[7] 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.

[9][18] Dietary measures include increasing soluble fiber intake, or a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs).

Tricyclic antidepressants such as amitriptyline or nortriptyline and medications from the selective serotonin reuptake inhibitor (SSRI) group may improve overall symptoms and reduce pain.

[24] The primary symptoms of IBS are abdominal pain or discomfort in association with frequent diarrhea or constipation and a change in bowel habits.

[30][31] Recent findings suggest that an allergy triggered peripheral immune mechanism may underlie the symptoms associated with abdominal pain in patients with irritable bowel syndrome.

[37] Genetic defects in innate immunity and epithelial homeostasis increase the risk of developing both post-infectious as well as other forms of IBS.

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.

[43] 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.

[44] Small intestinal bacterial overgrowth (SIBO) occurs with greater frequency in people who have been diagnosed with IBS compared to healthy controls.

[55][56] Vitamin D is involved in regulating triggers for IBS including the gut microbiome, inflammatory processes and immune responses, as well as psychosocial factors.

[58][59] The resulting defect leads to disruption in bowel function, by affecting the Nav1.5 channel, in smooth muscle of the colon and pacemaker cells.

[63] Dysregulated brain-gut axis, abnormal serotonin/5-hydroxytryptamine (5-HT) metabolism, and high density of mucosal nerve fibers in the intestines have been implicated in the mechanisms of IBS.

However, red flag conditions may not always contribute to accuracy in diagnosis; for instance, as many as 31% of people with IBS have blood in their stool, many possibly from hemorrhoidal bleeding.

[74] Investigations are performed to exclude other conditions:[citation needed] Colon cancer, inflammatory bowel disease, thyroid disorders (hyperthyroidism or hypothyroidism), and giardiasis can all feature abnormal defecation and abdominal pain.

[75] Conditions that may present similarly include celiac disease, bile acid malabsorption, colon cancer, and dyssynergic defecation.

[7] Ruling out parasitic infections, lactose intolerance, small intestinal bacterial overgrowth, and celiac disease is recommended before a diagnosis of IBS is made.

[77] People with IBS are at increased risk of being given inappropriate surgeries such as appendectomy, cholecystectomy, and hysterectomy due to being misdiagnosed as other medical conditions.

[104] In some individuals, IBS may have an acute onset and develop after an infectious illness characterized by two or more of: fever, vomiting, diarrhea, or positive stool culture.

[112] Symptoms most likely to improve on this type of diet include urgency, flatulence, bloating,[113] abdominal pain, and altered stool output.

[116] A low-FODMAP diet might help to improve short-term digestive symptoms in adults with irritable bowel syndrome,[120][114][121][20] but its long-term follow-up can have negative effects because it causes a detrimental impact on the gut microbiota and metabolome.

Musculotropics, such as mebeverine, act directly at the smooth muscle of the gastrointestinal tract, relieving spasm without affecting normal gut motility.

[141] Proton-pump inhibitors (PPIs) used to suppress stomach acid production may cause small intestinal bacterial overgrowth (SIBO) leading to IBS symptoms.

[148] Preliminary research shows that psychotherapeutic interventions are correlated with reductions in both autonomic nervous system dysregulation and gastrointestinal symptoms.

[111] An earlier meta-analysis suggested the results of peppermint oil were tentative as the number of people studied was small and blinding of those receiving treatment was unclear.

[109] Safety during pregnancy has not been established, however, and caution is required not to chew or break the enteric coating; otherwise, gastroesophageal reflux may occur as a result of lower esophageal sphincter relaxation.

[162] A comprehensive meta-analysis using twelve random trials resulted that the use of peppermint oil is an effective therapy for adults with irritable bowel syndrome.

GI propulsion, secretion, and inflammation in the gut are all modulated by the ECS (Endocannabinoid system), providing a rationale for cannabinoids as treatment candidates for IBS.

The bar graph at right shows the percentage of the population reporting symptoms of IBS in studies from various geographic regions (see table below for references).

[174] However, women in East Asian countries are not more likely than men to have irritable bowel syndrome, and there are conflicting reports about the female predominance of the disease in Africa and other parts of Asia.

[1] Studies of females with IBS show symptom severity often fluctuates with the menstrual cycle, suggesting hormonal differences may play a role.