Irritable Bowel Syndrome Pharmacotherapy
On this page
Direct answer
For irritable bowel syndrome with diarrhoea, the non-absorbed antibiotic rifaximin 550 mg three times daily for 14 days (repeat courses permitted, per the TARGET trials) improves bloating and loose stools without disturbing gut ecology the way systemic antibiotics do; eluxadoline and restricted alosetron are the heavier alternatives. For IBS with constipation, linaclotide 145-290 micrograms once daily (guanylate cyclase-C agonist, contraindicated under two years of age) and lubiprostone 8 micrograms twice daily (chloride channel activator) increase secretion and transit. Global therapy leans on stool-pattern-guided antispasmodics — mebeverine, dicyclomine, drotaverine beloved in Indian practice — and low-dose neuromodulators: amitriptyline 10 mg at night for diarrhoea-predominant or pain-heavy IBS, SSRIs when constipation or anxiety dominates. Peppermint oil sits legitimately in the list, and the diagnosis excludes every red flag before any of these prescriptions.
What you must remember
- IBS-D antibiotic: rifaximin 550 mg TDS for 14 days; response within a week; up to two repeat courses; also the hepatic-encephalopathy drug — a double-duty exam fact.
- IBS-D intensifiers: eluxadoline 100 mg BD (mixed opioid agonist-antagonist; contraindicated after pancreatitis, with heavy alcohol, or without a gallbladder); alosetron 1 mg BD (5HT3 antagonist, restricted programmes abroad for severe women-only diarrhoeal IBS because of ischaemic colitis).
- IBS-C secretagogues: linaclotide 145-290 mcg OD (contraindicated under 2 years — dehydration deaths in animals) and lubiprostone 8 mcg BD (nausea — take with food; 24 mcg BD for chronic idiopathic constipation).
- Neuromodulation: amitriptyline 10-25 mg nocte for IBS-D and pain; SSRIs (paroxetine-type) suit IBS-C with anxiety; both act at gut-nerve thresholds, far below antidepressant doses.
- Antispasmodics: mebeverine (anticholinergic-free smooth-muscle relaxant), dicyclomine, drotaverine — phosphodiesterase-inhibiting, popular across Indian OPDs for cramps.
- Red flags that revoke the IBS label: onset above 50, rectal bleeding, nocturnal symptoms, weight loss, anaemia, family history of colorectal cancer or coeliac disease — check coeliac serology once.
- Indian context: drotaverine-plus-antacid combinations and probiotic courses dominate real-world prescribing; rifaximin and secretagogues are available but self-funded, so diet-first, cheap-spasmolytic-second remains the national default the exam rarely states.
Building the prescription around the stool chart
Start with a 32-year-old woman, years of recurrent cramps relieved by defecation, bloating worse after meals, alternating stools but predominantly loose, no red flags, coeliac serology negative. Rome IV criteria met — IBS-D. First line: diet scaffolding (low-FODMAP trial where feasible, caffeine and lactose audit), drotaverine or mebeverine for cramps, and reassurance with explanation, which trials show is therapeutic. Inadequate response at six weeks: add rifaximin 550 mg TDS × 14 days — a course that helps roughly one in ten more than placebo achieve adequate relief, with bloating the best responder symptom. If pain and urgency persist: amitriptyline 10 mg nocte, increasing to 25 mg, warning about morning sedation. Her mirror-image patient — constipation, hard pellets, straining — travels the other road: fibre escalation to tolerable doses or osmotic polyethylene glycol, then linaclotide 145 mcg fasting each morning (30 minutes before breakfast), lubiprostone if nausea tolerable, SSRI if anxiety threads the story.
Eluxadoline and alosetron belong to the severe, specialist end of the diarrhoeal road, after the simpler ladder has been genuinely tried.
Where students slip
The pattern-matching traps are reliable. First, rifaximin's double life: the same molecule treats hepatic encephalopathy and IBS-D, and questions exploit the surprise. Second, linaclotide's paediatric contraindication — under two years, contraindicated absolutely (the boxed warning abroad) — a number-less fact students skip. Third, alosetron's ischaemic colitis: the reason for restricted prescribing and the answer to "which IBS drug causes ischaemic colitis". Fourth, eluxadoline's pancreatitis contraindications — no gallbladder, prior pancreatitis, or heavy alcohol — sphincter of Oddi spasm is the mechanism; a mechanism-with-contraindication pairing that vivas reward. Fifth, the antidepressant-dose confusion: neuromodulators in IBS are dosed for nerve sensitivity, not depression, and the TCA choice tilts diarrhoeal, the SSRI choice constipated. Indian exam flavour: tegaserod's withdrawal-and-return story (5HT4 agonist, cardiovascular restriction abroad), the over-the-counter probiotic economy, and drotaverine — a drug Indian formularies know well and Western textbooks barely mention — as the expected local answer among antispasmodics.
Frequently asked questions
What is the rifaximin regimen for IBS with diarrhoea?
550 mg three times daily for 14 days, with up to two repeat courses if symptoms recur.
Which drugs are first-line secretagogues for IBS with constipation?
Linaclotide 145-290 micrograms once daily and lubiprostone 8 micrograms twice daily.
Why is linaclotide contraindicated in young children?
Fatal dehydration occurred in juvenile animal studies, so it is contraindicated below two years of age.
Which adverse effect restricted alosetron's use?
Ischaemic colitis and severe constipation, leading to a restricted-access prescribing programme for severe diarrhoea-predominant IBS in women.
Why is low-dose amitriptyline used in IBS?
It raises visceral pain thresholds and slows transit at 10-25 mg nightly, making it most useful in diarrhoea-predominant or pain-dominant IBS.