In two large studies, 41 percent of the patients who took the antibiotic rifaximin said their symptoms substantially improved, compared to 32 percent of those who got fake pills. Their relief lasted for up to 10 weeks.
Experts want to see if that translates into a longer term benefit. But the fact that relief extended beyond the two weeks of treatment suggests that "we've actually touched on the cause of IBS, as opposed to just covering up symptoms," said Dr. Mark Pimentel, of Cedars-Sinai Medical Center in Los Angeles, who led the research in the U.S. and Canada.
Irritable bowel syndrome, or IBS, is a common gastrointestinal ailment, affecting as many as 1 in 5 Americans. It is more common in women than men. Main symptoms are abdominal pain, bloating and diarrhea or constipation, or both.
What causes the disorder has been a mystery; sensitivity to certain foods or stress are among the theories. Patients are typically told to change their diet, reduce stress and take medicines or fiber supplements to ease symptoms.
Some scientists think an overgrowth of bacteria in the gut is behind the problem, but studies testing that have had mixed results. Pimentel and his colleagues tested rifaximin. Unlike other antibiotics, this one is not absorbed very well so it stays in the gut and is less likely to develop resistance. It's also been safely used elsewhere for more than two decades.
In the U.S., rifaximin is approved for traveler's diarrhea and for a complication of liver disease. Salix Pharmaceuticals, which markets rifaximin under the name Xifaxan in the U.S., is seeking to expand its use to IBS. A decision from the Food and Drug Administration is expected in March, said William Forbes, executive vice president and chief development officer for Salix.
The price for IBS treatment hasn't been determined, he said. But the price listed at Drugstore.com suggests it's not cheap, with a two-week supply of the dosage used in the study costing $910, or about $21 a pill.
While the response to rifaximin may seem low to outsiders, it's in the range seen with other effective IBS treatments, said Dr. Jan Tack of the University of Leuven in Belgium. Other findings suggest that a larger number of patients may have had some benefit - just not to the level of "adequate relief" that was used in these studies to measure response, he said.
"In this condition, where treatment options are really limited, anything that works is always welcomed," said Tack, who wrote an accompanying editorial. "Rifaximin does not treat all patients, but the results are definitely novel and important."
Still, until more research is done, he said the antibiotic should be restricted to those with confirmed bacterial overgrowth or patients who haven't responded to other IBS treatments.
The studies were paid for by the Salix, based in Raleigh, North Carolina. Some of the researchers were Salix employees and others had received consulting and other fees from the company. Cedars-Sinai holds a patent on the use of rifaximin for irritable bowel syndrome.