Irritable bowel syndrome (IBS) is estimated to affect 10% to 15% of adults in the United States. Some patients with IBS experience diarrhea as the most common manifestation (IBS-D), and others experience constipation as the most commonmanifestation (IBS-C). IBS-D is a functional bowel disorder that is characterized by chronic abdominal pain and frequent diarrhea (loose or watery stools at least 25% of the time). Although loperamide may help to control diarrhea, it does not provide adequate relief of symptoms in many patients. The serotonin subtype 3 receptor antagonist alosetron (e.g., Lotronex) is the only agent that has been available with a labeled indication for the treatment of patients with IBS-D. However, it is indicated only for the treatment of women with severe IBS-D and is available only under the provisions of a risk management program owing to its potential to cause serious gastrointestinal adverse events (e.g., ischemic colitis, serious complications of constipation). Eluxadoline (ViberzidAllergan) is a mu-opioid receptor agonist that also acts as an agonist at kappa-opioid receptors and as an antagonist at delta-opioid receptors. It is indicated for the treatment of adult patients with IBS-D. Unlike alosetron, which was evaluated in studies in which a large majority of the patients were women, eluxadoline is indicated for use in men as well as women. The effectiveness of eluxadoline was demonstrated in 2 placebo-controlled studies that included almost 2500 patients. The new agent was more effective than placebo in simultaneously reducing abdominal pain and improving