Chapter 58: Drugs Affecting Gastrointestinal Motility

Drugs Affecting Gastrointestinal Motility outlines the pharmacology of drugs designed to modulate gastrointestinal (GI) motility, addressing the dysfunctions of constipation and diarrhea. Treatment agents are categorized into laxatives, GI stimulants, and antidiarrheals. Laxatives are used for temporary relief of constipation and are classified based on their mechanism of action. Chemical stimulants, such as senna and castor oil, increase activity by chemically irritating the GI wall and stimulating local reflexes; due to its irritant effects, castor oil is contraindicated during pregnancy as it may precipitate labor. Bulk stimulants, exemplified by psyllium (often preferred for older adults) and methylcellulose, expand the fecal matter, drawing in fluid, which stretches the GI lining and promotes movement; patients must ingest sufficient water with these agents to prevent esophageal obstruction. Osmotic laxatives, including magnesium citrate and lactulose, function by increasing the osmotic concentration within the GI tract, pulling water from the systemic circulation into the lumen, thereby increasing bulk and stimulating motility. Caution is advised when administering magnesium-based osmotic agents to patients with renal insufficiency due to systemic absorption and clearance. Lubricants, such as docusate and mineral oil, facilitate defecation by softening the stool and easing its passage, which is beneficial for patients needing to avoid straining, like those with hemorrhoids or post-surgery. Regular use of mineral oil, however, is known to interfere with the absorption of fat-soluble vitamins (A, D, E, K). Specialized laxatives like methylnaltrexone, naloxegol, and naldemedine are opioid mu-receptor antagonists that treat opioid-induced constipation by acting peripherally in the GI tract, specifically sparing the opioid’s analgesic effects in the central nervous system. Crucially, the chronic overuse of any laxative can result in cathartic dependence, requiring progressively stronger stimuli for normal bowel function. In contrast, GI stimulants like metoclopramide increase overall GI tone and movement by stimulating parasympathetic activity or increasing sensitivity to acetylcholine. Antidiarrheal agents work to slow excessive GI motility, allowing greater time for water and nutrient absorption. These drugs may slow local reflexes (bismuth subsalicylate), act directly on GI muscles (loperamide), or affect CNS centers to cause spasm (opium derivatives). Bismuth subsalicylate is effective for treating traveler’s diarrhea, although the antibiotic rifaximin is specifically approved for this condition caused by noninvasive E. coli. Lastly, treatments for Irritable Bowel Syndrome (IBS) are symptom-specific: eluxadoline and alosetron target diarrhea, while lubiprostone, a chloride channel activator, treats IBS associated with constipation. Nursing care emphasizes administering laxatives only as a temporary measure, promoting diet, fluid intake, and exercise, and monitoring patients carefully for fluid and electrolyte imbalances.