Developed by Solvay Pharmaceuticals, cilansetron is a 5-HT3 antagonist indicated for the treatment of diarrhoea-predominant irritable bowel syndrome (IBS). 5-HT(3) receptor antagonists such as cilansetron have been shown to affect gastrointestinal motility. With Phase III registration trials on cilansetron completed, Solvay filed for regulatory approval in Europe and the US in 2004. To ensure that cilansetron is only prescribed to patients with diarrhoea-predominant IBS, Solvay’s regulatory submission included an extensive appropriate use plan. In April 2005, however, Solvay received a “non-approvable” letter from the FDA and a request for additional data to support product registration in the US. Towards the end of 2005, the company announced that it had suspended registration of cilansetron in the US. Meanwhile, discussions continue with the UK’s MHRA about European marketing approval for cilansetron. In 2005, the MHRA also declined to approve cilansetron. Both the agencies requested additional clinical data to further assess the risk-benefit ratio of the compound. Despite the drug being rejected for approval, Solvay believed in the product and felt that the clinical data demonstrated important benefits for men and women suffering from diarrhoea-predominant IBS. However, taking into account the amount of clinical work requested and other business considerations, the company decided to end the development and regulatory activities for cilansetron. The clinical efficacy and safety of cilansetron was established in a series of clinical trials, including a large-scale international Phase III programme involving over 4,000 patients. Overall, results from these trials showed that cilansetron is significantly more effective than placebo in male and female patients with diarrhoea-predominant IBS, an important finding given the traditionally high placebo response rates seen in clinical trials of IBS drugs. Overall responder rates (adequate relief in at least 50% of weekly responses) for patients treated with cilansetron ranged from 52% to 61% compared with 37% to 46% for placebo recipients. The most common side effect of cilansetron is constipation, which is seen in 3-12% of subjects at 6 months. Ischemic colitis, a side effect associated with previous drugs of this class, has been seen in eight subjects (six women and two men) to date. All of these ischemic colitis events have been self-limited and did not require surgery. Because of its high degree of efficacy, the fact that it was well tolerated by the overwhelming majority of patients and that it showed efficacy in both genders, cilansetron represented a major advance in the treatment of irritable bowel syndrome with diarrhea predominance.