TeacherOfMedicine: Antimotility drugs

As a symptomatic treatment for diarrhoea, usually in the context of irritable bowel syndrome or viral gastroenteritis.

Loperamide is an opioid that is pharmacologically similar to pethidine.
However, unlike pethidine, is does not penetrate the central nervous system (CNS), so has no analgesic effects.
It is an agonist of the opioid µ-receptors in the gastrointestinal tract.
This increases non-propulsive contractions of the gut smooth muscle but reduces propulsive (peristaltic) contractions.
As a result, transit of bowel contents is slowed and anal sphincter tone is increased.
Slower gut transit also allows more time for water absorption, which (in the context of watery diarrhoea) has a desirable effect in hardening the stool.
Other opioids (e.g. codeine phosphate) have similar effects but, unless analgesia is also required, there is little reason to prefer them over loperamide.

In itself, loperamide is a safe drug with few adverse effects.
These are mostly gastrointestinal effects predictable from its mechanism of action (e.g. constipation, abdominal cramping and flatulence).
Indirectly, adverse effects may arise from the inappropriate inhibition of peristalsis.
Where CNS-penetrating opioids are used (e.g. codeine phosphate), there is a risk of opioid toxicity and dependence (see Opioids, weak).

Loperamide should be avoided in acute ulcerative colitis where inhibition of peristalsis may increase the risk of megacolon and perforation.
For the same reason, it should be avoided where there is a possibility of Clostridium difficile colitis, including in patients who develop diarrhoea in association with broad-spectrum antibiotic use.
It should not be used in acute bloody diarrhoea (dysentery) because this may signify bacterial infection. Particularly worrying in this context is Escherichia coli, as certain strains of this can cause a serious condition called haemolytic–uraemic syndrome (HUS). Use of antimotility drugs appears to increase the risk of HUS.

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