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Loperamide is a non absorbed opiod which acts on µ- opiod receptors within the myenteric plexi of the gut nervous system. This leads to a reduction in the tone and subsequent motility of the circular and londitudinal smooth muscle of the gut, which allows more time for salt and water to be absorb as food and digestive juices move through the gut. It is generally effective for those causes of diarrhoea where reduced transit or increased secretion is an issue. These include gastroenteritis and irritable bowel syndrome. It is however not recommended for patients with active severe  colitis since it may increase the risk of toxic megacolon, a potentially life-threatening condition associated with inflammatory bowel disease.

Side effects include constipation, bloating and abdominal pain as well as drowsiness.

Other Drugs

A number of other drugs including codeine (another opiate which is absorbed to a greater degree), amitriptyline, which slows gut transit through its anti-cholinergic effects and ondansetron, which is also used for nausea may also reduce gut transit and can be used alone or in combination with other drugs to help manage a combination of nausea (for which ondansetron is licensed and loose stools, for which there is no license). Similarly amitriptyline may have positive effects on pain. A number of drugs, of a different class are used to treat bile salt malabsorption, which also commonly causes loose watery diarrhoea.

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