. Not yet recruiting at 1 site in Thailand. Open to participants aged 18 Years to 80 Years. Per ClinicalTrials.gov, last updated 2026-10-27.
Sponsored by Mahidol University · Phase 3, Interventional, and Treatment
This randomized, double-blind, placebo-controlled trial will evaluate the effects of a 14-day course of rifaximin on bloating symptoms, gut microbiota, microbial metabolites, and intestinal gases in adults with Rome V irritable bowel syndrome with constipation (IBS-C) or functional abdominal bloating (FAB). Participants with predominant, bothersome bloating despite adequate bowel movement management will be randomly assigned in a 1:1 ratio to receive rifaximin 550 mg three times daily or matching placebo for 2 weeks. The primary endpoint is the proportion of participants with a bloating response, defined as at least a 1-point reduction from baseline on a 7-point Likert bloating scale at the end of treatment. Additional assessments will include other gastrointestinal symptoms, quality of life, psychological symptoms, gut microbiota diversity and composition, fecal short-chain fatty acids, plasma bile acids and gut-derived uremic toxins, and breath hydrogen and methane levels. Associations between biological changes and clinical response will also be examined.
Abdominal bloating is a common and bothersome symptom in disorders of gut-brain interaction, including irritable bowel syndrome with constipation (IBS-C) and functional abdominal bloating (FAB). Its mechanisms may include altered gastrointestinal motility, visceral hypersensitivity, microbial fermentation, and changes in intestinal microbiota. Symptoms may persist despite adequate management of constipation, and treatment options specifically targeting bloating remain limited.
Rifaximin is a minimally absorbed oral antibiotic that may improve bloating by modifying intestinal microbiota and microbial metabolic activity. This randomized, double-blind, placebo-controlled, parallel-group trial will evaluate its effects on bloating and associated biological measures in adults aged 18-80 years with Rome V IBS-C or FAB. Eligible participants must have predominant, bothersome bloating, with a score of at least 3 on a 7-point Likert scale, despite adequate bowel movement management.
Participants will be randomly assigned in a 1:1 ratio to receive rifaximin 550 mg three times daily or matching placebo for 14 days. Computer-generated randomization will use blocks of four, with stratification by diagnostic subgroup. Participants, treating physicians, and investigators will remain blinded to treatment allocation.
Baseline assessments will include demographic and clinical characteristics, gastrointestinal symptoms, bowel habits, quality of life, and depression, anxiety, and stress symptoms. Stool samples will be collected for gut microbiota profiling using shotgun metagenomics and measurement of fecal short-chain fatty acids. Blood samples will be collected to measure plasma bile acids and gut-derived uremic toxins. Lactulose breath testing will assess hydrogen and methane levels. Clinical and laboratory assessments will be repeated at the end of treatment.
During treatment, participants will record daily bloating and abdominal pain scores, bowel movement frequency, stool consistency using the Bristol Stool Form Scale, and rescue medication use. Prespecified rescue medications will be permitted for breakthrough symptoms. A telephone assessment at week 1 will evaluate symptoms, quality of life, psychological symptoms, treatment satisfaction, medication adherence, and adverse events.
The primary endpoint is the proportion of participants with a bloating response, defined as at least a 1-point reduction from baseline on the 7-point Likert bloating scale at the end of treatment. Secondary outcomes include abdominal pain, IBS symptom severity, bowel habits, quality of life, depression, anxiety and stress scores, treatment satisfaction, and rescue medication use. Biological outcomes include changes in gut microbiota diversity and composition, fecal short-chain fatty acids, plasma bile acids and gut-derived uremic toxins, and breath hydrogen and methane levels. Analyses will also examine associations between biological measures and clinical outcomes and identify baseline factors associated with treatment response.
1,116 studies on the registry are indexed under Irritable Bowel Syndrome; 208 are open to participants now.
This study's planned enrollment of 78 is above the median of 70 across 896 interventional studies indexed under Irritable Bowel Syndrome.
Browse Irritable Bowel Syndrome studies →Mahidol University is the lead sponsor of 747 studies on the registry; 119 are open to participants now.
Counted across the registry records on this site, refreshed daily.
The criteria were revised on the registry on Oct 27, 2026. See what changed
Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Malignancy means cancer. Criteria about another or earlier malignancy ask whether you have had a different cancer, usually within a set number of years.
Some are commonly allowed, such as treated skin cancers other than melanoma and early cancers removed completely — check the exceptions this study lists.
Cancer (malignancy)
Malignancy means cancer. Criteria about another or earlier malignancy ask whether you have had a different cancer, usually within a set number of years.
Some are commonly allowed, such as treated skin cancers other than melanoma and early cancers removed completely — check the exceptions this study lists.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Participants randomized to this arm receive rifaximin 550 mg orally three times daily for 2 weeks.
Drug: Rifaximin (drug)
Participants randomized to this arm receive an identical placebo orally three times daily for 2 weeks.
Drug: Placebo
Participants randomized to this arm receive rifaximin 550 mg orally three times daily for 2 weeks. Allocation is randomized and double-blinded.
Matching placebo administered orally three times daily for 14 days.
Bloating responder rate
Bloating responder is defined as at least a 1-point reduction from baseline to week 2 in bloating symptom severity measured using a 7-point Likert scale for bothersomeness of bloating (range 0 to 6; higher scores indicate worse symptoms). Score categories are 0 = not at all, 1 = hardly, 2 = somewhat, 3 = moderately, 4 = a good deal, 5 = a great deal, and 6 = a very great deal.
Time frame: From enrollment to the end of treatment at 2 weeks
Abdominal pain response rate
Abdominal pain responder is defined as at least a 1-point reduction from baseline to week 2 in abdominal pain severity measured using a 7-point Likert scale for bothersomeness of abdominal pain (range 0 to 6; higher scores indicate worse symptoms). Score categories are 0 = not at all, 1 = hardly, 2 = somewhat, 3 = moderately, 4 = a good deal, 5 = a great deal, and 6 = a very great deal.
Time frame: From enrollment to the end of treatment at 2 weeks
Change in Global IBS Symptoms (IBS-C patients)
Change in IBS Symptom Severity Score (IBS-SSS) from baseline, assessing overall IBS symptom burden, abdominal pain, bloating, and bowel habit changes. The IBS-SSS scale ranges from 0 to 500, with higher scores indicating more severe IBS symptoms.
Time frame: From enrollment to the end of treatment at 2 weeks
Treatment satisfaction
Overall treatment satisfaction, measured using a Visual Analog Scale (VAS) ranging from 0 to 10: 0 = Completely Dissatisfied 10 = Completely Satisfied Higher scores indicate greater satisfaction with treatment. The difference between baseline and post-intervention scores will be analyzed.
Time frame: End of treatment at 2 weeks
Change in Psychological Symptoms
Change in Depression, Anxiety, and Stress Scores (DASS-21), a validated self-reported scale measuring psychological distress. The DASS-21 consists of three subscales: Depression (0-21) Anxiety (0-21) Stress (0-21) Each subscale score ranges from 0 (normal) to 21 (severe symptoms), with higher scores indicating greater psychological distress. The total score is calculated by summing individual subscale scores.
Time frame: From enrollment to the end of treatment at 2 weeks
Change in Quality of Life (IBS-C patients)
Change in IBS-specific Quality of Life (IBS-QoL) score, which assesses the impact of IBS on daily activities, emotional well-being, and social functioning. The IBS-QoL score ranges from 0 to 100, with higher scores indicating better quality of life.
Time frame: From enrollment to the end of treatment at 2 weeks
Change in quality of life (FAB patients)
The quality of life of patients with FAB/D was measure with 36-Item Short Form Health Survey (SF-36). The SF-36 assesses health-related quality of life across 8 domains: physical functioning, role limitations due to physical health, role limitations due to emotional problems, energy/fatigue, emotional well-being, social functioning, pain, and general health. Each domain score is transformed to a 0 to 100 scale, with higher scores indicating better health status.
Time frame: From enrollment to the end of treatment at 2 weeks
Stool microbiota diversity and composition
Change from baseline in stool microbiota alpha diversity, beta diversity, and taxonomic abundance measured by shotgun metagenomics.
Time frame: From enrollment to the end of treatment at 2 weeks
Change in intestinal gas measurement (hydrogen)
Change from baseline in hydrogen gas production measured in part per million unit during lactulose hydrogen breath testing.
Time frame: From enrollment to the end of treatment at 2 weeks
Change in intestinal gas measurement (methane)
Change from baseline in methane gas production measured in part per million unit during lactulose hydrogen breath testing.
Time frame: From enrollment to the end of treatment at 2 weeks
Proportion of patients with positive lactulose hydrogen breath test
A positive lactulose hydrogen breath test is defined as either a rise in breath hydrogen of 20 ppm or more above baseline within 90 minutes or a breath methane concentration of 10 ppm or more at any time during the test after ingestion of 10 g lactulose. Breath samples are collected every 15 to 30 minutes for up to 180 minutes. Higher proportions indicate a worse outcome (more abnormal breath test results).
Time frame: From enrollment to the end of treatment at 2 weeks
Rescue medication use
Frequency and dose of rescue medications used for breakthrough symptoms during the treatment period.
Time frame: From enrollment to the end of treatment at 2 weeks
Adverse events
Incidence of treatment-emergent adverse events.
Time frame: From enrollment to the end of treatment at 2 weeks
Association between baseline bloating symptom severity score and bloating response at 2 weeks
Exploratory analysis of whether baseline bloating symptom severity predicts treatment response. Baseline bloating symptom severity is assessed using a 7-point Likert scale for bothersomeness of bloating (score range 0 to 6; higher scores indicate worse symptoms). The association between baseline score and bloating responder status at 2 weeks will be evaluated using logistic regression and reported as an odds ratio. Bloating responder is defined as at least a 1-point reduction from baseline in bloating symptom severity score.
Time frame: Baseline predictor with treatment response assessed at 2 weeks
Association between baseline fecal microbiota alpha diversity and bloating response at 2 weeks
Exploratory analysis of whether baseline fecal microbiota alpha diversity predicts treatment response. Baseline stool microbiota is assessed using 16S rRNA sequencing. Alpha diversity will be assessed using the Shannon diversity index. The association between baseline Shannon diversity index and bloating responder status at 2 weeks will be evaluated using logistic regression and reported as an odds ratio. Bloating responder is defined as at least a 1-point reduction from baseline in bloating symptom severity score.
Time frame: Baseline predictor with treatment response assessed at 2 weeks
Association between baseline positive diagnosis of small intestinal bacterial overgrowth and bloating response at 2 weeks
Exploratory analysis of whether baseline small intestinal bacterial overgrowth (SIBO) status predicts treatment response. Baseline SIBO status will be assessed using a lactulose hydrogen-methane breath test after ingestion of 10 g lactulose, with breath samples collected every 15 to 30 minutes for up to 180 minutes. A positive SIBO diagnosis is defined as a rise in breath hydrogen of 20 parts per million or more above baseline within 90 minutes. The association between baseline positive SIBO status (yes/no) and bloating responder status at 2 weeks will be evaluated using logistic regression and reported as an odds ratio. Bloating responder is defined as at least a 1-point reduction from baseline in bloating symptom severity score.
Time frame: Baseline predictor with treatment response assessed at 2 weeks.
Association between baseline positive diagnosis of intestinal methanogen overgrowth and bloating response at 2 weeks
Exploratory analysis of whether baseline intestinal methanogen overgrowth (IMO) status predicts treatment response. Baseline IMO status will be assessed using a lactulose hydrogen-methane breath test after ingestion of 10 g lactulose, with breath samples collected every 15 to 30 minutes for up to 180 minutes. A positive IMO diagnosis is defined as a breath methane concentration of 10 parts per million or more at any time during the test. The association between baseline positive IMO status (yes/no) and bloating responder status at 2 weeks will be evaluated using logistic regression and reported as an odds ratio. Bloating responder is defined as at least a 1-point reduction from baseline in bloating symptom severity score.
Time frame: Baseline predictor with treatment response assessed at 2 weeks.
Changes in fecal short-chain fatty acid concentrations
Between-group differences in changes from baseline in fecal short-chain fatty acid concentrations following rifaximin or placebo treatment.
Time frame: From enrollment to the end of treatment at 2 weeks
Changes in plasma bile acid concentrations
Between-group differences in changes from baseline in plasma bile acid concentrations following rifaximin or placebo treatment.
Time frame: From enrollment to the end of treatment at 2 weeks
Changes in gut-derived uremic toxin concentrations
Between-group differences in changes from baseline in plasma gut-derived uremic toxin concentrations following rifaximin or placebo treatment.
Time frame: From enrollment to the end of treatment at 2 weeks
Bloating response at weeks 4
Proportion of participants in each treatment group with at least a 1-point reduction from baseline on the 7-point Likert bloating scale, assessed by telephone at weeks 4.
Time frame: From enrollment to 4 weeks
Bloating response at weeks 8
Proportion of participants in each treatment group with at least a 1-point reduction from baseline on the 7-point Likert bloating scale, assessed by telephone at weeks 8.
Time frame: From enrollment to 8 weeks
Plan to share: No — To protect participant confidentiality and comply with institutional and regulatory requirements, no IPD will be shared unless with reasonable request.
Start is estimated for Dec 2026; primary completion for Jun 2028, 18 months later.
Of 23 phase 3 interventional studies under Irritable Bowel Syndrome, half reached primary completion within 30 months of starting (the median), counting those still running; a quarter within 19 and three quarters within 55. This study's planned 18 months is below that.
Since we started tracking on Sep 25, 2026, the dates moved once, on Oct 27, 2026: start Apr 2026 → Dec 2026, primary completion Oct 2027 → Jun 2028 and completion Dec 2027 → Dec 2028.
Earlier changes are on the record history on ClinicalTrials.gov ↗
How these figures are worked out, and which studies are left out of them.
From the registry record's own update history. This site started tracking changes on Sep 25, 2026; for anything earlier, see the record history on ClinicalTrials.gov ↗
This study is not yet recruiting, as verified in Oct 2026. You cannot join it, but the record below documents what was studied.
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Studies of the drug tested here: Rifaximin