Pharmacology of Laxatives and Antidiarrheal

Laxatives

• Laxatives are used to accelerate the movement of food through the GI tract, treating constipation
• Classification of laxatives is based on their mechanism of action
• Risks of laxatives include:
  • Loss of pharmacologic effect of poorly absorbed, delayed-acting, and extended-release oral preparations
  • Electrolyte imbalances with chronic use
  • Dependency risk for the user

Types of Laxatives

• Purgatives: stronger than laxatives, used for full cleaning of the GI tract, e.g., in radiological and endoscopic procedures
• Irritants and Stimulants:
  • Senna: a widely used stimulant laxative, causes evacuation of the bowels within 8-10 hours
  • Bisacodyl: a potent stimulant of the colon, available as suppositories and enteric-coated tablets
  • Castor oil: broken down in the small intestine to ricinoleic acid, irritating the stomach and increasing peristalsis
• Saline and Osmotic Laxatives:
  • Saline cathartics, e.g., magnesium citrate and magnesium hydroxide, hold water in the intestine by osmosis
  • Electrolyte solutions containing polyethylene glycol (PEG) used for colonic lavage and as an oral laxative
  • Lactulose: a semisynthetic disaccharide sugar, acting as an osmotic laxative, also used for hepatic encephalopathy treatment
• Stool Softeners:
  • Emollient laxatives or surfactants, e.g., docusate sodium and docusate calcium, produce softer feces and ease passage
• Lubricant Laxatives:
  • Mineral oil and glycerin suppositories facilitate the passage of hard stools
• Chloride Channel Activators:
  • Lubiprostone: increases fluid secretion in the intestinal lumen, easing stool passage with minimal electrolyte imbalance and no tolerance or dependency issues

Antidiarrheals

• Major Factors in Diarrhea:
  • Increased motility of the GI tract
  • Decreased absorption of fluid
• Antidiarrheal Drugs:
  • Antimotility Agents:
    • Diphenoxylate and Loperamide: opioid-like actions on the gut, inhibit acetylcholine release and decrease peristalsis
  • Adsorbents:
    • Aluminum hydroxide and methylcellulose: adsorb intestinal toxins or microorganisms and/or coat or protect the intestinal mucosa
  • Agents that Modify Fluid and Electrolyte Transport:
    • Bismuth subsalicylate: decreases fluid secretion in the bowel, used for traveler's diarrhea

This quiz covers the mechanism of action of laxatives, their effects on the GI tract, and their interactions with other medications. It also explores the role of antidiarrheal drugs.