Irritable Bowel Syndrome

A common, relapsing functional bowel disorder with reduced quality of life. Pathophysiology is poorly understood — contributing factors include genetic predisposition, environmental exposures (childhood stress, learned illness behaviour), post-infective onset (including gastroenteritis and COVID-19), visceral afferent hypersensitivity, altered intestinal transit, and psychological distress/ongoing stress.

Diagnostic criteria — Rome V (2026), replacing Rome IV

The Rome Foundation released Rome V in May 2026, the first update since Rome IV (2016). The diagnostic threshold for IBS is essentially unchanged from Rome IV, though the overall framework has been renamed from “disorders of gut-brain interaction” to simply “bowel disorders”:

Recurrent abdominal pain, on average ≥1 day/week for the last 3 months (with symptom onset ≥6 months before diagnosis), plus ≥2 of:

• Pain related to defecation (relieved or exacerbated)
• Onset associated with a change in stool frequency
• Onset associated with a change in stool form/appearance

IBS is not a diagnosis of exclusion — this definition is most useful for patients <50yrs with long-standing symptoms and no red flags.

Symptoms

Gut: abdominal pain (crampy or sharp, minutes to hours, typically lower abdomen), altered stool passage (constipation or diarrhoea, incomplete defecation, urgency), bloating/distension, mucus per rectum, symptoms often meal-related, faecal incontinence can occur with diarrhoea/urgency.

Non-gut (common and often overlooked): fatigue, dysmenorrhoea, dyspareunia, headache, bladder irritability, fibromyalgia, sleep dysfunction.

Red flags

Age >50 with a recent bowel habit change; rectal bleeding; iron deficiency; abdominal mass; unexplained weight loss; persistent vomiting; haematemesis or melaena.

History and examination

Ask about comorbidities, recent infection/gastroenteritis (possible precipitant), travel, diet (fluid/fibre intake, irregular meals, food triggers), family history (coeliac disease, IBD, bowel cancer), extra-intestinal symptoms suggesting IBD (arthritis, eye inflammation, rashes), and psychological distress/anxiety/ depression.

Other diagnoses to consider: coeliac disease, IBD, lactose intolerance, thyroid disease, GI malignancy, microscopic colitis, GI infection (Giardia, Clostridium), diverticular disease, drug-induced GI symptoms, bile acid malabsorption, laxative abuse, depression/anxiety, gynaecological conditions (endometriosis, PID, malignancy).

Examine vital signs, look for anaemia, measure BMI, examine the abdomen (and pelvis if a gynaecological cause is suspected). Rectal exam only if clinically indicated (rectal bleeding/pain, or severe constipation with incomplete emptying/ straining — to assess for pelvic floor dysfunction, e.g. paradoxical sphincter tightening on straining).

Investigations

Arrange: FBC, EUC, LFT, CRP, coeliac serology (tTG-IgA + DGP-IgG — ensure the patient is eating gluten, ≥2 slices of bread/day for 6 weeks before testing), iron studies, calcium, TFTs. If diarrhoea: stool MCS, C. difficile toxin (if risk factors), stool O&P (especially with travel history or worsening/new symptoms). Consider faecal calprotectin (MBS-funded if <51yrs, symptoms >6 weeks, infective causes excluded, no red flags — low/normal calprotectin with typical symptoms and normal other tests supports an IBS diagnosis; elevated levels make organic disease more likely and warrant referral). Update bowel cancer screening if not current (but do not use FOBT to investigate symptoms — it’s a screening test for asymptomatic patients only, and should not be used if rectal bleeding is present). Consider pelvic ultrasound in women to exclude ovarian cancer (often non-specific symptoms: increased abdominal girth, pelvic/back pain, early satiety, new urinary symptoms).

Tests not routinely recommended in IBS

• Faecal PCR — useful for acute gastroenteritis, not chronic symptoms (tests organisms of questionable pathogenicity, e.g. Blastocystis, Dientamoeba).
• Colonoscopy/endoscopy — low diagnostic yield when typical symptoms, no red flags, no significant family history, normal exam and limited investigations (especially <50yrs) — invasive with non-negligible risk.
• CT abdomen — not indicated; meaningful radiation exposure for no benefit.
• FOBT — a screening test only, not validated for symptom investigation.
• Coeliac gene test (HLA DQ2/DQ8) — ~40% of the general population carries these haplotypes, so a positive result isn’t useful; a negative result effectively excludes coeliac disease and may help patients unwilling to reintroduce gluten for serology.
• Faecal microbiota testing — limited evidence, not Medicare-funded.
• Carbohydrate breath testing — interpretation issues, doesn’t predict response to dietary change, expensive; a low FODMAP trial is a better way to assess carbohydrate intolerance.

Management

Emergency assessment if:

Melaena, haematemesis, or large-volume/haemodynamically-significant vomiting; severe bloody diarrhoea or profound dehydration/systemic illness.

Urgent gastroenterology referral if:

Age ≥40 with a positive FOBT; or age ≥50 with a negative FOBT but rectal bleeding, bowel-habit change, unexplained iron deficiency/anaemia, or ≥5%/≥10% unexplained weight loss (1/6 months).

Education and reassurance

Communicate the diagnosis clearly and empathetically — IBS is a legitimate, defined medical condition, not a “label” for normal investigations. Explain the chronic relapsing-remitting course and benign nature, the gut-brain connection, and agree on red flags that should prompt review. Empathic communication itself improves outcomes.

Lifestyle

Regular physical activity (a sedentary lifestyle worsens constipation/bloating); weight loss if overweight; limit chewing gum/carbonated drinks (air swallowing); avoid sorbitol (osmotic diarrhoea); limit caffeine (discomfort/reflux/diarrhoea, and can cause rebound constipation if stopped abruptly); smoking cessation (nicotine increases gut transit/peristalsis, worsening diarrhoea/reflux); limit alcohol; regular meals (missed meals → a large rushed next meal → pain/bloating/ nausea); good sleep hygiene.

Diet

Regular meals, adequate fluid, reduce fatty food/caffeine/artificial sweeteners/ alcohol.

Low FODMAP diet — the best-evidenced dietary intervention, leading to marked improvement in up to 75% of people with IBS. Trial for 6–8 weeks; if symptoms improve, reintroduce FODMAP groups one at a time to establish individual tolerance. Ideally taught by a dietitian (difficult to implement alone) — consider a GP Chronic Conditions Management Plan to facilitate this.

Lactose intolerance — a clinical diagnosis (malabsorption without symptoms doesn’t need treatment); may be transient. If suspected: 2-week dietary exclusion, then gradual reintroduction to find the tolerated amount (small amounts, up to ~250mL milk/day, usually don’t cause symptoms). Lactase supplements (e.g. Lacteeze) can help, especially when eating out. Lactose breath testing is not routinely recommended (tests malabsorption, not intolerance, and results still need correlating with symptoms).

Psychosocial

Address stress; relaxation techniques (breathing, meditation, T’ai Chi); hypnotherapy has evidence of benefit; refer for CBT if stress/anxiety is a major driver.

Symptom-targeted pharmacotherapy (most patients don’t need medication)

Diarrhoea: loperamide first-line — titrate the dose to stool consistency/frequency (aim 1 soft formed stool/day; doses as low as a quarter tablet can work, others need up to 8/day; night-time dosing if morning-predominant diarrhoea). Preferred over codeine (dependence/tolerance/interaction risks). Second-line: ondansetron, especially with coexisting nausea. Third-line: a bile-acid sequestrant (e.g. colestyramine) for persistent diarrhoea.

Constipation: stop opioids if possible; optimise lifestyle (activity, responding to urge especially post-meal/on waking, toilet positioning, hydration, fibre). Bulk-forming laxatives first — psyllium/ispaghula (soluble, may ferment — less useful if bloating/wind prominent) or sterculia (insoluble, less fermented, often better if bloating/wind are the issue); Benefiber is another soluble option, easier to take (doesn’t gel). Macrogols (Movicol 1–3 sachets/day, Osmolax 2 scoops up to 3×/day) or bisacodyl 5–15mg at night if bulking agents are insufficient. Osmotic/stimulant/emollient laxatives are commonly used but have limited RCT evidence specifically in IBS.

Abdominal pain/bloating: antispasmodics (mebeverine) or peppermint oil (enteric-coated capsules, e.g. Mintec, 1–2 capsules three times daily 30 minutes before meals for 2 weeks then reduce to lowest effective dose — antispasmodic, antiflatulent, mild analgesic; may worsen reflux). Opiates are contraindicated for IBS pain (dependence, long-term ineffectiveness, constipation).

Antidepressants (analgesic/motility mechanism, not primarily for mood): low-dose TCA (amitriptyline/nortriptyline 10–25mg nocte, up titrate slowly over 3–4 weeks to 50–75mg nocte if needed; avoid if constipation predominates; unlicensed indication, taper on ceasing). SSRI — controversial evidence, may suit constipation-predominant or where psychological symptoms predominate (paroxetine’s anticholinergic action may help; 10mg daily, up-titrate weekly to 50mg; unlicensed/non-PBS indication).

Complementary: probiotics (many formulations, limited quality evidence, not funded — trial 4 weeks at the manufacturer’s dose); Iberogast (9-herb extract including peppermint oil, ~$20/50mL, 1mL three times daily, trial 6 weeks). No evidence for other complementary treatments; rifaximin may improve non-constipating IBS bloating/flatulence but is a specialist-initiated, non-PBS option.

Ongoing care

Regular review of treatment response; monitor for new red flags/change in pattern. Non-urgent gastroenterology referral for longstanding symptoms suggestive of IBS with no critical factors (anaemia, low albumin, raised ESR/CRP, iron deficiency, significant weight loss) or abnormal investigations. Consider dietitian referral (food avoidance/exclusion diets) and psychology referral (CBT) where relevant.