Constipation is defined as the difficulty of passing stools, incomplete passage, or infrequent passage of hard stools. It can be further defined as having less than three stools per week for women and five for men despite a high residual diet, or a period greater than 3 d without a bowel movement. It can be caused by gastrointestinal disorders, metabolic and endocrine disorders, pregnancy, neurogenic and psychogenic problems, or it could be drug induced.
Laxative Mechanisms of Action
Laxatives promote bowel evacuation by decreasing water and electrolyte absorption, increasing intraluminal osmolarity, or increasing hydrostatic pressure in the gut. Chronic use of laxatives, particularly stimulants, may lead to laxative dependency. Laxative dependency, in turn, may result in fluid and electrolyte imbalances, steatorrhea, osteomalacia, and vitamin and mineral deficiencies. Known as laxative abuse syndrome (LAS), it is difficult to diagnose. Laxative abuse syndrome is often seen in women with anorexia nervosa, depression, and personality disorders and also in elderly patients with quasimedical concerns about their bowel movements. Table: Properties of Laxatives outlines important properties of six types of laxatives.
Table: Properties of Laxatives
|Laxatives||Onset of Action (h)||Site of Action|
|Bulk-forming||12-24 (up to 72)||Small and large intestine|
|Stool softeners/surfactants||24-72||Small and large intestine|
|Saline cathartics||0.5-3||Small and large intestine|
|Evacuant||0.25-0.5||Local irritation, hyperosmotic action|
Bulk-forming agents are used to promote regularity and are equally indicated for both constipation and diarrhea. The mechanism of action (MOA) is to provide fiber that is not digested or absorbed. This adds bulk to the stool and retains some water in the lumen of the gastrointestinal tract.
Side effects can include fluid and electrolyte imbalance. Specific drugs and usual dosages are as follows:
1. Methylcellulose (Citrucel ®): 4-6 g/d
2. Polycarbophil (FiberCon ®, Mitrolan ®): 4-6 g/d
3. Psyllium (Fiberall ®, Metamucil ®, Konsyl ®, etc.): Dose varies with product
These agents provide detergent activity and facilitate admixture of fat and water to soften stool. They also will retain water in the lumen of the gastrointestinal tract. They do not add volume to the stool, but they do prevent hardening of the stool and may prevent pain on defecation. They may be used postoperatively to decrease discomfort caused by defecation and for patients with heart disease to prevent Valsalva’s maneuver efforts upon defecation, which can produce cardiac arrhythmias. Commonly used stool softeners and surfactants include docusate sodium (Colace ®, Doxinate ®), 50-360 mg/d; docusate calcium (Surfak ®), 50-360 mg/d; and docusate potassium (Dialose ®, Diocto-K ®, Kasof ®, etc.), 100-300 m/d.
These agents attract and retain water in intestinal lumen, increasing intraluminal pressure and cholecystokinin release. These drugs contain an anion or cation that is poorly absorbed and remains in the lumen of the gastrointestinal tract. In an effort to maintain equal osmotic pressure on both sides of the cell membranes of the gastrointestinal tract, water will be secreted and not resorbed within the lumen. Agents and their dosages include magnesium citrate (Citrate of Magnesia ®, Citroma ®), 4 oz to 1 full bottle 120-300 mL; magnesium hydroxide (Phillips’™ Milk of Magnesia), 5-15 mL or 650 mg to 1.3 g tablets up to 4 times/d as needed; magnesium sulfate (Epsom salts ®), 10 to 15 g in a glass of water; and sodium phosphate (Fleet ®), 20-30 mL as a single dose.
These agents act to ease passage of stool by decreasing water absorption and lubricating the intestine. One agent is mineral oil (Kondremul ®). Dosage for adults and children >12 years of age is 15 to 45 mL once daily or divided dose. For children 6 to <12 years of age, dosage is 5 to 15 mL once daily or divided dose.
Note: All use of mineral oil, especially chronically, poses a significant nutritional problem, since mineral oil reduces absorption of the lipid-soluble vitamins (e.g., vitamins A, D, E, and K). Use in elderly patients, particularly those who exhibit high risk for aspiration, is not appropriate. Orally administered mineral oil can produce lipid pneumonia in these patients, a fatal complication. Prolonged, frequent, or excessive use may result in dependence or decrease absorption of fat-soluble vitamins.
These agents directly act on intestinal mucosa, stimulate myenteric plexus, and alter water and electrolyte secretion. Specific agents and usual dosages are bisacodyl (Dulcolax ®), 5 to 15 mg (usually 10 mg) as a single dose daily; senna, dose varies with formulation; and casanthranol (Dialose Plus ®, Peri-colace ®), dose varies with formulation.
Local irritation and hyperosmotic action are produced by these agents. Examples and usual dosage include two types of agents. The first type is glycerin, adults and children <12 years of age, one suppository high in the rectum and retained 15 to 30 minutes; it need not melt to produce laxative action. A second type is lactulose (Cephulac ®, Chronulac ®), adults and children <12 years of age, 15 to 30 mL (10 to 20 g) daily, increased to 60 mL/d if necessary.
A third agent is the sugar alcohol, sorbitol 70%, 30-50 g/d. See bowel-cleansing (also called bowel preparation) agents for a discussion of GoLytely ® (polyethylene glycol-electrolyte solution). Combination products include docusate and casanthranol (Peri-Colace ®), one or two at bedtime with a full glass of water.
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