IBS: low-FODMAP diet, gut-directed therapy, and pharmacotherapy — AU GP

What IBS actually is

Irritable bowel syndrome (IBS) is a chronic functional gastrointestinal disorder characterised by recurrent abdominal pain linked to altered bowel habit, in the absence of structural or biochemical disease. It is defined using the Rome IV criteria — the internationally accepted clinical framework — and affects approximately 10–15% of Australian adults, with women twice as commonly affected as men.

IBS sits within a biopsychosocial model: brain-gut axis dysregulation, visceral hypersensitivity, altered motility, and microbiome changes interact with psychological, dietary, and lifestyle factors to produce and maintain symptoms. Understanding this framework — and communicating it clearly to patients — is foundational to effective management. The diagnosis is positive, not one of exclusion: when Rome IV criteria are met and alarm features are absent, targeted limited investigation is appropriate, and the working diagnosis can be stated confidently.

The condition carries a significant burden of quality of life impact. Approximately 3–4% of those with IBS have severe symptoms affecting daily function. Around 10% of people develop IBS following acute gastroenteritis — post-infectious IBS (PI-IBS) — and this subset often presents to general practice in the aftermath of a clear trigger event.

A. Core clinical — the AU general practice framework

Defining IBS: Rome IV criteria

eTG Gastrointestinal, GESA, and the Rome Foundation align on the Rome IV diagnostic criteria:

Recurrent abdominal pain, on average at least one day per week in the last three months, associated with two or more of:

• Related to defaecation (pain improves or worsens with bowel motion).
• Associated with a change in stool frequency.
• Associated with a change in stool form (Bristol Stool Chart).

Symptoms must have been present for six or more months prior to diagnosis. The Bristol Stool Chart subtype drives treatment:

• IBS-D (diarrhoea-predominant) — ≥25% loose stools, < 25% hard.
• IBS-C (constipation-predominant) — ≥25% hard stools, < 25% loose.
• IBS-M (mixed) — ≥25% both loose and hard.
• IBS-U (unsubtyped) — insufficient loose or hard stools to classify.

Alarm features — investigate these

Any of the following require investigation before or instead of an IBS diagnosis:

• Unintentional weight loss (>3–5 kg unexplained).
• Rectal bleeding or melaena (black stool).
• Iron-deficiency anaemia (particularly in older adults and males).
• Nocturnal symptoms — diarrhoea or pain that wakes from sleep (IBS does not typically wake patients).
• Family history of colorectal cancer, inflammatory bowel disease, or coeliac disease.
• Fever accompanying bowel symptoms.
• Palpable abdominal mass or lymphadenopathy on examination.
• Age ≥50 with new-onset bowel symptoms — lower threshold for colonoscopy.
• Recent travel with persistent diarrhoea (parasitic infection differential).

History and examination

History: bowel pattern (Rome IV criteria); Bristol Stool Chart; pain character and relationship to defaecation; dietary triggers (lactose, gluten, caffeine, alcohol, fatty foods, large meals); stress and mental health (anxiety, depression — bidirectional with IBS); sleep quality; travel history and recent gastroenteritis; family history of bowel cancer or IBD; drug history (laxative use, opioids, magnesium supplements, iron supplementation); gynaecological symptoms (endometriosis is an important differential in women with IBS-D — cyclical timing, dysmenorrhoea, and deep dyspareunia are discriminating features).

Examination: generally normal or shows only mild diffuse abdominal tenderness. Check BMI and document weight. Mass, hepatosplenomegaly, or lymphadenopathy are alarm findings. Perianal examination for fissure, fistula, or IBD signs if IBS-D. Pelvic examination if endometriosis is suspected.

Investigations

Per eTG and GESA, limited first-line investigations are appropriate — do not over-investigate in the absence of alarm features:

• FBC — anaemia screen (iron-deficiency anaemia is an alarm feature, not an IBS feature).
• CRP — elevated CRP favours IBD over IBS; normal CRP is reassuring.
• Anti-tTG IgA + total IgA — coeliac serology; coeliac disease is a treatable IBS mimic presenting with bloating, altered stool pattern, and abdominal pain.
• Faecal calprotectin — the single most useful test to differentiate IBS from IBD. A result under 50 µg/g has high sensitivity for ruling out IBD; above 50 µg/g prompts IBD workup. This avoids unnecessary colonoscopy in the majority of patients with typical IBS presentations.
• Stool MCS and ova and parasites — if travel, recent acute gastroenteritis, or persistent diarrhoea.
• TSH — hyperthyroidism causes diarrhoea, urgency, and weight loss that can mimic IBS-D.
• Iron studies — if anaemia is present.

Colonoscopy is not routine. Reserve for alarm features, age ≥50 with new-onset symptoms, or chronic watery diarrhoea in an older patient where microscopic colitis (diagnosed on biopsy) must be excluded.

Differential diagnosis

Condition	Key discriminator
Inflammatory bowel disease (UC, Crohn’s)	Calprotectin ↑; rectal bleeding; nocturnal symptoms; weight loss; colonoscopy + biopsy
Coeliac disease	Anti-tTG IgA positive; gastroscopy + duodenal biopsy
Microscopic colitis	Older adult; chronic watery diarrhoea; normal colonoscopy but biopsy positive
Bile acid diarrhoea	IBS-D pattern; response to cholestyramine trial (~25–30% of IBS-D)
Endometriosis	Cyclical bowel symptoms; dysmenorrhoea; dyspareunia; pelvic MRI
Lactose or fructose intolerance	Symptom diary; breath testing; dietary trial
Hyperthyroidism	TSH suppressed; weight loss; palpitations
Colorectal cancer	Age, alarm features; colonoscopy
Functional dyspepsia	Upper GI predominant; gastroscopy

B. The FODMAP framework — Monash evidence and diet approach

What are FODMAPs?

FODMAPs (Fermentable Oligo-, Di-, Mono-saccharides And Polyols) are a group of short-chain carbohydrates poorly absorbed in the small intestine. They draw water osmotically into the gut lumen, pass into the colon, and are rapidly fermented by colonic bacteria — producing gas, distension, bloating, and altered motility. In people with IBS and visceral hypersensitivity, the threshold for symptom production from this process is lower than in the general population.

Common high-FODMAP foods: onion, garlic, wheat and rye products, apples, pears, watermelon, milk and soft cheeses, legumes, and most artificial sweeteners (polyols). The Monash FODMAP Diet Program at Monash University (Melbourne) originated this approach, maintains the food composition database, and produces the Monash FODMAP app.

The evidence

Multiple randomised controlled trials support the low-FODMAP diet for IBS. The consistent finding is approximately 70% symptom response in people with IBS (compared with ~50% for standard dietary advice), with improvements in pain, bloating, and stool consistency. eTG and GESA recommend it as the first-line dietary intervention in IBS.

Crucially, the Monash low-FODMAP diet is structured in three stages — it is not simply a “low-FODMAP forever” diet:

1. Stage 1 — Restriction (4–6 weeks): strict low-FODMAP diet to establish baseline symptom control. About 70% of people with IBS respond in this phase.
2. Stage 2 — Reintroduction: systematic one-at-a-time testing of FODMAP subgroups to identify which specific categories trigger symptoms for that individual.
3. Stage 3 — Personalisation: long-term least-restrictive diet removing only confirmed personal triggers. This phase is critical — prolonged restriction without personalisation risks nutritional inadequacy and unnecessary dietary limitation.

A Monash-trained dietitian guides all three stages and is best referred via a GP Chronic Condition Management Plan (GPCCMP) (5 allied health visits per year, 10 for Aboriginal and Torres Strait Islander patients). The Monash FODMAP app provides an interactive food guide and meal planning support for patients.

Gut-directed psychological therapy — equivalent to diet

The Peters et al. Monash University randomised controlled trial (Alimentary Pharmacology and Therapeutics, 2016) randomised 74 patients with IBS to gut-directed hypnotherapy versus the low-FODMAP diet. At primary endpoint, both groups showed equivalent and clinically significant IBS symptom improvement — approximately 70–80% response — with durability confirmed at follow-up. This finding positions gut-directed psychological therapy as an equivalent, not merely adjunctive, intervention.

Gut-directed hypnotherapy works through the brain-gut axis — retraining how the central and enteric nervous systems process and respond to gut sensations. In Australia, the Nerva app (developed by Australian company Mindset Health) delivers a structured 6-week gut-directed hypnotherapy program designed with clinical oversight. For patients who prefer face-to-face therapy, trained clinicians can provide sessions rebatable under a Better Access Mental Health Care Plan.

Gut-directed CBT — structured cognitive and behavioural therapy targeting IBS-specific thought patterns and avoidance behaviours. Supported by meta-analyses of multiple RCTs (Ford et al., ACG). Accessible via Better Access psychology referral (10 subsidised sessions per year via MHCP items 2715/2717) or online via This Way Up — IBS course (Australian, evidence-aligned, low-cost).

The patient-facing explanation matters: “IBS is a real condition — not ‘all in your head’ — involving the brain-gut connection. Your gut has become more sensitive than usual, and signals between the brain and bowel are amplified. We treat both the diet and the brain-gut axis.” Patients who receive this explanation show better engagement with non-pharmacological approaches and better outcomes.

C. Pharmacotherapy — symptom-targeted treatment

Medication in IBS is symptom-targeted and used alongside dietary and psychological approaches — not as a substitute for them. Per eTG and AMH:

Pain and cramping

Hyoscine butylbromide (Buscopan) 10–20 mg as needed — anticholinergic antispasmodic; over the counter; fastest-acting for acute spasm. Use cautiously in glaucoma and urinary retention.

Enteric-coated peppermint oil 0.2 mL three times daily — antispasmodic; over the counter; supported by multiple RCTs; use enteric-coated formulation to avoid oesophageal reflux. eTG acknowledges this as near-mainstream with a positive evidence profile.

Mebeverine 135 mg three times daily — smooth muscle relaxant; PBS general benefit.

Low-dose amitriptyline 10–25 mg at night — tricyclic antidepressant used at sub-antidepressant doses for visceral pain modulation via central sensitisation. Particularly useful in IBS-D (constipating side effect is advantageous). The Ford ACG meta-analyses consistently support low-dose TCAs for IBS pain. Start at 10 mg nocte; may titrate to 25–50 mg. Monitor QTc and anticholinergic effects (dry mouth, constipation, urinary retention, drowsiness).

IBS-D (diarrhoea-predominant)

Loperamide 2 mg as needed — over the counter; reduces diarrhoea frequency and urgency; does not treat IBS pain.

Cholestyramine 4 g twice to four times daily — bile acid sequestrant; trial for suspected bile acid diarrhoea (~25–30% of IBS-D presentations). Response to cholestyramine is both diagnostic and therapeutic when SeHCAT scanning is unavailable (limited in Australia).

Ondansetron 4–8 mg as needed — 5-HT₃ antagonist; off-label; modest emerging evidence for IBS-D urgency component.

Rifaximin — PBS Authority for hepatic encephalopathy; not PBS-listed for IBS-D; cost-limiting; reserve for specialist setting.

IBS-C (constipation-predominant)

Polyethylene glycol (PEG, Movicol) — osmotic laxative; 1 sachet daily to twice daily; over the counter and PBS general benefit; well-tolerated long-term.

Soluble fibre (psyllium, ispaghula) — IBS-C and IBS-M; avoid insoluble fibre (wheat bran) which can worsen IBS symptoms.

Linaclotide (Constella) 290 mcg daily, 30 minutes before food — guanylate cyclase-C agonist; PBS Authority for chronic constipation (not IBS-C specifically — PBS criteria are specific); specialist initiation preferred.

Prucalopride (Resolor) 2 mg daily — selective 5-HT₄ agonist; PBS Authority for chronic constipation refractory to laxatives; particularly for women.

Comorbid anxiety, depression, or global IBS

SSRIs (escitalopram, sertraline) — dual benefit in IBS with comorbid anxiety or depression: direct gut effect via 5-HT modulation (particularly useful in IBS-C due to prokinetic serotonergic effect) and mood/anxiety improvement. eTG and AMH support their use when psychological comorbidity is present.

SNRIs (duloxetine) — when pain is prominent alongside depression. Duloxetine has pain-modulating properties beyond mood effects.

Probiotics

Specific strains — Bifidobacterium infantis 35624, multistrain products — show modest evidence for IBS symptom reduction in systematic reviews. Considerable heterogeneity exists between studies and products. A trial of 4–8 weeks with a defined strain is reasonable; cease if no benefit at 8 weeks.

D. Australian operations

MBS item numbers

Per MBS Online:

• Standard consultations — items 23, 36, 44.
• GPCCMP — items 965/967 (replaced items 721/723/732 from 1 July 2025); chronic IBS with functional impairment or comorbidity qualifies; dietitian referral (Monash-trained for FODMAP, 5 visits/year, 10 for ATSI patients).
• Better Access Mental Health Care Plan (MHCP) — items 2700/2701/2715/2717; psychologist for gut-directed CBT or hypnotherapy; 10 sessions/year.
• Pathology — FBC (65070), CRP (66509), anti-tTG IgA (66648), faecal calprotectin (66521), iron studies (66600), TSH (66716), stool MCS.
• Colonoscopy — item 32090 range; alarm features only; specialist referral.
• 75+ Health Assessment — item 705; ATSI Health Assessment — item 715.
• Telehealth — suitable for symptom review, dietary and psychological progress review with established treating relationships.

PBS-listed agents

Per PBS:

• Hyoscine butylbromide, peppermint oil, loperamide, PEG, psyllium — over the counter.
• Mebeverine, amitriptyline, SSRIs (escitalopram, sertraline) — general benefit schedule.
• Cholestyramine — Authority Required for some indications.
• Linaclotide (Constella) — Authority Required for chronic constipation (specific PBS criteria apply; not IBS-C by name).
• Prucalopride (Resolor) — Authority Required for chronic constipation refractory to laxatives.
• Rifaximin — PBS Authority for hepatic encephalopathy; not listed for IBS-D.
• Eluxadoline (Truberzi) — limited PBS availability; specialist.

Patient resources (AU)

HealthDirect — IBS, Better Health Channel — IBS, Monash FODMAP app, Nerva gut-directed hypnotherapy app, GESA patient resources, Crohn’s and Colitis Australia (for differential education).

NPS MedicineWise has consumer IBS information aligned with eTG.

E. Special populations

Women. IBS affects women at twice the rate of men, and the IBS-D and IBS-M subtypes are more common in women. Endometriosis bowel involvement is a critical differential — cyclical pain patterns, dysmenorrhoea, and dyspareunia warrant gynaecological assessment. SSRI caution in perimenopause and pregnancy; involve obstetric or gynaecology specialist when needed. The low-FODMAP diet is generally safe in pregnancy under dietitian supervision.

Older adults. New-onset IBS-type symptoms after age 50 require colonoscopy to exclude colorectal cancer and microscopic colitis before attributing symptoms to IBS. Beware polypharmacy — opioids, anticholinergics, calcium channel blockers, and iron supplements all cause constipation; review medication list before diagnosing IBS-C. Low-dose TCA caution in older adults: falls risk, QTc prolongation, urinary retention, cognitive effects.

Children and adolescents. Functional abdominal pain and IBS are common in children. Paediatric gastroenterology guidance applies — Rome IV paediatric criteria differ slightly. eTG paediatric sections guide investigation and management. Psychological approaches (CBT, hypnotherapy) are particularly effective in this age group.

People with eating disorders. Before recommending the low-FODMAP diet or any restrictive dietary intervention, screen for eating disorder history — restriction can reinforce disordered eating patterns. A Monash-trained dietitian skilled in both IBS and eating disorder management is essential for this group.

Comorbid anxiety and depression. The brain-gut axis means these conditions reinforce each other bidirectionally. IBS symptoms worsen during periods of psychological stress; anxiety and depression are more prevalent in people with IBS. Integrated management — gut-directed psychological therapy alongside SSRI if indicated — produces better outcomes than treating each in isolation.

When to escalate

Refer or escalate when:

• Alarm features present (weight loss, rectal bleeding, anaemia, nocturnal symptoms, family history of IBD or colorectal cancer) — urgent or semi-urgent gastroenterology referral for colonoscopy.
• Acute severe abdominal pain or GI bleeding — emergency department.
• Suspected IBD — gastroenterology for colonoscopy with biopsy; Crohn’s and Colitis Australia is a useful patient resource during the diagnostic period.
• Suspected coeliac disease — gastroscopy with duodenal biopsy; continue gluten-containing diet until biopsy completed.
• Refractory IBS after optimising diet, psychology, and pharmacotherapy — gastroenterology to re-confirm diagnosis and consider biofeedback, pelvic floor physiotherapy (IBS-C), or elemental diet.
• Endometriosis suspected — gynaecology referral.
• Severe comorbid psychiatric disorder — psychiatry or specialist mental health input.

What this article is and is not

This is general health information drawn from current Australian general practice guidelines — Therapeutic Guidelines (eTG), GESA, Australian Medicines Handbook (AMH), and the Monash University FODMAP Diet Program — and the Peters et al. 2016 Australian RCT comparing gut-directed hypnotherapy with the low-FODMAP diet. It is not personal medical advice and does not create a doctor–patient relationship. Decisions about investigation, dietary therapy, pharmacotherapy, and referral are made with your own GP and treating clinicians based on your subtype, symptom burden, comorbidities, and response to treatment.

For Australian consumer-friendly resources: HealthDirect — IBS, Better Health Channel — IBS, Monash FODMAP, Nerva app.

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Sources cited

1. Therapeutic Guidelines (eTG) — Gastrointestinal: IBS
2. Gastroenterological Society of Australia (GESA)
3. Australian Medicines Handbook (AMH)
4. Monash University FODMAP Diet Program
5. Rome Foundation — Rome IV criteria
6. Peters SL et al. — Gut-directed hypnotherapy vs low-FODMAP for IBS (Aliment Pharmacol Ther 2016)
7. Ford AC et al. — ACG IBS meta-analyses
8. NPS MedicineWise
9. HealthDirect — IBS
10. Better Health Channel — IBS
11. Nerva — Gut-directed hypnotherapy app
12. Crohn’s and Colitis Australia
13. MBS Online
14. PBS — IBS pharmacotherapy listings
15. This Way Up — IBS course