Diagnosis and management of irritable bowel syndrome

SUMMARY

Irritable bowel syndrome (IBS) is a disorder of gut–brain interaction characterised by recurrent abdominal pain or discomfort that is often related to defaecation or associated with a change in stool frequency or form.

IBS is common in Australia and affects women more than men. The exact pathophysiology remains unclear, though it can be multifactorial and relate to gastrointestinal dysmotility, post-infection and microbial changes, and a patient's psychosocial background.

A positive clinical diagnosis can be made when the Rome V criteria are met, alarm features are absent and simple screening tests (full blood count, C-reactive protein, coeliac serology) are negative. If alarm features are present, further examination, laboratory testing or imaging may be helpful in assessing for organic pathology.

Management involves lifestyle and dietary modifications, and psychological and pharmacological therapies. Pharmacological therapy should be individualised to the patient's IBS subtype and symptoms. Examples of drug classes that are used include antispasmodics, osmotic laxatives, antidiarrhoeal drugs and neuromodulators (tricyclic antidepressants).

There is insufficient evidence to support the use of therapies such as probiotics, faecal microbiota transplantation, and mesalamine.

 

Introduction

Irritable bowel syndrome (IBS) is a common disorder of gut–brain interaction and is characterised by chronic or intermittent abdominal pain or discomfort directly linked to a change of bowel habits. It is one of the most commonly diagnosed gastrointestinal conditions, affecting at least 3.5% of Australians and 4.1% of the global population.¹ IBS usually presents in young to middle-aged adults but can occur at any age. It is more common in women than men and is associated with lower quality of life and more frequent healthcare presentations.² There is no definitive evidence to suggest a single underlying cause for IBS, and it was previously labelled a 'functional gastrointestinal disorder'; however, this nomenclature has changed to 'a disorder of gut–brain interaction', emphasising the bidirectional relationship between the gut (including the immune system and microbiome) and the central nervous system.

This article provides an overview of the diagnosis and management of IBS, including lifestyle and dietary modifications, psychological and pharmacological therapies.

 

Pathophysiology

The aetiology and pathophysiology of IBS remain uncertain, but there are multiple underlying contributors, including gastrointestinal dysmotility, visceral hypersensitivity, and post-infection changes (bacterial, protozoal or viral gastroenteritis). Low-grade intestinal inflammation with increased mast cells has been observed in some IBS cohorts.^3,4

A dysbiotic or less diverse gut microbiome may be present in IBS, and small intestinal bacterial overgrowth occurs in a subset of patients; however, routine testing is not of clinical value.

Many studies support a strong psychosocial foundation for IBS, particularly seen in patients who report higher levels of daily stress and those with increased anxiety, depression, and somatic symptoms.⁴ There is often a genetic predisposition for IBS. Food intolerance plays a significant role (which is why diet modification often improves symptoms).

 

Clinical features

IBS is characterised by abdominal pain, which may be crampy or constant, and importantly, the pain is related to defaecation.^3,4 For example, many patients report that their pain improves with defaecation, while others report that defaecation triggers or exacerbates the pain. Some patients notice a change in stool form when pain begins. Meals also often trigger symptoms of IBS. All patients experience an alteration in their bowel habits, which varies from constipation to diarrhoea or both. Bloating is also common, and some patients demonstrate visible abdominal distension. Patients with IBS may also report symptoms of gastro-oesophageal reflux, dyspepsia, and systemic symptoms such as fatigue or headache.

 

Diagnosis

IBS is not a diagnosis of exclusion and should be a positive clinical diagnosis even at the first visit. Patients who meet the Rome criteria (Box 1) and have no alarm features (Box 2) have a high pretest probability of IBS and a low probability of organic disease. The Rome diagnostic criteria for IBS were updated in February 2026.⁵

All patients who present with altered bowel habits and abdominal pain should have a comprehensive history and examination (Box 3), as there is significant overlap between the symptoms of IBS and a range of other gastrointestinal diseases.⁶ Most importantly, these include inflammatory bowel disease (IBD), coeliac disease (the great mimic) and malignancy (even in younger patients with the increasing incidence of colon cancer in younger people).⁷ All new patients with suspected IBS should have a full blood count, C-reactive protein and coeliac serology ordered; other tests depend on the clinical presentation.

When alarm features (Box 2) are encountered in the history or physical examination, this should prompt further evaluation and consideration of referral to a specialist for endoscopic assessment. Further clinical examination, laboratory testing or imaging should be undertaken based on the patient's symptoms and clinical concern (Table 1). Electrolytes, urea and creatinine may be ordered to assess hydration status and electrolyte loss in patients with diarrhoea. Faecal calprotectin, a test that measures white blood cells in faeces, is used to investigate possible IBD and is rebated by Medicare. Endoscopic assessment is the gold standard investigation for colorectal malignancy and IBD. Biopsies of apparently normal-appearing colonic mucosa may identify microscopic colitis as a cause of IBS-like symptoms in older patients. A pelvic ultrasound may be considered in older women to investigate potential gynaecological pathology, as ovarian cancer can present with gastrointestinal symptoms.

Table 1 Clinical examination and investigations for organic pathology in patients presenting with altered bowel habits and abdominal pain

Examination or investigation	Justification
Rectal examination	Exclude faecal loading and screen for pelvic floor dysfunction, which is a major cause of chronic constipation
Full blood count	Exclude infection or anaemia
C-reactive protein	Assess for inflammation or infection
Iron studies	Low iron suggests possible gastrointestinal bleeding and necessitates upper and lower endoscopy
Thyroid-stimulating hormone	Assess for thyroid disease, which can present with either constipation or diarrhoea; test if suspicious clinical history or signs
Coeliac serology, including tissue transglutaminase (tTG)	Exclude coeliac disease (and check for IgA deficiency if tTG negative)
Faecal viral multiplex, Clostridioides difficile toxin, culture, and ova, cysts and parasites in stool	Investigate infectious causes (e.g. giardia) in patients with diarrhoea, recent travel, infectious contacts, or contaminated water use
Faecal calprotectin	Elevated concentrations indicate possible inflammatory bowel disease, with differentials of infection, malignancy or small bowel bleeding (e.g. angioectasias, polyps, malignancy)
Pelvic ultrasound	Helps to exclude gynaecological pathology (e.g. ovarian cancer) in older women with abdominal or pelvic pain or constipation

Testing for colorectal cancer should be performed in patients aged 45 years or older if not already undertaken (e.g. through the National Bowel Cancer Screening Program).

Avoid routine CT scanning in patients who meet the criteria for IBS (Box 1) without alarm features (Box 2), as this usually adds no diagnostic value, incurs a radiation burden, and can result in false-positive findings that lead to harmful further testing or interventions.

The Bristol Stool Form Scale is a useful objective tool to classify whether the stool pattern is diarrhoea or constipation (or a mixture of both), especially if the patient records their stool form over a week. The subtypes of IBS are characterised based on the patient's bowel habits (Box 4).

Referral to a gastroenterologist should occur if IBS symptoms worsen, standard therapy fails, or any alarm symptoms develop.

 

Management

Management of IBS requires a multifaceted patient-centred approach with a solid foundation of patient–clinician trust, symptom validation and understanding.⁸ Given the gut–brain interaction that underpins IBS, patients need to be cared for in an environment that fosters understanding and helps to manage expectations. Once organic disease has been excluded, the clinician should strongly reassure the patient, provide an explanation of the symptoms, and establish clear patient expectations with emphasis on the low likelihood that there is a 'quick fix' or 'wonder drug' that will resolve all symptoms. Disease management requires psychological, social, emotional and physical elements. In complex cases, best outcomes are achieved by an integrated care model approach including dietician, psychologist and gastroenterologist input.⁹

Lifestyle modifications

Lifestyle modifications are an important part of IBS management. For example, adequate sleep, regular exercise, stress reduction where possible, and minimisation of smoking and alcohol are essential components of IBS management.^3,4

Dietary modifications

Dietary triggers should be avoided, and it is prudent to ask patients to keep a diet-symptom diary to identify and track potential triggers. Common food triggers include caffeine, alcohol, spicy food, lactose, wheat, insoluble fibre, and high-fat foods. Patients should be advised to stay hydrated, reduce alcohol, coffee and fizzy drinks, eat regular, smaller meals more frequently, avoid high-fat foods, avoid gas-producing foods, such as beans, onions and cabbage, and, if lactose intolerance is suspected, avoid a high-lactose intake.¹⁰

It is important to note that patient self-imposed overly restrictive diets may indicate an underlying eating disorder and that encouraging these diets to continue without strong clinical justification may lead to malnutrition.

While some patients respond to specific diets, and others will improve with soluble fibre supplementation, such as psyllium (with the dose slowly increased as tolerated), a low FODMAP* diet has the most robust evidence of benefit in patients with IBS.¹¹ Advice should be sought from a dietitian to assist with implementation of the low FODMAP diet in all cases with more than mild IBS or, if this is unavailable, the Monash University FODMAP Diet app can be used.

Psychological therapies

The use of psychological therapies in IBS directly addresses the known gut–brain interaction. Identifying and managing psychiatric conditions can be transformative for a patient's symptoms. Psychological therapies can range from simple patient-led mindfulness when eating or experiencing symptoms, to psychologist-led cognitive behavioural therapy or hypnotherapy, depending on the degree of benefit the patient experiences. In a randomised controlled trial, women who underwent an 8-week mindfulness course showed a significantly greater reduction in symptom severity compared with those who attended a support group, and these results were sustained at 3 months follow-up.¹² Though not the primary outcome, there was also a significant improvement in IBS–Quality of Life (a scale that measures changes in physical and psychosocial functioning because of IBS and its treatment), which was also sustained to 3 months post intervention. Though more intensive, the benefits of cognitive behavioural therapy have been demonstrated in multiple randomised controlled trials.^8,13 Some studies suggest that internet-delivered cognitive behavioural therapy could be as effective as face-to-face sessions and may be more accessible and affordable compared with in-person cognitive behavioural therapy.¹⁴

Pharmacological therapies

Pharmacological therapies should be tailored to the individual's IBS subtype and symptoms.^4,8 For example, in patients with IBS-C subtype (IBS with predominant constipation)s, trial soluble fibre, such as psyllium, as first line; if this does not help, an osmotic laxative, such as polyethylene glycol, or a stimulant laxative, such as bisacodyl, may be used. In patients with IBS-D subtype (IBS with predominant diarrhoea), a trial of loperamide in the morning may be considered. If symptoms persist, loperamide can also be taken before meals. Pain is not helped by these treatments. If symptoms respond, therapy can be tailored to the lowest effective dose for 3 months, after which a drug holiday can be considered.

Peppermint oil may provide benefit for abdominal pain, constipation or diarrhoea and has minimal adverse effects (although reflux may be precipitated). It can be taken in capsules (e.g. 30 minutes before meals). The effectiveness of peppermint oil was evaluated in 7 randomised controlled trials that found a statistically significant improvement in IBS symptoms favouring peppermint oil over placebo (number needed to treat of 4).¹⁵ However, there was significant heterogeneity between the trials. As peppermint oil is safe, affordable and well tolerated, it is worth trialling in patients with IBS whose symptoms do not respond to lifestyle or dietary modifications.

Smooth muscle relaxants or antispasmodics (e.g. mebeverine) are of minimal or no benefit for many patients, but some find they help reduce abdominal pain. A meta-analysis of 2811 patients comparing 13 different antispasmodics with placebo found that fewer patients managed with antispasmodics had persistent global symptoms or abdominal pain.¹⁵ It should be noted that there was significant heterogeneity across the studies, including the antispasmodics used and the patients' IBS subtypes. Anticholinergics (e.g. hyoscine butylbromide) in higher doses can provide benefit through smooth muscle relaxation, though they should be avoided in patients with IBS-C because of the risk of worsening constipation. Other adverse effects include dry mouth, visual disturbances, sleep disturbances, somnolence and urinary retention, so they should also be avoided in older patients.

Tricyclic antidepressants, in low (non-antidepressant) doses, reduce visceral hypersensitivity, which leads to an improvement in abdominal pain and, at high doses, treats mood-related symptoms.¹⁶ They are associated with many adverse effects including anticholinergic effects, so these should be used with caution in older patients. Amitriptyline is often well tolerated in younger patients, while nortriptyline may be considered for older patients because it has fewer anticholinergic adverse effects.

Selective serotonin reuptake inhibitors such as fluoxetine and sertraline are less efficacious than tricyclic antidepressants in IBS and are best reserved for patients with IBS and comorbid depression or anxiety.

Poorly absorbable antibiotic therapy with rifaximin has been shown to reduce bloating and flatulence and can be used in patients with IBS-D or mixed IBS,¹⁷ though symptom relapse is universal and the drug is expensive. The use of rifaximin for IBS is off label in Australia.

Antihistamine therapy (e.g. loratadine) may have benefit in IBS-D, as may ondansetron (for diarrhoea and nausea but not pain).⁸

Therapies with limited or controversial evidence include acupuncture, Chinese herbal medicines, probiotics,¹⁸ faecal microbiota transplantation, and mesalamine.

 

Conclusion

IBS is a common disorder of gut–brain interaction and is characterised by recurrent abdominal pain or discomfort related to defaecation and altered stool frequency or form. A positive diagnosis of IBS can be made if diagnostic criteria are met and there are no alarm features. Effective management requires a holistic, patient-centred approach that addresses lifestyle, dietary, psychological and pharmacological aspects. Pharmacological treatment should be individualised to the patient's IBS subtype and symptoms.

*FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols. These are short-chain carbohydrates that are poorly absorbed in the small intestine and can contribute to symptoms of IBS.

This article was finalised on 17 April 2026.

Conflicts of interest: Savannah Morrison has no conflicts of interest to declare.

Nicholas Talley is the Chief Investigator of the Centre of Research Excellence in Transforming Gut Health, which is funded by the National Health and Medical Research Council. He is the President of the Asia Pacific Association of Medical Journal Editors and Board Chair of Doctors for the Environment Australia. He is an Emeritus Editor-in-Chief of the Medical Journal of Australia and former Board Director of the Gastroenterological Society of Australia.

Nicholas has received support from Brown University and the US Agency for Healthcare Research and Quality (fibre and laxation systematic review), the Rome Foundation (member of the gastroduodenal committee), Biocodex (functional dyspepsia diagnostic tool), Microba (microbiome consulting), Comvita Manuka Honey (functional dyspepsia trial consulting), BluMaiden (microbiome advisory board), Schwabe (functional dyspepsia and irritable bowel syndrome consulting), and Abbott (lectures on functional dyspepsia, gastroparesis and prokinetics).

In addition, Nicholas has developed questionnaires: Nepean Dyspepsia Index 1998, licensed to MAPI, and Talley Bowel Disease Questionnaire, licensed to Mayo Clinic. He also holds the following patents: Performance of a biomarker panel for irritable bowel syndrome (European Patent Number EP2710383B1), 'Diagnostic marker for functional gastrointestinal disorders' (US Patent Application Number 20240272175A1 [provisional]), 'Methods and compositions for treating age-related neurodegenerative disease associated with dysbiosis' (US Patent Application Number 63/537,725), and 'Compositions and methods for the treatment of oesophageal disorders' (Australian Provisional Patent ID 2025902002).

This article is peer reviewed.

 

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