Participants were assigned to take 550 milligrams of rifaximin or dummy pills for two weeks, three times a day. For the next four weeks, they were asked if their symptoms improved. Overall, 41 percent of those on rifaximin reported “adequate relief” for at least two weeks, compared to 32 percent on dummy pills. The benefits lasted for the 10 weeks they were followed. 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, N.C. 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.