KEY:T, increased; decreased; ?, no data available; —, no effect on this parameter. Modified from Kreek, 1994, with permission.

KEY:T, increased; decreased; ?, no data available; —, no effect on this parameter. Modified from Kreek, 1994, with permission.

bacterial mass. The usual dose is 2.5-4 g (1-3 teaspoonfuls in 250 mL of fruit juice), titrated upwards until the desired goal is reached. A variety of semisynthetic celluloses—e.g., methylcellu-lose (citrucel, others) and the hydrophilic resin calcium polycarbophil (fibercon, fiberall, others), a polymer of acrylic acid resin—also are available. These poorly fermentable compounds absorb water and increase fecal bulk.

Osmotically Active Agents

SALINE LAXATIVES Laxatives containing magnesium cations or phosphate anions commonly are called saline laxatives: magnesium sulfate, magnesium hydroxide, magnesium citrate, sodium phosphate. Their cathartic action is believed to result from osmotically mediated water retention, which then stimulates peristalsis. Magnesium-containing laxatives may stimulate the release of cholecystokinin, which leads to intraluminal fluid and electrolyte accumulation and to increased intestinal motility. For every additional mEq of Mg2+ in the intestinal lumen, fecal weight increases by ~7 g. The usual dose of magnesium salts contains 40-120 mEq of Mg2+ and produces 300-600 mL of stool within 6 hours.

Table 37-3

Classification and Comparison of Representative Laxatives

Laxative Effect and Latency in Usual Clinical Dosage

Softening of Feces, 1-3 Days

Soft or Semifluid Stool, 6-8 Hours

Watery Evacuation, 1-3 Hours

Bulk-forming laxatives Bran

Psyllium preparations Methylcellulose Calcium polycarbophil Surfactant laxatives Docusates Poloxamers Lactulose

Stimulant laxatives

Diphenylmethane derivatives Bisacodyl

Anthraquinone derivatives Senna

Cascara sagrada

Osmotic laxatives* Sodium phosphates Magnesium sulfate Milk of magnesia Magnesium citrate Castor oil

*Employed in high dosage for rapid cathartic effect and in lower dosage for laxative effect.

640 SECTION VI Drugs Affecting Gastrointestinal Function Table 37-4

Properties of Different Dietary Fibers

Type of Fiber

Water Solubility % Fermented



Poor Poor

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