Irritable bowel syndrome is a functional disorder in which recurrent abdominal pain is linked to altered defecation, without structural or biochemical abnormalities visible on routine testing. Symptoms wax and wane for years, often begin in adolescence or early adulthood, and can dominate daily schedules while leaving life expectancy untouched.

1. Core diagnostic features
   • Abdominal pain: crampy, aching, or sharp, usually located in the lower abdomen; episodes last minutes to hours and are relieved—at least partially—by passing stool.
   • Change in stool frequency: patients report more than three loose stools daily or fewer than three stools weekly; some alternate between the two extremes within days.
   • Change in stool form: using the Bristol scale, types 6–7 (mushy or watery) during diarrhea spells and types 1–2 (hard or lumpy) during constipation spells are typical.
   • Sense of incomplete evacuation: a feeling that “something is still inside,” prompting repeated toilet visits.
   • Visible abdominal distension: waistbands tighten by evening, sometimes accompanied by measurable increase in abdominal girth.
2. Predominant subtypes
   • IBS-D: loose or watery stools ≥25 % of the time, hard stools <25 %.
   • IBS-C: hard or lumpy stools ≥25 %, loose stools <25 %.
   • IBS-M (mixed): both hard and loose stools ≥25 % each.
   • IBS-U: insufficient criteria for the above categories.
3. Associated non-intestinal symptoms
   • Upper-gut complaints: early satiety, epigastric burning, nausea overlapping with functional dyspepsia.
   • Urgency and frequency of urination, especially in women.
   • Dyspareunia or worsening pain mid-cycle, often mislabeled as “gynecologic” pain.
   • Fatigue, non-restorative sleep, headache, and low-back stiffness independent of stool pattern.
4. Trigger patterns
   • Food: fatty meals, caffeine, onions, garlic, legumes, lactose, polyol-sweetened gums, large single-day fiber load.
   • Stress: examinations, job interviews, or family conflict may precipitate flares within hours.
   • Hormonal: many women note perimenstrual exacerbation.
   • Post-infectious: roughly 10 % of acute gastroenteritis cases evolve into IBS through persistent mucosal immune activation and altered microbiota.
5. Red-flag features that exclude IBS
   Age over 50 at first onset, nocturnal pain awakening from sleep, bloody stools, unintentional weight loss, anemia, elevated inflammatory markers, or family history of colorectal cancer, celiac disease, or inflammatory bowel disease—these mandate further investigation.

Summary table