22.8 : 消化管運動に影響を及ぼす薬剤：下痢止め薬としてのオピオイド

Diarrhea is characterized by increased gut motility and secretion of water and electrolytes, leading to frequent loose stools.

It may be caused by viral or bacterial infections, food poisoning, anxiety, or as a side effect of radiation or drug therapy.

Opioids, such as loperamide, diphenoxylate, and difenoxin, are widely utilized to manage diarrhea. They primarily bind to μ - or δ-opioid receptors, reducing intestinal motility and enhancing fluid absorption.

Loperamide, an over-the-counter synthetic opioid, shows poor CNS penetration. It acts rapidly after oral intake by increasing anal sphincter tone and delaying fecal transit, relieving diarrhea.

However, overdosing can cause constipation, CNS depression, and paralytic ileus.

Another opioid, diphenoxylate, once ingested, undergoes de-esterification to form difenoxin, an active metabolite.

Their excessive intake can result in severe colonic inflammation, known as toxic megacolon.