An antibiotic called rifaximin has shown promise in treating irritable bowel syndrome (IBS) in two large clinical trials by targeting bacteria in the gut, researchers found.

Four weeks after the beginning a two-week-long treatment with the drug, 41 percent of people who took it said they had relief from their IBS symptoms including bloating, abdominal pain and loose and watery stools. Of those who took the placebo, 32 percent reported relief, the study showed.

With "the other medications studied in the past, you have to stay on them, otherwise one week after stopping, you're back the way you were," said study researcher Dr. Mark Pimentel, director of the Gastrointestinal Motility Program and Laboratory at Cedars-Sinai Medical Center in Los Angeles. But with rifaximin, "you treat it, it's kind of a one-and-done, for at least a period of time."

Rifaximin is currently approved by the Food and Drug Administration (FDA) to treat traveler's diarrhea and hepatic encephalopathy, a condition that results when the liver is unable to remove toxic substances from the blood. The FDA will likely make a decision about approving rifaximin to treat IBS within the next two to three months, Pimentel said.

The new studies will be published tomorrow (Jan. 6) in the New England Journal of Medicine. Salix Pharmaceuticals, Inc., the manufacturer of rifaximin, provided funding for the study. Pimentel is a consultant to Salix, Inc., and serves on its scientific advisory board.

Finding relief

IBS is one of the most common disorders in the United States, according to the National Institute of Diabetes and Digestive and Kidney Diseases. As many as one in five Americans has the condition, which is characterized by severe cramps, bloating, constipation, diarrhea and abdominal pain.

IBS can be caused by excessive amounts of bacteria in the small intestine , Pimentel said. Current medications for IBS may relieve symptoms, but to provide lasting relief from the condition, a drug has to kill the gut bacteria that causes it.

Pimentel and his colleagues tested rifaximin in two trials with more than 600 patients in each.

The people in the trials all had IBS with mild to moderate diarrhea and bloating. Some were assigned to take 550 milligrams of rifaximin three times a week for two weeks, and others were assigned a placebo .

Pimentel said patients experienced relief even eight weeks after they stopped taking the drug, whereas other IBS drugs only provide relief if the patient is currently taking the drug.

Rifaximin versus other drugs

Rifaximin is different from other antibiotics one might take for a urinary tract infection or a sinus infection, Pimentel said. He said it less likely to create antibiotic resistance because 99 percent of it passes out of the body in the feces, rather than being absorbed into other areas of the body outside the gut.

People who took the drug to treat IBS also didn't have any more or less side effects than people who took the placebo, Pimentel said.

Pass it on: The antibiotic rifaximin could soon be approved for use in treating irritable bowel syndrome.

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