The irritable bowel syndrome (IBS) is a common disorder that affects more than 60 million Americans ( IBS is about twice as common in women compared to men. IBS is a gastrointestinal disorder in which patients experience chronic disruptions in bowel habit (diarrhea, constipation or both diarrhea and constipation) and abdominal pain. Psychological stress worsens these symptoms. The causes of IBS are not clear but it is likely that it is a multifactorial disorder involving changes in neural and hormonal signaling, local changes in the gut wall leading to inflammation and psychosocial factors.

There is good scientific evidence that the neurotransmitter/hormone, serotonin (aka 5-hydroxytryptamine, 5-HT) is a major player in causing IBS symptoms. This conclusion is based in part on the known effectiveness against IBS symptoms of some drugs which either block or stimulate serotonin receptors. However, serotonergic drugs have also caused adverse consequences for a small percentage of patients receiving these medications resulting in voluntary withdrawal of the drugs from the market by the manufacturers. This situation has left IBS patients wanting for effective drug treatments of their symptoms.

The gastrointestinal tract is home to a native population of bacteria which are an important contributor to normal gastrointestinal health and function. However, disruption in the balance of different types of bacteria or the total number of bacteria in the gut can cause gastrointestinal upset including, bloating, discomfort and altered motor function leading to constipation or diarrhea. These are all symptoms of IBS. A number of investigators have proposed that IBS symptoms, in a subset of patients, are caused by a disruption in the normal bacterial population in the gut. The results of a study reported at a recent meeting of the American College of Gastroenterology ( support this conclusion ( In this study, IBS patients with diarrhea as their most prominent symptom received the antibiotic, rifaximin ( Just over 50% of the patients receiving the drug reported that they experienced “adequate relief” from their symptoms. Interestingly, about 40% of the patients receiving placebo (a drug free pill) also reported adequate relief of symptoms. This study highlights two important points. Firstly, the effectiveness of the antibiotic suggests that disturbances in the normal bacterial population of the gut can contribute to IBS symptoms in a subset of patients. Secondly, the large placebo effect observed in this study is typical of clinical drug trials targeting IBS. The high placebo response raises the bar for a new drug when its effects are compared to those observed in the placebo-treated group. In the end, the results of this study provide some hope that new drug treatments will be available for IBS patients. However, additional research on the causes of IBS is needed in order to develop new safe and effective IBS drugs.