Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a chronic, relapsing disorder of gut-brain interaction (the current terminology for what was previously called a functional gastrointestinal disorder) characterized by recurrent abdominal pain or discomfort together with altered bowel habits — constipation, diarrhea, or an unpredictable mix of both — without any structural or biochemical abnormality that fully explains the symptoms.^[1]

There is no biomarker for IBS. The underlying pathophysiology is multifactorial, involving visceral hypersensitivity, altered gastrointestinal motility, low-grade mucosal immune activation, disruption of the gut microbiota, and dysregulated signalling along the brain-gut axis, often compounded by psychological comorbidity. Because no test can confirm the diagnosis, IBS is identified through a positive, symptom-based diagnostic process — the current Rome V criteria — combined with a limited, targeted set of investigations to exclude the small number of alternative diagnoses that can mimic it, rather than an open-ended process of exclusion.^[2]

Symptoms fluctuate over time, and a substantial proportion of patients migrate between subtypes as their bowel habit pattern changes, so both diagnosis and treatment selection need to be revisited whenever the predominant pattern shifts rather than fixed permanently at the first visit.

Typical presentation

Recurrent abdominal pain or discomfort, often crampy and variable in location, that is characteristically related to defecation — improved, worsened, or otherwise clearly linked to a bowel movement — together with a change in stool frequency, a change in stool form/appearance, or both. Bloating and abdominal distension are extremely common accompanying complaints, as is a sensation of incomplete evacuation and passage of mucus with stool. Symptoms typically wax and wane over weeks to months and are frequently triggered or worsened by meals, stress, or hormonal changes.

Physical examination

Usually unremarkable. Mild, diffuse abdominal tenderness on palpation may be present, but a normal examination is expected and does not argue against the diagnosis. An abdominal mass, ascites, or focal peritoneal signs is not consistent with uncomplicated IBS and should prompt urgent further assessment.

• Family history of IBS or another disorder of gut-brain interaction.
• Personal history of anxiety, depression, or other psychological comorbidity.
• History of an acute gastrointestinal infection — post-infectious IBS can follow bacterial, viral, or parasitic gastroenteritis.
• Early-life adverse events or significant psychosocial stress/trauma.
• Female sex.
• Overlapping disorders of gut-brain interaction or centrally mediated conditions — fibromyalgia, chronic fatigue syndrome, interstitial cystitis/bladder pain syndrome, temporomandibular disorder, chronic pelvic pain.
• Reported intolerance or sensitivity to specific dietary components, particularly fermentable carbohydrates and wheat-containing foods.
• Prior antibiotic exposure.

Conditions commonly confused with IBS

Several conditions produce a similar symptom picture to IBS, particularly IBS-D and IBS-M, and should be considered before finalizing an IBS diagnosis, especially if any alarm feature is present or first-line therapy fails.

Diagnostic criteria — Rome V (current since June 2026)

Symptom onset should be at least 6 months before diagnosis. This is a meaningful loosening of the prior Rome IV threshold, which required pain at least 1 day per week and did not recognize discomfort as a qualifying symptom on its own — a patient who did not strictly meet Rome IV criteria may now meet Rome V criteria, and real-world validation shows Rome V identifies a somewhat different population than Rome III/IV rather than simply a larger version of the same population.^[4]

Treat these criteria as a framework rather than a rigid checklist: a patient who falls just short of literal criteria fulfillment but otherwise has a classic, alarm-feature-free presentation can still reasonably be managed as IBS.

History

• Bowel movement frequency and stool appearance (Bristol Stool Form Scale) to establish subtype.
• Abdominal pain characteristics — location, frequency, severity, duration, and relationship to defecation — plus associated bloating, distension, or flatulence.
• Potential symptom triggers: specific foods, personal stress, relationship or occupational stressors.
• Family history of IBS or other gastrointestinal disorders, specifically screening for the red-flag family-history conditions (colorectal cancer, IBD, celiac disease).
• Comorbid medical history, including gynecologic and urologic conditions that can mimic or overlap with IBS symptoms.
• Current medications that may be contributing to constipation or diarrhea.
• Screen specifically for reproducible food-allergy symptoms — routine testing for food allergy or food sensitivity is not indicated in patients without them.^[2]

Targeted investigations

Diagnose using a positive, symptom-based strategy rather than a strategy of exhaustive exclusion — this improves both time to starting effective therapy and overall cost-effectiveness without missing clinically important alternative diagnoses, provided alarm features are absent.^[2]

• Build a strong, ongoing patient-clinician relationship as the foundation of management.
• Reduce abdominal pain/discomfort and normalize bowel habits to a level the patient finds acceptable — IBS management targets symptom control, not a cure.
• Tailor the treatment plan to the patient's specific subtype, predominant/most bothersome symptom, and personal preferences.
• Identify and treat coexisting psychological comorbidity when present.
• Improve quality of life and support normal social and occupational functioning.
• Avoid unnecessary escalation of testing or medication once alarm features have been appropriately excluded.

The patient-clinician relationship

A strong therapeutic relationship measurably improves outcomes in IBS and is itself part of treatment, not just a courtesy before it. Key elements: active listening to the patient's concerns, not interrupting, using empathy, setting realistic expectations about what treatment can achieve, and using non-verbal techniques such as eye contact and open body posture. A multidisciplinary approach — involving a pharmacist, dietitian, and psychologist as needed — is recommended.^[3]

Lifestyle

• Encourage regular physical activity; sustained increases in activity have long-term benefit for both GI and associated psychological symptoms.
• Limit caffeine intake to 400 mg/day or less, similar to general-population guidance — evidence specific to IBS is limited.
• Counsel against binge drinking specifically (rather than light/moderate alcohol intake), which is associated with worsened GI symptoms, particularly in IBS-D.

Diet

• Low FODMAP diet: a limited trial (a restriction phase followed by structured, systematic reintroduction) improves global IBS symptoms for many patients and is recommended as a dietary option, ideally with dietitian involvement to guide reintroduction and avoid unnecessary long-term restriction and nutritional deficiency.^[2]
• Fibre: soluble fibre (psyllium, calcium polycarbophil) has consistent evidence of benefit for global IBS symptoms; insoluble fibre (e.g., wheat bran) does not and should not be recommended as an IBS treatment.^[2] Start fibre at a nominal dose and titrate gradually over several weeks to a total daily intake around 20-30 g, since starting at a high dose worsens bloating and gas.
• Gluten-free diet: not recommended in the absence of confirmed celiac disease — test for celiac disease before considering gluten restriction rather than restricting empirically.
• IgG-based elimination diets: not recommended — the supporting evidence is very low quality.

Gut microflora

• Probiotics: evidence is heterogeneous and of very low overall quality across strains/formulations; current ACG guidance advises against routine use for global IBS symptoms, a shift from the earlier, broader positioning of probiotics as generally recommended.^[2]
• Prebiotics: evidence is limited to isolated trials (e.g., guar gum) and does not support routine recommendation.
• Fecal microbiota transplantation: not recommended for IBS outside of a research setting given the current evidence base.^[2]

Psychological treatment

Gut-directed psychotherapies — cognitive behavioural therapy, gut-directed hypnotherapy, and mindfulness-based therapy — are recommended to improve global IBS symptoms and can be offered at any point in management, not only after pharmacologic options have failed.^[2]

Pediatrics

IBS occurs in children and adolescents and is diagnosed using pediatric-adapted symptom criteria; management emphasizes nonpharmacologic measures and dietary review first. Psyllium and polyethylene glycol have established pediatric dosing. Linaclotide, plecanatide, eluxadoline, and rifaximin are not established for routine pediatric use in IBS — refer to pediatric gastroenterology rather than extrapolating adult dosing.

Older adults

Use caution with the anticholinergic burden of TCAs and antispasmodics (dry mouth, constipation, cognitive effects, falls risk). Avoid supratherapeutic loperamide dosing, which carries a real risk of cardiac dysrhythmia at doses above the labelled maximum. Prucalopride and eluxadoline both require reduced starting doses at age 65 and older.

Renal and hepatic impairment

Prucalopride requires dose reduction in severe renal impairment (creatinine clearance 30 mL/min or less) and severe hepatic impairment. Eluxadoline requires the lower 75 mg twice-daily starting dose in mild-to-moderate hepatic impairment and should not be used in severe hepatic impairment (Child-Pugh class C). Rifaximin needs no dose adjustment in mild-to-moderate hepatic impairment but should be used cautiously in severe (Child-Pugh class C) impairment given increased systemic absorption.

Reassess symptom response at an interval appropriate to the agent chosen — most pharmacologic options should show a meaningful benefit within 4 weeks; discontinue and reassess the plan if there is no benefit by then, and specifically discontinue eluxadoline sooner if severe constipation persists beyond 4 days. Re-screen for newly developed alarm features at every follow-up, since a stable, long-standing IBS pattern can still be interrupted by a new, unrelated diagnosis. Monitor adherence and agent-specific adverse effects — anticholinergic burden with TCAs/antispasmodics, electrolyte status with heavy or prolonged laxative use, and gallbladder/alcohol-use status before continuing eluxadoline. Revisit IBS subtype classification whenever the bowel habit pattern changes, since this can change the appropriate drug class. Screen for psychological comorbidity on an ongoing basis, not only at diagnosis, and reinforce the value of gut-directed psychotherapy at any point that symptoms remain bothersome.^[2]

1. Reconfirm the diagnosis: re-screen for alarm features and, if not already done, complete celiac serology (IBS-D/IBS-M) and inflammatory markers (fecal calprotectin/CRP) before assuming the current therapy has simply failed.
2. Confirm adherence and correct administration — e.g., linaclotide taken on an empty stomach, fibre separated from other oral medications by the required interval, loperamide dose/timing relative to anticipated triggers.
3. Optimize within the subtype-appropriate pathway before switching classes: for IBS-C, step from fibre to a guanylate cyclase-C agonist; for IBS-D, step from loperamide to rifaximin or eluxadoline.
4. Add a low-dose tricyclic antidepressant if abdominal pain remains the dominant, poorly controlled symptom despite subtype-directed therapy.
5. Offer gut-directed psychotherapy if not yet tried, particularly when psychological comorbidity or high symptom-related anxiety is present.
6. Investigate specifically for bile acid malabsorption in refractory IBS-D (fecal bile acid testing, SeHCAT where available, or an empiric cholestyramine trial), and for pelvic floor dysfunction in refractory IBS-C (anorectal physiology testing).
7. Refer to gastroenterology for symptoms that remain refractory despite optimized multimodal therapy, or whenever the diagnosis itself becomes uncertain.

• Diagnosing IBS only after an extensive negative exclusion workup instead of using a positive, Rome-based diagnostic strategy in a patient without alarm features.
• Missing an alarm feature (GI bleeding, unexplained weight loss, nocturnal symptoms, onset after age 50, relevant family history) and treating presumptively as IBS.
• Treating every patient the same way rather than tailoring therapy to subtype and predominant symptom.
• Recommending insoluble fibre (e.g., wheat bran) expecting the same benefit as soluble fibre.
• Relying on continuous, scheduled loperamide as a complete IBS-D treatment — it controls stool consistency/urgency but does not reliably improve pain or global symptoms.
• Starting a tricyclic antidepressant at a full antidepressant-range dose instead of the much lower IBS-specific starting dose.
• Prescribing eluxadoline in a patient without a gallbladder, with a history of pancreatitis, or with heavy alcohol use — an absolute contraindication given pancreatitis/sphincter of Oddi risk.
• Assuming probiotics are a reliable, evidence-based fix for IBS symptoms — the evidence is heterogeneous and of very low overall quality, and current guidance recommends against routine use.
• Missing bile acid malabsorption as a contributor in refractory IBS-D.
• Failing to re-triage a patient whose previously stable, typical IBS pattern has newly changed.

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Bulk-forming agents (soluble fibre)

Soluble fibre has consistent trial evidence for global IBS symptoms across subtypes; insoluble fibre (e.g., wheat bran) does not and should not be recommended.

Osmotic laxatives

For constipation specifically, not for global IBS symptom relief; add on to fibre rather than substituting for it when pain/bloating persist alongside constipation.

Guanylate cyclase-C agonists

Recommended for global IBS-C symptoms including pain, not just constipation itself. Lubiprostone, a related chloride channel activator with similar evidence, is not currently marketed in Canada.

Serotonin 5-HT4 receptor agonist (prokinetic)

Prucalopride is approved for chronic idiopathic constipation and may be considered specifically for the constipation component of IBS-C when other measures are inadequate; evidence for improving global IBS-C symptoms is weaker than for guanylate cyclase-C agonists. Tegaserod, an older 5-HT4 agonist once used for IBS-C, is not currently marketed in Canada.

Antispasmodics

Evidence for global symptom control is low quality; current ACG guidance recommends against routine use for this purpose, while short-term use for pain/cramping episodes around meals or anticipated triggers remains a reasonable option.

Antidiarrheals

First-line symptomatic control of stool consistency and urgency in IBS-D; do not rely on this class alone for global symptom control.

Gut-selective antibiotic

Mixed opioid agonist/antagonist

Bile acid sequestrants

Off-label for IBS-D with suspected bile acid malabsorption; a therapeutic trial can support the diagnosis even without confirmatory testing.

Serotonin 5-HT3 receptor antagonist

Off-label, special-situation use for refractory IBS-D diarrhea/urgency not adequately controlled by first- and second-line options; not included in the 2021 ACG graded recommendations, so reserve for specialist-directed use. Alosetron, a 5-HT3 antagonist approved in the United States for severe IBS-D in women who failed conventional therapy, is not marketed in Canada.

Tricyclic antidepressants (low-dose, pain modulation)

Used at doses well below the antidepressant range. The anticholinergic effect can specifically benefit IBS-D by slowing transit; use cautiously if constipation-predominant.

Selective serotonin reuptake inhibitors

Used at standard antidepressant doses (unlike TCAs, which are used at much lower doses for IBS). The 2021 ACG guideline found that the benefit of antidepressant therapy on abdominal pain was attributable to TCAs, not SSRIs — reserve SSRIs for patients with comorbid depression/anxiety or TCA intolerance rather than as a primary pain-modulating choice.

Stimulant laxatives (short-term use, including in pregnancy)

Reserve for short-term use (generally under 1 week) when bulk-forming fibre alone is inadequate for IBS-C constipation.