Efficacy of Rifaximin vs Placebo in Reducing Symptoms in Adults With IBS

David A. Johnson, MD, FACG, FACP


November 10, 2006

The Effect of a Nonabsorbed Oral Antibiotic (Rifaximin) on the Symptoms of the Irritable Bowel Syndrome: A Randomized Trial

Pimentel M, Park S, Mirocha J, Kane SV, Kong Y

Ann Intern Med. 2006;145:557-563


Irritable bowel syndrome (IBS) is a prevalent condition that has been labeled as a "functional bowel disorder." By this delineation, its cause has been thought to be indefinable. The primary symptoms of this disorder include constipation, diarrhea, abdominal bloating, and cramping. Recent attention has focused on a potential infectious component of IBS.

In this study, Pimental and colleagues investigated the use of rifaximin in patients diagnosed with IBS as defined by the Rome I criteria. Rifaximin is a nonabsorbable, gut-selective antibiotic derived from the rifamycin family that may reduce bacterial overgrowth due to its broad-spectrum activity in vitro against gram-positive, gram-negative, aerobic, anaerobic, and microaerophilic bacteria.

This was a prospective, double-blind randomized controlled trial that involved 43 patients who received 400 mg of rifaximin 3-times daily and 44 who received placebo for 10 days. Patient symptom and stool diaries were completed prior to entry, during the study, and the week following treatment.

Global improvement was found to vary widely across weeks for most patients, and the data were reported as an averaging of the symptoms over all 10 weeks of the study. This percentage was significant as a function of group (P = .020), but not as a function of week (P = .78) or group-by-week (P = .96). The patients in the rifaximin group had a 36.4% improvement in global symptoms compared with 21.0% for the placebo group.

The rifaximin group also reported significantly less bloating on a visual analog scale compared with the placebo group (P = .010), a difference that persisted after controlling for differences between groups in baseline pain scores (P = .001). Besides bloating, none of the secondary endpoints improved with treatment compared with placebo on the visual analog scale: abdominal pain (P = .32), diarrhea (P = .67), and constipation (P = .069).


Rifaximin is currently US Food and Drug Administration-approved for the treatment of traveler's diarrhea, although these study findings suggest a more expanded potential role for this antibiotic. However, before recommending widespread globalized use of this medication for all patients with IBS, healthcare providers should be aware of several limitations of this study. First, although this was a "multicenter study," there was a considerable imbalance between enrollment from the 2 study sites (83 participants vs 3 participants). Second, because the global measure includes pain, it is important to note that there was an imbalance in baseline pain scores between the 2 patient groups at entry. The imbalance (higher baseline pain scores in the rifaximin group) potentially favors the reduction in the global scores. Additionally, this primary outcome measure is unique in the spectrum of the recent treatment intervention evaluations for IBS trials, and therefore makes comparisons with other trials somewhat difficult. Third, the study authors relied solely on the use of the lactose breath test to define small bowel bacterial overgrowth. Future tests in this area should evaluate the use of other breath tests to define bacterial overgrowth in the small intestine. Fourth, the data suggest that rifaximin may be of more help in the subset of patients with bloating. In IBS, abdominal bloating is reported in more than 50% of patients, and a recent study suggests that changes in abdominal girth can reach 12 cm in more than 50% of patients.[1]

Clearly, this study highlights a new concept in the potential pathogenesis of IBS. An infectious cause may offer a tremendous opportunity to manage an otherwise somewhat frustrating disease -- both for patients and their treating physicians.



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