Chronic diarrhea means >3 loose or watery stools per day for more than four weeks. It is not a disease itself but a common end-point of many mechanisms. Here is a rapid overview from cause to clinic.

1. Osmotic diarrhea

Unabsorbed solutes (lactose, fructose, sorbitol, Mg-antacids) hold water in the lumen; stool output falls markedly when the patient stops eating the offending sugar or salt.

2. Secretory diarrhea

The mucosa actively pours water and electrolytes into the gut; fasting does not stop the flood. Think hyperthyroidism, adrenal insufficiency, gastrinoma, VIPoma, bile-acid malabsorption or chronic alcohol abuse.

3. Malabsorption / steatorrhoea

Insufficient pancreatic enzymes, bile acids or damaged villi (coeliac, short-bowel) leave fat and protein undigested. Stools are bulky, greasy, foul-smelling and often accompanied by weight loss.

4. Inflammatory / exudative diarrhea

Mucosal ulceration leaks protein, blood and pus. Typical offenders: Crohn’s disease, ulcerative colitis, microscopic colitis, radiation enteritis, intestinal tuberculosis, eosinophilic gastro-enteritis.

5. Dysmotility

Rapid transit gives contents too little contact time with the mucosa. Causes include hyperthyroidism, diabetic autonomic neuropathy, post-infectious IBS and pro-kinetic drugs.

6. Dysbiosis & small-intestinal bacterial overgrowth (SIBO)

Broad-spectrum antibiotics, long-term acid suppression or altered anatomy allow colonic-type bacteria to colonise the small bowel, producing gas, altering bile salts and generating watery, bloating diarrhea.

7. Food intolerance & dietary factors

Lactose, fructose, FODMAPs, caffeine, chilli or high-fat meals can trigger osmotic or secretory episodes; true food allergy may add urticaria or eosinophilic infiltrate.

8. Drugs & chronic toxins

NSAIDs, metformin, ACEI/ARB, colchicine, PPIs, Mg-containing antacids, heavy metals, organic phosphorus compounds and chronic alcohol all have “drug-induced or toxic diarrhea” on their label.

9. Systemic disease

Diabetes, hyperthyroidism, adrenal insufficiency, SLE, scleroderma and hypogammaglobulinaemia disturb electrolyte transport or neuromuscular control through endocrine, immune or neural pathways.

10. Neoplasia

Colorectal cancer, lymphoma, VIPoma, medullary thyroid cancer and villous adenoma may secrete peptides, cause partial obstruction or bleed, leading to nocturnal pain, weight loss and anaemia.

Quick look-up table

Category	Clinical tip	First-line test	Simple management
Osmotic	Stops when patient fasts	Stool osmotic gap	Remove offending sugar / Mg
Secretory	Watery, persists fasting	Serum VIP/gastrin, TSH	Treat endocrine tumour
Steatorrhoea	Greasy, floats, weight↓	Faecal fat, coeliac serology	Pancreatic enzymes, gluten-free
Inflammatory	Blood/pus, fever	Colonoscopy + biopsy, faecal calprotectin	Anti-inflammatory / immunosuppressant
Dysmotility	Urgency, no blood	TSH, HbA1c, transit study	Regulate motility, control glucose
SIBO	Bloating after meals	Breath test, culture	Antibiotic + probiotic taper
Food intolerance	Symptom food-linked	Elimination diary	Avoid trigger diet
Drug-induced	New drug→new diarrhea	Drug history, withdrawal	Substitute or stop drug
Systemic	Multi-system signs	Auto-screen, endocrine panel	Treat primary disease
Neoplastic	Nocturnal pain, anaemia	Tumour markers, imaging, scope	Surgery / chemo / targeted