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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- IBS = recurrent abdominal pain/discomfort associated with altered bowel habit for ≥6 months, with no organic cause
- Subtypes: IBS-D (diarrhoea-predominant), IBS-C (constipation-predominant), IBS-M (mixed)
- Diagnosis of exclusion: screen with FBC, CRP, coeliac serology (tTG-IgA), faecal calprotectin (to exclude IBD) before diagnosing IBS
- Management: lifestyle advice → antispasmodics (mebeverine/peppermint oil) → low FODMAP diet → TCAs (amitriptyline) or SSRIs → psychological therapy
- Red flags: weight loss, rectal bleeding, family history of CRC/IBD, onset >50, nocturnal symptoms, anaemia → investigate further, do not diagnose IBS
Overview
Irritable bowel syndrome is the most common functional gastrointestinal disorder, characterised by chronic abdominal pain associated with defaecation or altered bowel habit (diarrhoea, constipation, or both) in the absence of structural or biochemical abnormalities. The pathophysiology is multifactorial, involving visceral hypersensitivity, altered gut motility, gut-brain axis dysfunction, low-grade mucosal inflammation, altered gut microbiome, and psychosocial factors. Post-infectious IBS (following gastroenteritis) is a well-recognised trigger.
Epidemiology
IBS affects approximately 10–20% of the UK population. It is twice as common in females as males. Peak onset is in the 20–30s age group. IBS accounts for up to 50% of all gastroenterology outpatient referrals. It is associated with significant reduction in quality of life and increased healthcare utilisation. There is substantial overlap with other functional disorders (functional dyspepsia, chronic fatigue syndrome, fibromyalgia).
Clinical Features
Symptoms
Recurrent abdominal pain or discomfort (often relieved by defaecation)
Altered bowel habit: diarrhoea, constipation, or alternating between the two
Abdominal bloating and distension (very common)
Urgency and incomplete evacuation
Mucus per rectum
Symptoms worse with stress
Weight loss, rectal bleeding, or nocturnal symptoms
Onset after age 50
Signs
Usually normal examination
Mild diffuse abdominal tenderness (especially left iliac fossa)
Bloating/distension may be visible
Any abnormal findings on examination should prompt investigation for organic disease
Investigations
First-line
FBCExclude anaemia (would suggest organic pathology)
CRP/ESRShould be normal in IBS. Raised values suggest IBD or other inflammatory cause
Coeliac serology (tTG-IgA + total IgA)Must exclude coeliac disease in ALL patients with IBS-type symptoms (NICE CG61)
Faecal calprotectin<100 µg/g effectively excludes IBD. Very useful to differentiate IBS from IBD without colonoscopy
Second-line
Thyroid function testsExclude hyper/hypothyroidism as cause of altered bowel habit
Stool MC&SIf diarrhoea-predominant — exclude infective cause
Specialist
ColonoscopyOnly if red flags present or age ≥50 with new symptoms — NOT routinely required for IBS diagnosis
SeHCAT scan or serum 7αC4If diarrhoea-predominant and not responding to treatment — screen for bile acid malabsorption (BAM), which mimics IBS-D
Hydrogen breath testIf lactose intolerance or SIBO suspected
1
General lifestyle advice (first-line)
- Regular meals, adequate hydration (8 cups/day), limit caffeine and alcohol
- Limit fresh fruit to 3 portions/day
- Adjust fibre intake: soluble fibre (oats, ispaghula) may help; insoluble fibre (bran) may worsen symptoms
- Regular physical activity
- Stress management and relaxation techniques
2
Pharmacological — symptom-directed
- Pain/bloating: antispasmodics — mebeverine 135 mg TDS, hyoscine butylbromide 10 mg TDS, or peppermint oil capsules
- Constipation-predominant: osmotic laxatives (macrogol). Avoid lactulose (increases bloating)
- Diarrhoea-predominant: loperamide (titrate to stool frequency)
- Bloating: simethicone
3
Second-line pharmacological
- Tricyclic antidepressants (TCAs): amitriptyline 5–10 mg ON (low-dose) — first-line for refractory pain. Titrate up to 30 mg. Review at 4 weeks then 6–12 monthly
- SSRIs: if TCAs ineffective or not tolerated (e.g. fluoxetine 20 mg OD)
- Note: these are used at low doses for visceral pain modulation, NOT for depression
4
Dietary therapy
- Low FODMAP diet: supervised by trained dietitian. Effective in ~70% — involves elimination phase (4–6 weeks) then gradual reintroduction
- FODMAPs = Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols
- Consider if first-line dietary advice and pharmacotherapy insufficient
5
Psychological therapy
- CBT (cognitive behavioural therapy): evidence-based for IBS
- Gut-directed hypnotherapy: effective, especially in specialist centres
- Consider for refractory symptoms or significant psychological comorbidity
Complications
- Reduced quality of life: Significant impact on work, social activities, and mental health
- Psychological comorbidity: Anxiety and depression are common (up to 50%) — bidirectional relationship
- Unnecessary investigations: Important to make a positive diagnosis rather than a diagnosis of exclusion to avoid repeated testing
- Misdiagnosis: Bile acid malabsorption and coeliac disease commonly misdiagnosed as IBS — always screen
UKMLA Exam Tips
- 1IBS is a POSITIVE diagnosis (based on symptom criteria + exclusion of key differentials), NOT a diagnosis of exclusion by colonoscopy
- 2MUST test coeliac serology (tTG-IgA) in ALL patients before diagnosing IBS (NICE CG61)
- 3Faecal calprotectin <100 µg/g = very unlikely to be IBD (high negative predictive value)
- 4Low FODMAP diet is supervised by a trained dietitian — not self-directed
- 5Amitriptyline at LOW dose (5–10 mg ON) is for visceral pain modulation in IBS — NOT as an antidepressant
- 6Bile acid malabsorption mimics IBS-D — consider SeHCAT scan if diarrhoea-predominant and not responding to treatment
- 7Red flags in "IBS" that should trigger further investigation: rectal bleeding, weight loss, onset >50, nocturnal symptoms, anaemia, family history CRC/IBD
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