In May, the US Food and Drug Administration (FDA) approved two drugs for irritable bowel syndrome (IBS) with diarrhoea.

One approval went to eluxadoline, which was developed by Actavis (which last month rebranded itself as Allergan). Eluxadoline is a mixed opioid receptor agonist that acts on the nervous system to reduce bowel contractions. The most common side-effects of the drug included constipation, nausea and abdominal pain. Annual global sales of the drug could reach US$450 million by 2020, shows Cortellis consensus sales forecasts.

A second approval went to Valeant's rifaximin. Rifaximin is an antibiotic derived from rifampin that is thought to act by changing the bacterial make-up of the gastrointestinal tract. Common side-effects include nausea and an increase in alanine aminotransferase levels. The drug is also already approved for travellers' diarrhoea caused by Escherichia coli, and for recurrences of hepatic encephalopathy. Drug sales were around $400 million in 2014, but are set to rise to $1.75 billion by 2019, suggests Cortellis consensus sales forecasts.

At least two other drugs are in Phase III development for IBS. The Menarini Group's ibodutant blocks the NK2 tachykinin receptor, modulating intestinal motility and reducing the hyper-responsiveness that occurs following intestinal inflammation. Synergy Pharmaceuticals' plecanatide is an analogue of a natural gastrointestinal hormone that activates the guanylate cyclase 2C receptor, promoting fluid and ion transport in the gastrointestinal tract.