The antibiotic, called rifaximin, reduced symptoms such as bloating, cramps, diarrhea and constipation for as long as 10 weeks after it was given in a 10-day course, the researchers report in this week's issue of the Annals of Internal Medicine.

As many as 20 percent of U.S. adults suffer from irritable bowel syndrome, also known as spastic colon, according to the National Institutes of Health. It can be disabling in severe cases.

Its cause is unknown but may be related to bacteria that naturally live in the gut. In patients with irritable bowel syndrome, the muscles in the colon do not function normally and may spasm.

Dr. Mark Pimentel and colleagues at Cedars-Sinai Medical Center in Los Angeles gave 87 sufferers either 400 mg of rifaximin three times a day for 10 days or a placebo.

The patients filled out a weekly questionnaire about their symptoms.

Use of rifaximin helped overall symptoms and the benefits lasted for the full 10 weeks of follow-up, Pimental's team reported. Rifaximin, an antibiotic approved for travelers' diarrhea, is made by Salix Pharmaceuticals, Inc. under the brand name Xifaxan.

"The fact that the benefit of the targeted antibiotic continued even after it was stopped provides evidence that the antibiotic was acting on a source of the problem: excess bacteria in the gut," he said in a statement.

"This finding offers a new treatment approach -- and a new hope -- for people with IBS."