About Irritable Bowel Syndrome

Irritable bowel syndrome is a common medical condition affecting around 1 in 10 people worldwide where the intestinal tract’s normal function is disturbed.

Keep reading to learn more.

Irritable bowel syndrome, known as IBS for short, is a gastrointestinal disorder affecting the large intestine. The exact cause of IBS remains unclear, but it is thought to involve abnormal muscle contractions in the colon, heightened pain sensitivity, changes in the gut microbiome, and disrupted communication between the brain and the gut. 1

Symptoms of IBS vary but commonly include:2

Abdominal pain






Although the symptoms of IBS often change over time, people tend to develop their own pattern. For example, some people have mostly diarrhoea, some have mostly constipation and others have abdominal pain without a major change in bowel movements. Triggers can differ from person-to-person.


There is no single test for IBS. It is largely classified as a condition of exclusion. In other words, IBS is usually diagnosed after all other causes of symptoms, such as infection or disease, are ruled out. The Rome Criteria – in its fourth iteration as of 2016 – is a set of defined criteria to more accurately diagnose IBS and other functional gastrointestinal disorders, which involve the brain as well as the gut. The guidelines outline symptoms and apply parameters such as frequency and duration make a possible a more accurate diagnosis of IBS.3

The Rome IV Criteria for IBS are:4

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

Related to defecation

Associated with a change in frequency of stool

Associated with a change in form (appearance) of stool

*Criteria fulfilled for the last three months with symptom onset at least six months prior to diagnosis.

Far from being a homogeneous condition, IBS is a spectrum and people can experience different forms of it, including diarrhoea-predominant and constipation-predominant, and alternating between constipation and diarrhoea.5

IBS Subtypes

Predominant bowel habits are based on stool form on days with at least one abnormal bowel movement.*
IBS with predominant constipation (IBS-C):
> ¼ (25%) of bowel movements with Bristol stool types 1 or 2 and < ¼ (25%) of bowel movements with Bristol stool types 6 or 7.
IBS with predominant diarrhoea (IBS-D):
> ¼ (25%) of bowel movements with Bristol stool types 6 or 7 and < ¼ (25%) of bowel movements with Bristol stool types 1 or 2.
IBS with mixed bowel habits (IBS-M):
> ¼ (25%) of bowel movements with Bristol stool types 1 or 2 and > ¼ (25%) of bowel movements with Bristol stool types 6 or 7.
IBS Unclassified (IBS-U):
Patients who meet diagnostic criteria for IBS but whose bowel habits cannot be accurately categorised into one of the three groups above should be categorised as having IBS-U.
*IBS subtypes related to bowel habit abnormalities (IBS-C, IBS-D and IBS-M) can only be confidently established when the patient is evaluated off medications used to treat bowel habit abnormalities.

Bristol Stool Form Scale

The Irritable Bowel Syndrome Severity Scoring System

To assist with clinical assessment and investigation of irritable bowel syndrome, a simple, easy-to-use severity scoring system was developed to reliably score patients previously classified as mild, moderate or severe. In its assessment it incorporates abdominal pain severity, abdominal pain frequency, satisfaction with bowel habits, abdominal distension severity and interference with quality of life / global well-being.6

The maximum achievable score was 500. Mild, moderate and severe cases were indicated by scores of 75 to 175, 175 to 300 and > 300 respectively. Controls scored below 75 and patients scoring in this range can be considered to be in remission. There was a highly significant difference between controls and patients as a whole (P = 0.0001) as well as significant differences (P < 0.01) between all severity categories. Scores repeated within 24 h were very reproducible and sensitivity to change was also extremely good (P < 0.001) with a change of 50 reliably indicating improvement.

The global prevalence of IBS

IBS affects between 5-10% of the world's population. 7

Approximately, 2 in every 3 IBS sufferers are female. 7

Most people affected by IBS are under the age of 50 although many older adults suffer. 7

Approximately 40% of those with IBS have mild symptoms, 35% have moderate IBS, and 25% have severe IBS. 8

IBS and links with stress & anxiety

Stress and anxiety have been found to influence the functioning of the gut-brain axis, a bidirectional communication system between the brain and the gut.9

Psychological stress can trigger physiological changes in the gut, leading to increased sensitivity, altered motility, and abnormal contractions.1   These changes can contribute to the development or exacerbation of IBS symptoms.

A study that looked at more than 1.2 million IBS patient hospitalisations from 4,000 U.S. hospitals over a three-year period, found that over 38% had anxiety, and over 27% had depression. Both figures were double the rate of anxiety and depression found in those without IBS.10

The connection between stress and Irritable Bowel Syndrome (IBS) involves many different factors including the release of stress hormones, gut bacteria and intestinal permeability. Stress and anxiety can disrupt the balance of gut bacteria, impacting gut function and contributing to IBS symptoms.

Stress and anxiety can form a vicious circle, starting with triggers such as a traumatic event or infection, leading to changes in bowel function, resulting in disruptions to daily routines or eating habits. These concerns can grow and start to affect work or social lives and cause further stress and anxiety. 11


1 Ford AC, Sperber AD, Corsetti M, Camilleri M. Irritable bowel syndrome. Lancet. 2020 Nov 21;396(10263):1675-1688. doi: 10.1016/S0140-6736(20)31548-8. Epub 2020 Oct 10. PMID: 33049223. 

2 Irritable bowel symptoms, https://www.nhs.uk/conditions/irritable-bowel-syndrome-ibs/symptoms/

3 Drossman DA. Functional gastrointestinal disorders: history, pathophysiology, clinical features, and Rome IV. Gastroenterology. 2016;150(6):1262-1279. doi:10.1053/j.gastro.2016.02.032

4 Rome Foundation. Rome IV Criteria – https://theromefoundation.org/rome-iv/rome-iv-criteria/

5 https://www.ncbi.nlm.nih.gov/books/NBK534810/#:~:text=IBS%20typically%20consists%20of%20abdominal,and%20stool%20pattern%20with%20time

6 Francis CY, Morris J, Whorwell PJ. The irritable bowel severity scoring system: a simple method of monitoring irritable bowel syndrome and its progress. Aliment Pharmacol Ther. 1997 Apr;11(2):395-402. doi: 10.1046/j.1365-2036.1997.142318000.x. PMID: 9146781.

7 IBS Facts and Statistics, https://aboutibs.org/what-is-ibs/facts-about-ibs/

8 Drossman DA, Chang L, Bellamy N, et al. Severity in irritable bowel syndrome: a Rome Foundation Working Team report. Am J Gastroenterol. 2011;106(10):1749-1760. doi:10.1038/ajg.2011.201

9 The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4367209/

10 Tarar ZI, Farooq U, Zafar Y, et al. Burden of anxiety and depression among hospitalized patients with irritable bowel syndrome: a nationwide analysis. Ir J Med Sci. 2023;192(5):2159-2166. doi:10.1007/s11845-022-03258-6

11 The Vicious Circle, IBS & Stress – Bladder & Bowel Community, https://www.bladderandbowel.org/news/vicious-circle-ibs-and-stress/