Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by a group of symptoms that commonly include abdominal pain, abdominal bloating, and changes in the consistency of bowel movements. These symptoms may occur over a long time, sometimes for years. IBS can negatively affect quality of life and may result in missed school or work or reduced productivity at work. Disorders such as anxiety, major depressive disorder, and myalgic encephalomyelitis/chronic fatigue syndrome are common among people with IBS, sometimes developing due to the condition.The cited review is based on sources ranging from 1988 to 2001 and is probably biased relative to more recent research.
The cause of IBS is not known, but multiple factors have been proposed to lead to the condition. Theories include combinations of "gut–brain axis" problems, alterations in gut motility, visceral hypersensitivity, infections including small intestinal bacterial overgrowth, neurotransmitters, genetic factors, and food sensitivity. Onset may be triggered by a stressful life event, or an intestinal infection. In the latter case, it is called post-infectious irritable bowel syndrome.
Diagnosis is based on symptoms in the absence of worrisome features, and once other potential conditions have been ruled out. Worrisome or "alarm" features include onset at greater than 50 years of age, weight loss, blood in the stool, or a family history of inflammatory bowel disease. Other conditions that may present similarly include celiac disease, microscopic colitis, inflammatory bowel disease, bile acid malabsorption, and colon cancer.
Treatment for IBS aims to improve symptoms. This may include dietary changes, medication, probiotics, and counseling. Dietary measures include increasing soluble fiber intake, or a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs). A low-FODMAP diet is meant for short to medium-term use and is not intended as a life-long therapy. The medication loperamide may be used to help with diarrhea while laxatives may be used to help with constipation. There is strong clinical-trial evidence for the use of antidepressants, often in lower doses than those used for depression or anxiety, even in patients without comorbid mood disorder. Tricyclic antidepressants such as amitriptyline or nortriptyline and medications from the selective serotonin reuptake inhibitor (SSRI) group may improve overall symptoms and reduce pain. Patient education and a good doctor–patient relationship are an important part of care.
About 10–15% of people in the developed world are believed to be affected by IBS. The prevalence varies according to country (from 1.1% to 45.0%) and criteria used to define IBS; the average global prevalence is 11.2%. It is more common in South America and less common in Southeast Asia. In the Western world, it is twice as common in women as men and typically occurs before age 45. However, women in East Asia are not more likely than their male counterparts to have IBS, indicating much lower rates among East Asian women. "However, some Asian studies fail to report significant gender differences in the prevalence of IBS.6" Similarly, men from South America, South Asia and Africa are just as likely to have IBS as women in those regions, if not more so. "In South Asia, South America, and Africa, rates of IBS in men are much closer to those of women, and in some cases higher. Consequently, if prevalence is stratified according to geographic region, no significant sex difference can be seen in these areas.80" The condition appears to become less common with age. IBS does not affect life expectancy or lead to other serious diseases. The first description of the condition was in 1820, while the current term irritable bowel syndrome came into use in 1944.
Signs and symptoms The primary symptoms of IBS are abdominal pain or discomfort in association with frequent diarrhea or constipation and a change in bowel habits. Symptoms usually are experienced as acute attacks that subside within one day, but recurrent attacks are likely. There may also be urgency for bowel movements, a feeling of incomplete evacuation (tenesmus) or bloating. In some cases, the symptoms are relieved by bowel movements.
People with IBS, more commonly than others, have gastroesophageal reflux, symptoms relating to the genitourinary system, fibromyalgia, headache, backache, and psychiatric symptoms such as depression, sleep disorders, and anxiety. About a third of adults who have IBS also report sexual dysfunction, typically in the form of a reduction in libido.
By definition, the symptoms of IBS are chronic (having been present for at least 6 months based on Rome IV diagnostic criteria). Symptoms such as unexplained weight loss, rectal bleeding, recent changes in bowel habits, anemia, or iron deficiency are usually not due to IBS and warrant additional testing. Some people with IBS with constipation also have pelvic floor dysfunction, which may present as needing to manually disimpact stool during bowel movements, or bladder symptoms such as incomplete bladder emptying, urination at night (nocturia), increased urinary frequency, or frequent urinary tract infections.
Cause While the causes of IBS are still not fully understood, it is believed that the entire gut–brain axis is affected.
Allergy Recent findings suggest that an allergy-triggered peripheral immune mechanism may underlie the symptoms associated with abdominal pain in patients with irritable bowel syndrome.
Risk factors People who are younger than 50, women, and those with a family history of the condition are more likely to develop IBS. Further risk factors are anxiety, depression, and stress. The risk of developing IBS increases six-fold after having a gastrointestinal infection (gastroenteritis). This is also called post-infectious IBS. The risk of developing IBS following an infection is further increased in those who also had a prolonged fever during the illness. Antibiotic use also appears to increase the risk of developing IBS. Genetic defects in innate immunity and epithelial homeostasis increase the risk of developing both post-infectious as well as other forms of IBS.
Stress The role of the brain–gut axis in IBS has been suggested since the 1990s and childhood physical and psychological abuse are often associated with the development of IBS. It is believed that psychological stress may trigger IBS in predisposed individuals.
Given the high levels of anxiety experienced by people with IBS and the overlap with conditions such as fibromyalgia and chronic fatigue syndrome, a potential explanation for IBS involves a disruption of the stress system. The stress response in the body involves the hypothalamic–pituitary–adrenal axis (HPA) and the sympathetic nervous system, both of which have been shown to operate abnormally in people with IBS. Psychiatric illness or anxiety precedes IBS symptoms in two-thirds of people with IBS. Psychological traits predispose previously healthy people to developing IBS after gastroenteritis. Individuals with IBS also report high rates of sleep disturbances such as trouble falling asleep and frequent arousal throughout the night.
Gastroenteritis
Approximately 10 percent of IBS cases are triggered by an acute gastroenteritis infection. The CdtB toxin is produced by bacteria causing gastroenteritis. The host may develop an autoimmunity when host antibodies to CdtB cross-react with vinculin. Genetic defects relating to the innate immune system and epithelial barrier, as well as high stress and anxiety levels, appear to increase the risk of developing post-infectious IBS. Post-infectious IBS usually manifests itself as the diarrhea-predominant subtype. Evidence has demonstrated that the release of high levels of proinflammatory cytokines during acute enteric infection causes increased gut permeability leading to translocation of the commensal bacteria across the epithelial barrier; this in turn can result in significant damage to local tissues, which can develop into chronic gut abnormalities in sensitive individuals. However, increased gut permeability is strongly associated with IBS regardless of whether an infection initiated IBS. A link between small intestinal bacterial overgrowth and tropical sprue has been proposed to be involved as a cause of post-infectious IBS.
Bacteria
Small intestinal bacterial overgrowth (SIBO) occurs with greater frequency in people who have been diagnosed with IBS compared to healthy controls. SIBO is most common in diarrhea-predominant IBS but also occurs in constipation-predominant IBS more frequently than healthy controls. Symptoms of SIBO include bloating, abdominal pain, diarrhea, or constipation, among others. IBS may be the result of the immune system interacting abnormally with gut microbiota, resulting in an abnormal cytokine signalling profile.
Certain bacteria are found in lower or higher abundance when compared with healthy individuals. Generally Bacteroidota, Bacillota, and Pseudomonadota are increased and Actinomycetota, Bifidobacteria, and Lactobacillus are decreased. Within the human gut, there are common phyla found. The most common is Bacillota. This includes Lactobacillus, which is found to have a decrease in people with IBS, and Streptococcus, which is shown to have an increase in abundance. Within this phylum, species in the class Clostridia are shown to have an increase, specifically Ruminococcus and Dorea. The family Lachnospiraceae presents an increase in IBS-with-diarrhea (IBS-D) patients. The second most common phylum is Bacteroidota. In people with IBS, the Bacteroidota phylum has been shown to have an overall decrease, but an increase in the genus Bacteroides. IBS-D shows a decrease for the phylum Actinomycota and an increase in Pseudomonadota, specifically in the family Enterobacteriaceae.
Gut microbiota
Alterations of gut microbiota (dysbiosis) are associated with the intestinal manifestations of IBS, but also with the psychiatric morbidity that coexists in up to 80% of people with IBS.
Studies in patients with IBS have shown notable changes in gut microbiota, including increases in Clostridia and Escherichia coli, along with reductions in Bacteroidia, Lactobacillus species, and Bifidobacterium species. Many patients also have a higher Firmicutes to Bacteroidetes ratio and reduced overall microbial diversity. The intricate relationship between intestinal microbes and the gut–brain axis has led to the emerging concept of a microbiota–gut–brain axis. Evidence supporting therapies that modify the microbiome remains mixed; antibiotics, probiotics, and prebiotics have documented roles, while fecal microbiota transplantation is still largely confined to research. Important questions remain, including what constitutes the optimal microbiome profile for fecal donors in patients with IBS.Microbiota-Gut-Brain Axis: Towards a Microbiota-Centred Approach for Irritable Bowel Syndrome. J Biol Res 2025, 98. https://doi.org/10.4081/jbr.2025.13873.
Protozoa
Protozoal infections can cause symptoms that mirror specific IBS subtypes, e.g., infection by certain substypes of Blastocystis hominis (blastocystosis). Many people regard these organisms as incidental findings, and unrelated to symptoms of IBS.
Blastocystis and Dientamoeba fragilis colonisation occurs more commonly in IBS-affected individuals, but their role in the condition is unclear.
Vitamin D Vitamin D deficiency is more common in individuals affected by IBS. Vitamin D is involved in regulating triggers for IBS including the gut microbiome, inflammatory processes and immune responses, as well as psychosocial factors.
Genetics