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Functional diarrhea occurs as part of the irritable bowel syndrome (IBS) and as an isolated symptom as functional (painless) diarrhea. Progress has been made in defining these disorders and in identifying new mechanisms involved in symptom production. A strong link exists between intestinal infection and IBS, as is the role of 5-hydroxytryptamine (5-HT). The importance of persistent subclinical inflammation is also emerging as a potential etiologic factor, particularly in post-infectious IBS. Although changes in the bacterial flora and bacterial overgrowth have been put forward as additional new contributors to symptom production, the case is not strong. These developments in pathogenesis have facilitated the introduction of new therapies. 5-HT(3) antagonists reduce bowel frequency and pain in women with diarrhea-predominant IBS, but their use is limited because of ischemic colitis. Prednisolone lacks efficacy, and early results with probiotics and herbal remedies are encouraging but require confirmation by larger trials.

The irritable bowel syndrome (IBS) is part of the spectrum of functional bowel disorders characterised by a diverse consortium of abdominal symptoms including abdominal pain, altered bowel function (bowel frequency and/or constipation), bloating, abdominal distension, the sensation of incomplete evacuation and the increased passage of mucus. It is not surprising therefore that no single, unifying mechanism has as yet been put forward to explain symptom production in IBS. The currently favoured model includes both central and end-organ components which may be combined to create an integrated hypothesis incorporating psychological factors (stress, distress, affective disorder) with end-organ dysfunction (motility disorder, visceral hypersensitivity) possibly aggravated by sub-clinical inflammation as a residuum of an intestinal infection. There is currently no universally effective therapy for IBS. Standard therapy generally involves a symptom-directed approach; anti-diarrhoeal agents for bowel frequency, soluble fibre or laxatives for constipation and smooth muscle relaxants and anti-spasmodics for pain. New drug development has focused predominantly on agents that modify the effects of 5-hydroxytryptamine (5-HT) in the gut, principally the 5-HT(3) receptor antagonists for painful diarrhoea predominant IBS and 5-HT(4) agonists for constipation predominant IBS. More speculative new therapeutic approaches include anti-inflammatory agents, antibiotics, probiotics, antagonists of CCK1 receptors, tachykinins and other novel neuronal receptors.