Irritable bowel syndrome (IBS) is a common, chronic condition affecting how the gut functions — causing recurrent abdominal pain along with changes in bowel habit, such as diarrhoea, constipation, or both. IBS is classified as a disorder of gut-brain interaction: the bowel itself is not inflamed or structurally damaged, but the way it moves, senses, and communicates with the brain is disrupted, producing genuine, often significant symptoms.
This is an important starting point, because IBS is frequently and understandably confused with inflammatory bowel disease (IBD), a different group of conditions involving visible inflammation and tissue damage in the gut. IBS does not cause inflammation, does not damage the intestine, and does not raise the risk of colorectal cancer — but it can still meaningfully affect daily life, and its symptoms deserve proper assessment and active management rather than being dismissed as “just stress” or something to simply live with.
How IBS Develops: The Gut-Brain Axis
IBS is understood to arise from a combination of factors affecting the communication network between the gut and the brain, known as the gut-brain axis:
- Visceral hypersensitivity: The nerves lining the gut become more sensitive than usual, so that normal amounts of gas, stretching, or movement are perceived as pain or discomfort.
- Altered gut motility: The muscular contractions that move contents through the intestine can become too fast (contributing to diarrhoea), too slow (contributing to constipation), or irregular.
- Gut microbiome changes: The balance of bacteria living in the intestine appears to differ in people with IBS, and may influence both gut sensitivity and motility, though the exact relationship is still being researched.
- Brain-gut signalling: The gut and brain communicate constantly via nerves, hormones, and immune signalling; in IBS, this two-way communication appears to be disrupted, which is also why psychological stress can worsen symptoms even though it does not cause the underlying condition.
IBS Subtypes
IBS is classified into subtypes based on the predominant bowel habit, using a standardised stool classification tool. Knowing the subtype matters, since treatment — particularly medication choice — is often tailored to it.
- IBS-C (Constipation-Predominant): Bowel movements are predominantly hard or lumpy, occurring on more than a quarter of symptomatic days, with looser stools occurring less than a quarter of the time.
- IBS-D (Diarrhoea-Predominant): Bowel movements are predominantly loose or watery, occurring on more than a quarter of symptomatic days, with harder stools occurring less than a quarter of the time.
- IBS-M (Mixed): Both hard and loose stools each occur on more than a quarter of symptomatic days — bowel habit alternates between constipation and diarrhoea.
- IBS-U (Unclassified): Bowel habit does not fit clearly into the other categories, but the core diagnostic criteria for IBS are still met.
IBS vs. IBD: Two Very Different Conditions
Because their names and some symptoms overlap, IBS is often confused with inflammatory bowel disease (IBD) — which includes Crohn's disease and ulcerative colitis. Understanding the difference matters, both for peace of mind and for making sure the right condition is actually being treated.
- IBS is functional, the bowel looks structurally normal on colonoscopy and biopsy; the problem lies in how it functions and how pain is perceived, not in visible tissue damage.
- IBD is inflammatory, it involves genuine, visible inflammation and, over time, structural damage to the bowel wall, detectable on colonoscopy, imaging, and blood or stool markers.
- IBS does not cause rectal bleeding, does not cause the bowel damage seen in IBD, and does not increase the risk of colorectal cancer. IBD carries all three of these risks to varying degrees.
A simple, non-invasive stool marker called faecal calprotectin, which detects intestinal inflammation, is increasingly used to help distinguish the two: a normal result makes IBD unlikely and supports an IBS diagnosis, while an elevated result points toward further investigation for IBD instead. This is one of the ways a confident, positive IBS diagnosis can often be reached without needing to proceed straight to colonoscopy in every case.
Symptoms of IBS
- Recurrent abdominal pain, often related to bowel movements
- Bloating and visible abdominal distension
- Constipation, diarrhoea, or an alternating pattern of both
- A sense of incomplete emptying after a bowel movement
- Mucus in the stool
- Excess gas
- Symptoms that fluctuate over time, often worsening with certain foods, stress, or hormonal changes
Causes and Triggers of IBS
The exact underlying cause of IBS is not fully understood, but several factors are recognised as common triggers that worsen symptoms in someone already predisposed to the condition:
- Certain foods and drinks — including high-FODMAP foods such as wheat, certain dairy products, some fruits, beans, and carbonated drinks; true food allergy is rarely the underlying cause, though intolerance-like reactions are common
- Stress and anxiety — can worsen symptoms via the gut-brain axis, without being the root cause of IBS
- Hormonal changes — many women notice symptoms fluctuate with the menstrual cycle
- A previous gut infection (post-infectious IBS) — some people develop IBS symptoms for the first time after a bout of gastroenteritis
- Family history — IBS appears to run in families, suggesting a genetic component alongside environmental triggers
How IBS Is Diagnosed
Modern IBS diagnosis is based on recognising a specific, well-defined symptom pattern — recurrent abdominal pain together with a change in bowel habit, present for a set period — combined with screening for red flag features that would suggest a different cause. This is a shift from older approaches, which treated IBS purely as a diagnosis reached only after exhaustively ruling out every other possibility.
- Clinical History Against Diagnostic Criteria: A detailed symptom history is assessed against standardised diagnostic criteria for IBS, alongside screening for red flag features that would prompt further investigation instead.
- Blood Tests: Checks for anaemia, coeliac disease, and thyroid dysfunction, all of which can cause symptoms that overlap with IBS and should be excluded.
- Stool Tests: Faecal calprotectin helps distinguish IBS from IBD by checking for intestinal inflammation, while stool tests for infection are used when a recent onset or infectious trigger is suspected.
- Colonoscopy: Not required for every patient, but recommended when red flag symptoms are present, when the presentation is atypical, or as part of routine age-appropriate colorectal cancer screening.
Living with IBS: Day-to-Day Management
- Keep a symptom and food diary to identify your personal triggers, ideally with the support of a dietitian rather than self-restricting broadly
- Build regular meal timing and physical activity into your routine, both of which support healthy gut motility
- Build regular meal timing and physical activity into your routine, both of which support healthy gut motility
- Stay well hydrated, particularly important for IBS-C
- Don't assume every new or changing symptom is “just the IBS” — seek reassessment if your pattern changes significantly or red flag symptoms develop
- Work with your care team on a personalised plan rather than relying solely on broad, indefinite dietary restriction, which can affect nutrition and quality of life unnecessarily
