An Antibiotic for Irritable Bowel Syndrome (IBS)?

Irritable bowel syndrome, or IBS, is a chronic gastrointestinal disorder that causes bloating, mucus in the stools, abdominal pain, and irregular bowel habits – diarrhea and/or constipation.  About 30 million Americans are known to be affected. Currently available treatments, such as fiber supplements or dietary manipulations, are not very satisfactory, as symptoms recur as soon as the treatment stops. Now the makers of an antibiotic, rifaximin, have completed two identically-designed double-blind, placebo-controlled trials in patients who had IBS without constipation. The New England Journal of Medicine contains a report of these studies authored by some of the investigators as well as representatives of the company, Salix.

Over 600 patients who had IBS without constipation were enrolled in each study, which took place at one of 179 investigational sites in the United States and Canada. Participants were randomly assigned to receive 550 mg of rifaximin or placebo, three times a day, for two weeks, with a follow-up period of 10 additional weeks. Each week the investigators determined the proportion of patients who had adequate relief of their IBS symptoms and the proportion that had adequate relief of IBS-related bloating, which was a second key study outcome.

More patients in the rifaximin groups than in the placebo groups had adequate relief of their symptoms during the first 4 weeks after treatment – 40.7% versus 31.7% in the two studies combined. Also, more patients in the rifaximin group had adequate relief of bloating – 40.2% versus 30.3% in the placebo groups. The incidence of side effects was similar in the two groups.

What's interesting about this study is the finding that the benefits of treatment, although only moderate, lasted four weeks after ending rifaximin. This suggests that the change in the intestinal flora brought about by the antibiotic persisted for a useful period. However, the benefits were not dramatic, although statistically significant. If one is to consider this form of treatment, would one have to re-treat after 10 weeks, or even less? The potential usefulness of rifaximin in IBS patients is not sufficiently demonstrated, to my mind, to justify a publication of these results in a journal like the New England Journal – especially when the drug manufacturer helped write the report. It will be interesting to see if the FDA approves the use of rifaximin for this indication (it is already approved for the treatment of hepatic encephalopathy and traveler’s diarrhea).