Laxatives and Antidiarrhoeal Agents
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Direct answer
Laxatives and antidiarrhoeal agents either speed up bowel transit or slow it, and they are classified by mechanism as osmotic, stimulant, bulk-forming or antimotility drugs. Loperamide is the classic antimotility agent, an opioid receptor agonist that slows peristalsis without significant central effects at usual doses. Oral rehydration solution is the most important antidiarrhoeal intervention, and the WHO low-osmolarity formula contains 75 mmol/L of sodium.
What you must remember
- Osmotic laxatives: lactulose, polyethylene glycol and magnesium salts draw water into the colon, and magnesium salts are unsafe in renal failure.
- Stimulant laxatives: bisacodyl and senna act on enteric nerves and cause cramps, and chronic use can lead to electrolyte loss and dependence.
- Bulk-forming laxatives: ispaghula husk and methylcellulose need adequate fluid, and they are unsuitable in intestinal obstruction because of impaction risk.
- Loperamide: a mu-opioid agonist acting on the myenteric plexus, which slows peristalsis and increases sphincter tone, and it is avoided in bloody diarrhoea.
- Racecadotril: an enkephalinase inhibitor that reduces intestinal secretion without slowing transit, and it is commonly prescribed in India for acute watery diarrhoea.
- Diphenoxylate with atropine: an opioid with few central effects at usual doses, combined with a subtherapeutic dose of atropine to discourage overdose and misuse.
- WHO low-osmolarity ORS: sodium 75 mmol/L, glucose 75 mmol/L, potassium 20 mmol/L and total osmolarity about 245 mOsm/L.
- Zinc: given with ORS for 14 days in children with diarrhoea, as part of the national programme.
A young woman with watery stools and mild dehydration
A 30-year-old woman has passed six watery stools a day for two days, with no fever and no blood. Her skin turgor is reduced and she is passing urine less often.
- Assess the degree of dehydration: Reduced skin turgor and urine output decide whether oral rehydration solution is enough or intravenous fluid is needed.
- Check for blood and fever: Bloody stools or fever suggest invasive infection, and antimotility drugs are withheld until that is excluded.
- Start oral rehydration: Low-osmolarity ORS is given in small, frequent sips, because the sodium and glucose co-transport makes it absorb even during diarrhoea.
- Add an antisecretory or antimotility drug: Racecadotril or loperamide is reasonable here, since the diarrhoea is non-bloody and afebrile.
- Reserve antibiotics: Antibiotics are kept for cholera, dysentery, Clostridioides difficile infection or a proven bacterial cause, not for routine watery diarrhoea.
- Review the cause if symptoms persist: A diarrhoea lasting beyond two weeks needs stool culture, coeliac serology and a search for malabsorption.
Where the examiner tests the wrong drug
Most errors come from giving a drug for a symptom without checking the cause.
- Trap: giving loperamide for bloody diarrhoea with fever Correct: antimotility drugs can trap invasive organisms and precipitate toxic megacolon, so they are avoided in dysentery and C. difficile colitis.
- Trap: choosing a magnesium-based laxative in renal failure Correct: magnesium accumulates and causes hypermagnesaemia, so polyethylene glycol is the safer osmotic option.
- Trap: treating ORS as simple sugar water Correct: the sodium and glucose ratio in the WHO formula is what drives absorption, so the standard formula is used.
Frequently asked questions
What is the composition of WHO low-osmolarity oral rehydration solution?
WHO low-osmolarity oral rehydration solution contains 75 mmol/L of sodium, 75 mmol/L of glucose, 20 mmol/L of potassium, 65 mmol/L of chloride and 10 mmol/L of citrate. Its total osmolarity is about 245 mOsm/L, and it is the standard formula for most cases of acute watery diarrhoea.
Why is loperamide avoided in bloody diarrhoea?
Loperamide slows intestinal transit, so it can trap invasive bacteria and toxins in the colon and worsen the illness. It is avoided in dysentery, suspected Clostridioides difficile colitis and toxic megacolon, where slowing motility is dangerous.
Which osmotic laxative is safest in chronic kidney disease?
Polyethylene glycol is generally the safer osmotic laxative in chronic kidney disease, because magnesium salts accumulate and cause hypermagnesaemia when renal function is poor. Lactulose is also used, but its safety depends on fluid balance and the underlying condition.
How does racecadotril differ from loperamide?
Racecadotril is an enkephalinase inhibitor that reduces intestinal secretion without slowing transit, whereas loperamide is an opioid agonist that slows peristalsis. Racecadotril is commonly prescribed in India for acute watery diarrhoea, and it does not trap organisms in the way an antimotility drug can.
Why is zinc given with oral rehydration in childhood diarrhoea?
Zinc supplementation shortens the duration and severity of childhood diarrhoea, and it is given for 14 days alongside ORS. It forms part of the national diarrhoea control approach in India, which relies on ORS and zinc together.