Bulk-Forming Laxatives These come in natural forms (psyllium) as well as synthetic form (Metamucil, Konsyl, Citrucel) and are felt to be the safest laxatives. They have mechanisms of action and side effects similar to that of fiber (162).
Hyperosmolar Laxatives These consist of poorly absorbed substances that increase intraluminal osmolarity and water absorption. This action results in greater stool volume with decreased consistency. Examples include nonabsorbable sugars (lactulose and sorbitol), glycerin, and polyethylene glycol (GoLytely, MiraLAX). Polyethylene glycol is a common preoperative bowel preparation. Side effects are diarrhea, increased ﬂatus, and abdominal cramping (162).
Emollient Laxatives These agents are divided into two subsets: docusate salts and mineral oil. The docusate salts have hydrophilic and hydrophobic properties similar to detergents. They soften stool and decrease surface tension by increasing stool water and lipid content. Examples include docusate calcium (Surfak), docusate potassium (Dialose, Kasof), and docusate sodium (Colace, Comfolax). They also improve the absorption of other laxatives and are combined in preparations with stimulant laxatives such as Correctol, Peri-Colace, and Feen-a-Mint. The limited absorption of mineral oil allows it to penetrate and soften the stool. It can be used orally or rectally. Prolonged daily use can lead to decreased absorption of the fat-soluble vitamins A, D, E, and K. Use of mineral oil should be avoided in elderly and debilitated patients, as well as in those with esophageal motility disorders because of the potential for aspiration pneumonia. Side effects include diarrhea, anal leakage, and pruritus ani (162–164).
Saline Laxatives These usually contain magnesium cations and phosphate anions that are relatively nonabsorbable and produce an osmotic gradient with increased water absorption. They also stimulate intestinal motility by increasing cholecystokinin release. Fast-acting effects can be seen with both oral (2–6 hours) and rectal (15 minutes) preparations. Examples include magnesium citrate, magnesium hydroxide (Milk of Magnesia), magnesium sulfate, sodium phosphate, and biphosphate (Phospho-soda, Fleet enema). Although generally well tolerated, electrolyte abnormalities can occur. These side effects should be avoided in patients with renal insufficiency because of the potential for magnesium toxicity (162–164).
Stimulant Laxatives These are found in three basic types: castor oil, anthraquinones, and diphenylmethanes. A metabolite of castor oil, ricinoleic acid, increases intestinal motility and secretion. Anthraquinones (cascara sagrada, senna [Senokot], casanthranol [aloe], and danthron) are absorbed by the small intestine and stimulate motility by increasing intraluminal ﬂuid and electrolyte content. Diphenylmethanes (phenolphthaleins [Feen-a-Mint, Correctol] and bisacodyl [Dulcolax]) have a mechanism of action similar to anthraquinones. These agents are potent and are intended for short-term use in cases refractory to bulk or osmotic laxatives. It has been a long-standing belief that prolonged use can lead to a dilated atonic colon known as cathartic colon syndrome, melanosis coli, or neuronal degeneration. A recent article refutes the theory that stimulant laxatives damage the autonomic nervous system when used at recommended doses (151). Other side effects include cramping, nausea, and abdominal pain (162–164).