Below is a concise “field manual†you can keep handy. It explains (1) why diarrhea happens and (2) what to do when it’s severe, step by step.
(Medical information evolves; the citations show current consensus and the most recent large reviews. Always involve a clinician if any red-flag sign appears.)
1 Why we get diarrhea – the four main mechanisms
| Mechanism | What’s going on | Typical examples |
| Osmotic | Un-absorbed molecules pull water into the gut | Lactose/sorbitol intolerance, some laxatives |
| Secretory | Gut cells actively pump sodium & chloride → water follows | Cholera, ETEC “travellers’ diarrheaâ€, certain hormones/drugs |
| Inflammatory / Exudative | Mucosal injury leaks protein, blood & fluid | Shigella, Salmonella, ulcerative colitis |
| Altered motility | Food races through before water can be re-absorbed | Irritable-bowel syndrome, hyperthyroidism |
Most acute, watery episodes are infectious; chronic (>4 weeks) demands a work-up for malabsorption, endocrine disease, medications, etc.
2 When is it “severe�
- ≥ 6–10 watery stools/day or obvious dehydration
- Blood / black stool, fever > 38.5 °C, severe cramps
- Unable to keep fluids down, dizziness on standing, little or no urine
- Infants, adults > 65 y, pregnancy, immunosuppression
Any of these ⇒ go straight to a clinician or emergency department.
3 Treatment playbook
| Priority | What to do | Practical details & doses | Evidence / notes |
| Re-hydrate first, fast | WHO low-osmolar ORS (1 L water + 6 tsp sugar + ½ tsp salt) or commercial sachet | Adults ≈ 200–400 mL after each loose stool; kids 10 mL/kg | ORS alone prevents ≈ 90 % of cholera deaths |
| IV Ringer’s lactate / normal saline | In hospital for severe dehydration, shock | ||
| Replace electrolytes & micronutrients | Zinc 20 mg daily for 10–14 d (children ≥ 6 mo); 10 mg if <6 mo | Shortens episode by ~½ day and lowers 2-month relapse risk | |
| Keep eating | Resume normal diet early; use easily digested starches (rice, potatoes, bananas, toast, oatmeal) + lean protein | Avoid high-fat, very sugary drinks, alcohol, caffeine; consider temporary lactose restriction | |
| Symptom relief (adults only, afebrile, no blood) | Loperamide 4 mg, then 2 mg after each loose stool (max 8 mg OTC / 16 mg Rx per day) | Never use if bloody stool, high fever, C. difficile risk, or in children | |
| Bismuth subsalicylate 524 mg every 30–60 min (≤ 8 doses/24 h) | Adds mild antimicrobial & anti-secretory effect; avoid in aspirin allergy, kids with viral illness | ||
| Racecadotril 100 mg TID (where available) | Decreases stool volume; safe in children & adults | ||
| Antibiotics – only for specific scenarios | – Traveller’s diarrhea with moderate/severe symptoms or blood/fever – Culture-proven bacterial dysentery – Cholera – Immunocompromised host | Azithromycin 1 g single dose or 500 mg daily × 3 d (first-line worldwide)Alternatives: Rifaximin 200 mg TID × 3 d (non-invasive), Ciprofloxacin 500 mg BID × 3 d where resistance low | Empiric azithromycin remains first-line because of rising fluoroquinolone resistance |
| Probiotics (optional) | Lactobacillus rhamnosus GG or Saccharomyces boulardii | May shorten illness by ~1 day; avoid if severely immunocompromised |
4 When to seek urgent help
| Symptom | Why it matters |
| No urine for ≥8 h, sunken eyes, listlessness | Severe dehydration – IV fluids needed |
| Blood, mucus, or tarry black stool | Possible dysentery, IBD flare, or upper-GI bleed |
| High fever, shaking chills | Invasive bacterial infection or sepsis risk |
| Severe abdominal pain or distension | Toxic megacolon, perforation |
| Recent antibiotic course | Danger of C. difficile colitis |
| Diarrhea >48 h in infants / >72 h in adults despite ORS | Needs evaluation & possible labs |
5 Staying out of trouble next time
- Hand-wash (soap & water 20 s) before eating, after toilet.
- Drink water that’s boiled, filtered, or sealed.
- “Boil it, cook it, peel it, or forget it†when travelling.
- Keep vaccines up to date (rotavirus for infants; cholera for high-risk travellers).
- Store leftovers ≤4 °C and re-heat thoroughly (≥75 °C).
- For chronic or recurrent episodes, ask about celiac serology, thyroid tests, inflammatory markers, and medications.
Bottom line
Diarrhea kills mainly through dehydration, so every treatment plan starts with prompt, adequate oral (or IV) re-hydration, then targets the cause or the worst symptoms. Most episodes settle in ≤ 3 days with ORS, zinc (for kids), and careful diet; medicines have supporting roles and antibiotics are selective tools – not defaults.
This guidance is for education, not a substitute for individualized medical care. If any red-flag feature emerges, contact a healthcare professional immediately.