Drug Therapy of Irritable Bowel Syndrome

Introduction

• Definition — Irritable bowel syndrome (IBS) is a chronic functional GI disorder ("disorder of brain–gut interaction") defined by recurrent abdominal pain with altered bowel habit — constipation, diarrhoea or both — with no structural or biochemical explanation; it affects up to 10–15% of the population.
• Pathophysiological basis — A disorder of visceral hypersensitivity plus GI motor abnormality, frequently with affective (anxiety/depression) comorbidity; because there is no single disease mechanism, treatment "remains empirical and symptom-based."
• Subtypes drive drug choice — IBS is stratified by predominant stool form into IBS-C (constipation), IBS-D (diarrhoea), IBS-M (mixed) and IBS-U (unsubtyped).
• Core therapeutic principle — Therapy is subtype-directed and symptom-targeted — correct the predominant bowel disturbance (loosen stool in IBS-C, firm/slow stool in IBS-D) while a separate analgesic/neuromodulatory layer (antispasmodics, low-dose antidepressants) treats the abdominal pain common to all subtypes.