Antacids are classified by their cationic metal, usually a sodium, magnesium or aluminum. Recall that sodium antacids should be avoided in hypertensive patients, that magnesium antacids can cause diarrhea, and that aluminum antacids can cause constipation. Both are structurally related to 4-phenylpiperidine opioid analgesics, but have negligible opiate effects and a low abuse potential. These agents act by reducing the propulsive activity of the intestinal tract, a common side effect of the opiate analgetic agents. In fact, codeine may also be used for this purpose, but is less frequently used due to its addition liability and potential for abuse.
Surfactants act by a detergent effect which enhances the mixing of fat and water to soften stools.
Irritant laxatives act by direct contact with the gastric mucosa, and the resulting irritation causes a decrease in transit time. Bulk laxative act by increasing stool volume, and include psyllium-based laxatives such as Metamucil , methylcellulose containing laxatives and polycarbophil. Lactulose , a simple disaccharide, is metabolized by colonic bacteria to lactate and small amounts of acetic and formic acids, resulting in acidification of the colonic contents and a resultant catharsis. Antiflatulent Agents Simethecone is the most commonly used antiflatulent, and acts by dispersing excess gas in the intestine before the patient has the opportunity to do so. It is a simple, methylated siloxane.