Possible Relief for Irritable Bowel

Local-acting antibiotic tops placebo in two trials.


By Nathan Seppa, Science News

An antibiotic typically used to combat traveler’s diarrhea might benefit some people with irritable bowel syndrome, a vexing condition that has few good treatment options, researchers report in the Jan. 6 New England Journal of Medicine.

There is no antibiotic currently approved for use against IBS, notes gastroenterologist Jan Tack of the University of Leuven in Belgium, writing in the same NEJM issue. Part of the reason stems from the mystery of IBS itself. Patients clearly have a problem with food movement in the gut—either too rapid (diarrhea) or too slow (constipation)—and experience pain and cramping.

That suggests the GI tract of some people might just sense poorly when to advance food, says William Chey, a gastroenterologist at the University of Michigan Health System in Ann Arbor and a coauthor of the new report. But an overgrowth of bacteria in the intestines apparently contributes to the problem, he says. That suggests an antibiotic could help, he says.

In two clinical trials, Chey and his colleagues pitted the drug rifaximin against a placebo in 1,260 patients who had IBS marked by cramping and diarrhea but not constipation. All participants received three pills a day for two weeks without knowing whether they were getting the drug or not. Over the following four weeks, 41 percent of those getting rifaximin had clear improvement of symptoms in at least two of those four weeks, compared with 32 percent of the people getting a placebo.

Although the benefits didn’t extend to all patients—or even a majority—the report is good news for people with IBS, Chey says. Approved IBS treatments mainly speed up or slow down motility, the movement of food through the gut, he says, whereas this drug takes on bacterial overgrowth. “This is just the initial salvo in this whole area,” he says, and should clear the way for further trials testing antibiotics against IBS.

The benefits of rifaximin lingered for 10 weeks after the two-week treatment. Although the percentage of people reporting improved IBS symptoms dropped off gradually in the later weeks, the average  scores of people taking the drug remained better than those in the placebo group.

Rifaximin is poorly absorbed through the intestines, a drawback against some diseases. For example, it wouldn’t work against pneumonia, says Herbert DuPont, an infectious disease physician at the Baylor College of Medicine in Houston. But remaining localized to the bowel, and particularly the small intestine, makes rifaximin valuable against traveler’s diarrhea and now potentially against IBS, he says.

The Food and Drug Administration is now considering licensing rifaximin for IBS, DuPont says, meaning insurance companies would be much more likely to cover it. The drug is sold as Xifaxan by its manufacturer, Salix Pharmaceuticals, which funded the new research.

Until more research is done, Tack says, it might be prudent to limit the use of rifaximin and similar antibiotics to IBS patients who have confirmed bacterial overgrowth in the intestines or who have diarrheal IBS that hasn’t responded to other treatments.


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