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PH6.5 | PH6.5 | Chronic Lower-GI Symptom Pharmacotherapy — SDL Guide — SDL Guide (Part 3)
IBS Pharmacotherapy
IBS pharmacotherapy is fundamentally different from IBD pharmacotherapy in both intent and mechanism. There is no mucosal inflammation to suppress, so immunosuppressants, corticosteroids, biologics, and aminosalicylates have no role in IBS — their use in IBS would cause toxicity without benefit. Instead, drugs target the functional abnormalities underlying IBS: abnormal gut motility, visceral hypersensitivity, altered gut-brain signalling, and in some patients, gut microbiome dysbiosis. The subtype of IBS — IBS-D, IBS-C, or IBS with dominant pain and bloating — determines which functional pathway to target and which pharmacological class to prescribe. This subtype-first approach to IBS prescribing mirrors the mechanism-first approach to antiemetic prescribing: the symptom is common but the correct drug depends entirely on which physiological pathway is causing it.
IBS pharmacotherapy is fundamentally different from IBD pharmacotherapy — there is no mucosal inflammation to suppress, so immunosuppressants, corticosteroids, and biologics have no role. Instead, drugs target the functional abnormalities of IBS: abnormal gut motility, visceral hypersensitivity, altered gut-brain signalling, and in some patients, dysbiosis. The subtype of IBS (IBS-D, IBS-C, or IBS with dominant pain/bloating) determines which pharmacological pathway to target.
IBS-D (diarrhoea-predominant) pharmacotherapy:
- Loperamide (peripheral mu-opioid agonist — reduces gut motility + stool frequency): effective for reducing diarrhoea but does not address abdominal pain. Used on-demand or regularly in IBS-D. Dose: 2 mg after each loose stool, max 16 mg/day. [Mechanism already covered in PH6.3.]
- Low-dose amitriptyline (tricyclic antidepressant, 10–30 mg at night): not used for its antidepressant effect but for its peripheral anticholinergic effect (slows gut transit, reduces bowel frequency — beneficial in IBS-D) and its central neuromodulatory effect (inhibits visceral pain signalling via noradrenergic/serotonergic modulation of descending pain inhibitory pathways). First-line drug for IBS-D with pain per NICE UK guidelines. ADRs: dry mouth, constipation (beneficial in IBS-D), sedation (take at night).
- Rifaximin (non-absorbable rifamycin antibiotic): targets gut microbiome — in a subset of IBS-D patients with small intestinal bacterial overgrowth (SIBO) or specific dysbiosis, rifaximin significantly reduces bloating and loose stools. Systemic bioavailability <0.4% — acts entirely within the gut. 550 mg three times daily × 14 days; repeatable if relapse. FDA-approved for IBS-D.
IBS-C (constipation-predominant) pharmacotherapy: Linaclotide, lubiprostone, and prucalopride (covered in PH6.4 — Constipation module). The key distinction: in IBS-C, linaclotide is preferred over lubiprostone because linaclotide's cGMP-mediated mechanism reduces both constipation AND abdominal pain (via submucosal sensory afferent inhibition) — a dual action that addresses the two defining symptoms of IBS-C simultaneously.
IBS pain/bloating (all subtypes) pharmacotherapy:
- Mebeverine (135–200 mg three times daily): first-line antispasmodic — direct smooth muscle relaxant (blocks sodium channels in colonic smooth muscle → reduces spastic contractions). No anticholinergic adverse effects (does not cross BBB, does not block M₃ receptors). Well tolerated; onset within 20–30 minutes. ADRs: rare allergic reactions.
- Hyoscine butylbromide (Buscopan) (20 mg): quaternary ammonium anticholinergic that does not cross the BBB; blocks M₃ receptors on gut smooth muscle → relaxation. Short-acting; used on-demand for acute spasm. Fewer systemic anticholinergic effects than atropine.
- Low-dose SSRIs (e.g. paroxetine 10–20 mg, fluoxetine 20 mg): useful in IBS-C (SSRIs accelerate GI transit via 5-HT signalling) and for anxiety-driven IBS. Also address the psychiatric comorbidity common in IBS.
- Peppermint oil (enteric-coated capsules): relaxes colonic smooth muscle via calcium channel antagonism + 5-HT3 receptor effects; evidence for reducing IBS pain; well tolerated; may cause heartburn (enteric coating reduces this).
Clinical Decision-Making — IBD and IBS
Clinical decision-making in IBD follows a structured stepwise (or occasionally accelerated top-down) strategy that escalates drug intensity according to disease severity, response to treatment, and mucosal healing endpoints. The most critical first decision is accurately characterising IBD severity at presentation — because under-treatment of moderate-severe IBD with aminosalicylates alone (when biologics are needed) leads to ongoing mucosal damage, complications, and surgery, while over-escalation to biologics in mild disease exposes patients to serious infection risks without proportionate benefit. The stepwise framework prevents both errors. For IBS, clinical decision-making is simpler in structure but requires the same principled approach: lifestyle modifications first, then targeted pharmacotherapy by subtype, with clear criteria for escalation to second-line agents.
Clinical decision-making in IBD follows a stepwise (step-up) or occasionally an accelerated (top-down) strategy depending on disease severity at diagnosis. The most critical first decision is distinguishing IBD severity — because under-treatment of moderate-severe IBD with 5-ASA alone (when biologics are needed) leads to mucosal damage, complications, and surgery, while over-escalation to biologics in mild disease exposes patients to serious infection risks unnecessarily.
IBD step-up management framework:
| Disease/Setting | Induction | Maintenance | Refractory/Severe |
|---|---|---|---|
| Mild-moderate UC | Oral + topical mesalazine | Oral mesalazine (indefinite) | Add prednisolone short-course |
| Moderate-severe UC | Prednisolone 40 mg/day × 8–12 weeks | Azathioprine (check TPMT) or mesalazine | Infliximab/vedolizumab; tofacitinib |
| Mild-moderate CD (ileal/ileocolonic) | Budesonide (ileal CD) or prednisolone | Azathioprine or methotrexate | — |
| Moderate-severe CD | Prednisolone | Azathioprine + infliximab (combination most effective) | Ustekinumab; vedolizumab |
| Perianal fistulising CD | IV infliximab (most evidence) | Azathioprine + infliximab combination | Surgery |
Top-down approach (start with biologics, not 5-ASA → steroids): used in patients with severe disease at presentation (deeply ulcerated mucosa, high CRP/calprotectin, predictors of poor outcome) to prevent early structural damage. Evidence-based in Crohn's disease (REACT2 trial).
IBS management framework:
1. Lifestyle and dietary first: Regular meals, avoid known triggers (fatty food, caffeine, alcohol), low-FODMAP diet (reduces fermentable carbohydrates that cause bloating/diarrhoea in sensitive individuals). 50–70% of IBS patients improve with dietary modification.
2. First-line pharmacotherapy by subtype: IBS-D → loperamide ± mebeverine; IBS-C → linaclotide; IBS pain → mebeverine first-line.
3. Second-line: Low-dose amitriptyline (IBS-D + pain), low-dose SSRI (IBS-C + anxiety), rifaximin (IBS-D with bloating).
4. Psychological therapies (CBT, hypnotherapy) are first-line for refractory IBS with significant psychiatric comorbidity — as effective as pharmacotherapy in head-to-head trials.
CLINICAL PEARL
The TB screening rule for anti-TNF biologics is absolute — and the consequences of omitting it are catastrophic:
Anti-TNF therapy reactivates latent Mycobacterium tuberculosis infection because TNF-α is essential for granuloma formation and maintenance — it is the cytokine that 'walls off' the dormant TB bacilli. When anti-TNF blocks this, granulomas disintegrate and dormant TB is released. The reactivated TB in biologic-treated patients is often extrapulmonary or disseminated (lymphadenopathy, peritoneal TB, miliary TB) rather than the classic pulmonary presentation — making it harder to diagnose. In India, where TB prevalence is among the highest globally (approximately 28% of Indians have latent TB infection), this is not a theoretical risk — it is a real, preventable clinical event.
The protocol: IGRA + chest X-ray before every anti-TNF prescription. If positive → prophylactic isoniazid 300 mg/day for 9 months, starting at least 4–8 weeks BEFORE the biologic. If the patient develops TB on anti-TNF, stop the biologic immediately and treat with a full TB regimen (6 months). After TB treatment is complete, switching to vedolizumab (gut-selective, does not affect systemic TNF-α) is a safer biologic option for IBD maintenance.
Self-Assessment — IBD and IBS Pharmacotherapy
The self-assessment scenarios below test your ability to apply the pharmacological framework for IBD and IBS to clinical decisions. The three questions focus on: (1) the pre-treatment safety requirement that distinguishes responsible from negligent prescribing of azathioprine; (2) the mandatory pre-treatment screen for anti-TNF biologics in an Indian patient; and (3) the correct first-line drug for IBS-D. Each question is designed to test mechanistic reasoning — knowing WHY a particular drug requires a particular screen, not just THAT it requires one. A student who understands the TPMT-myelosuppression mechanism will never forget to check TPMT before starting azathioprine; a student who only memorised 'check TPMT' may forget the step when the context changes.
Work through each question by applying the pharmacological knowledge from this module before reviewing the explanations.
SELF-CHECK
A 45-year-old Indian man with moderate-severe Crohn's disease requires infliximab. An IGRA test is performed — it is positive. What is the CORRECT next step?
A. Immediately start infliximab — the IGRA being positive does not contraindicate therapy
B. Withhold infliximab permanently and switch to azathioprine monotherapy
C. Start prophylactic isoniazid 300 mg/day for at least 9 months, beginning ≥4–8 weeks before infliximab
D. Perform a CT chest scan; if clear, proceed with infliximab immediately
Reveal Answer
Answer: C. Start prophylactic isoniazid 300 mg/day for at least 9 months, beginning ≥4–8 weeks before infliximab
A positive IGRA indicates latent TB infection — the patient has been exposed to Mycobacterium tuberculosis and is at high risk of reactivation if anti-TNF therapy is started. TNF-α is required to maintain granuloma integrity around dormant TB bacilli; blocking TNF-α allows the granuloma to break down and dormant bacteria to disseminate. The mandatory protocol is to start isoniazid prophylaxis (300 mg/day × 9 months) at least 4–8 weeks BEFORE the first infliximab infusion — this reduces reactivation risk significantly. Infliximab must not be started immediately with a positive IGRA. Withholding infliximab permanently is too restrictive — azathioprine monotherapy is inadequate for moderate-severe CD requiring biologic therapy. A clear CT chest does not exclude latent TB (which by definition has no active radiographic infiltrate).