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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Antidiarrhoeals

Medicines that slow the gut or reduce intestinal secretion to control diarrhoea. They never replace fluids and salts, which come first.

Updated 2026-11-01

## What they are

Symptomatic treatments for diarrhoea: opioid motility inhibitors that act on μ receptors in the gut wall (loperamide, diphenoxylate) and the enkephalinase inhibitor racecadotril, which reduces secretion. Rehydration remains the core of treatment.

## Mechanism of action

Gut μ-opioid receptor agonists reduce acetylcholine release from the myenteric plexus, slow propulsive peristalsis and give more time for water absorption. Racecadotril is a prodrug of thiorphan, which blocks enkephalinase (neprilysin) so the gut's own enkephalins reduce the secretion of water and electrolytes.

Target: Gut μ-opioid receptors (loperamide, diphenoxylate) and enkephalinase/neprilysin (racecadotril)

## Key points

- Fluids and electrolytes come first; antidiarrhoeals treat symptoms only.
- Avoid motility inhibitors in bloody diarrhoea with fever, invasive bacterial colitis, C. difficile colitis and acute ulcerative colitis: risk of toxic megacolon.
- Loperamide above the recommended dose can cause QT prolongation, torsades de pointes and cardiac arrest.
- Diphenoxylate carries a small amount of atropine to discourage misuse.
- Racecadotril reduces secretion without slowing transit much.

## Drugs in this class

- [**Loperamide** An opioid that acts on the gut wall and barely reaches the brain. It slows the bowel to control diarrhoea but does not… μ-opioid receptor of the myenteric plexus](https://durgical.com/en/pharmacology/loperamide/)
- [**Diphenoxylate and atropine** An opioid antidiarrhoeal related to pethidine, with a little atropine added to discourage misuse. Dangerous in young… μ-opioid receptors of the gut wall](https://durgical.com/en/pharmacology/diphenoxylate-and-atropine/)

## Others in the class

- Racecadotril
- Bismuth subsalicylate
- Crofelemer
- Eluxadoline
- Octreotide (secretory diarrhoea of tumours)

## Test yourself

Think of the answer, then tap the question to see it.

## Why are motility inhibitors avoided in dysentery (bloody stools and fever)?

Slowing the bowel keeps invasive bacteria and their toxins in contact with the mucosa and can lead to toxic megacolon or sepsis.

## Why is atropine added to diphenoxylate?

In a subtherapeutic amount, to cause unpleasant anticholinergic effects if large doses are taken and so discourage misuse.

## How does racecadotril reduce diarrhoea?

Its active metabolite thiorphan inhibits enkephalinase, so endogenous enkephalins last longer and reduce intestinal secretion of water and electrolytes.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A07D) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [DailyMed - LOPERAMIDE HYDROCHLORIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=5daab489-eb3f-eca1-e063-6394a90a99d5) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - DIPHENOXYLATE HYDROCHLORIDE AND ATROPINE SULFATE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=3d0e2877-7a74-433f-8a31-e3e54aa9dc9f) (DailyMed (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 54 (McGraw Hill, 2023).

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