2. Loperamide is an opioid drug. Like morphine, it has a high affinity for and exerts a direct action on opiate receptors in the gut wall. These drugs also increase sphincter tone, including the anal sphincter, and decrease secretory activity along the gastrointestinal tract. Decreased motility enhances fluid and electrolyte reabsorption and decreases the volume of intestinal contents.

Loperamide undergoes extensive first-pass metabolism, so very little reaches the systemic circulation and at the dosage permitted for non-prescription use it is unlikely to cause any of the side-effects associated with opiates.

In addition, loperamide does not penetrate the blood/brain barrier, so does not exert central opioid effects. Several controlled trials1 have shown it to be effective in reducing the duration of diarrhoea, although acute diarrhoea is in any case self-limiting and relatively short-lived.