Bulk-forming laxatives are indigestible, hydrophilic colloids that absorb water, forming a bulky, emollient gel that distends the colon and promotes peristalsis. Common preparations include natural plant products (psyllium, methylcellulose) and synthetic fibers (polycarbophil). Bacterial digestion of plant fibers within the colon may lead to increased bloating and flatus.
These agents soften stool material, permitting water and lipids to penetrate. They may be administered orally or rectally. Common agents include docusate (oral or enema) and glycerin suppository. In hospitalized patients, docusate is commonly prescribed to prevent constipation and minimize straining. Mineral oil is a clear, viscous oil that lubricates fecal material, retarding water absorption from the stool. It is used to prevent and treat fecal impaction in young children and debilitated adults. It is not palatable but may be mixed with juices. Aspiration can result in a severe lipid pneumonitis. Long-term use can impair absorption of fat-soluble vitamins (A, D, E, K).
The colon can neither concentrate nor dilute fecal fluid: fecal water is isotonic throughout the colon. Osmotic laxatives are soluble but nonabsorbable compounds that result in increased stool liquidity due to an obligate increase in fecal fluid.
These agents may be used for the treatment of acute constipation or the prevention of chronic constipation. Magnesium hydroxide (milk of magnesia) is a commonly used osmotic laxative. It should not be used for prolonged periods in patients with renal insufficiency due to the risk of hypermagnesemia. Sorbitol and lactulose are nonabsorbable sugars that can be used to prevent or treat chronic constipation. These sugars are metabolized by colonic bacteria, producing severe flatus and cramps.
High doses of osmotically active agents produce prompt bowel evacuation (purgation) within 1–3 hours. The rapid movement of water into the distal small bowel and colon leads to a high volume of liquid stool followed by bowel evacuation. Several purgatives are available, which may be used for the treatment of acute constipation or to cleanse the bowel prior to medical procedures (eg, colonoscopy). These include magnesium citrate, sulfate solution, and a proprietary combination of magnesium oxide, sodium picosulfate, and citrate (Prepopik). When taking these purgatives, it is very important that patients maintain adequate hydration by taking increased oral liquids to compensate for fecal fluid loss. Sodium phosphate also is available—by prescription—as a tablet formulation but is infrequently used due to the risk of hyperphosphatemia, hypocalcemia, hypernatremia, and hypokalemia. Although these electrolyte abnormalities are clinically insignificant in most patients, they may lead to cardiac arrhythmias or acute renal failure due to tubular deposition of calcium phosphate (nephrocalcinosis). Sodium phosphate preparations should not be used in patients who are frail or elderly, have renal insufficiency, have significant cardiac disease, or are unable to maintain adequate hydration during bowel preparation.