Kaolin powder and other hydrated aluminum silicate clays, often combined with pectin (a complex carbohydrate), are the most widely used adsorbent powders (e.g., Kaopectate). Kaolin is a naturally occurring hydrated aluminum silicate that is prepared for medicinal use as a very finely divided powder. The rationale behind its use in acute nonspecific diarrhea stems from its ability to adsorb some of the bacterial toxins that often cause the condition. It is almost harmless and is effective in many cases of diarrhea if taken in large enough doses (2-10 g initially, followed by the same amount after every bowel movement). The adsorbents are generally safe, but they may interfere with the absorption of some drugs from the GI tract.
Bismuth subsalicylate (Pepto-Bismol) also binds intestinal toxins and may coat irritated mucosal surfaces. This compound is a salicylate and may therefore produce signs of salicylate toxicity (e.g., ringing of the ears) if taken chronically, especially with aspirin. Bismuth is radiopaque and may interfere with radiological examinations. Its use may cause temporary gray-black discoloration of the stool and brown pigmentation of the tongue. High dose Pepto-Bismol (8 tablets/day) has been efficacious in some patients with diarrhea secondary to collagenous or lymphocytic colitis.
Hydrophilic substances such as calcium polycarbophil (FiberCon, Equalactin), methylcellulose (Citrucel), and various psyllium seed derivatives (Metamucil) are natural or synthetic fiber supplements that bind water and bile salts and may be useful in controlling diarrhea associated with the passing of excessively watery stools.
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