Drugs Acting on the Gastrointestinal Tract
David H. Shaw, in Pharmacology and Therapeutics for Dentistry (Seventh Edition), 2017
Bulk-Forming Agents
Bulk-forming agents include synthetic fibers (polycarbophil) and natural plant products (psyllium and methylcellulose). They possess the property of absorbing water and expanding, increasing the bulk of the intestinal contents. The elevated luminal pressure stimulates reflex peristalsis, and the increased water content softens the stool. These agents are not absorbed and do not interfere with the absorption of nutrients from the gastrointestinal tract. Several days of medication may be required to achieve the full therapeutic benefit, although the usual onset of action is 12 to 24 hours. Some patients prefer to add foods such as bran or dried fruit (e.g., prunes and figs) to their diet that exert the same effect rather than use a bulk-forming laxative. These laxatives have the advantage of having few systemic effects and are unlikely to produce laxative abuse. Cellulose agents may physically bind with other drugs if administered concurrently (e.g., salicylates, warfarin, digitalis glycosides) and hinder their absorption. Patients should not take a calcium polycarbophil laxative within 2 hours of taking tetracycline for the same reason.
Laxatives with psyllium come in a powdered mixture containing approximately 50% powdered psyllium seeds and 50% dextrose or sucrose. Sugar-free products are also available. Psyllium seeds are rich in a hemicellulose that forms a gelatinous mass with water. The refined hydrophilic colloid from the seeds is the most widely used form of this agent. Methylcellulose is indigestible and not absorbed systemically. Bloating and flatus have been reported after the use of psyllium products because of bacterial digestion of the plant fibers within the colon.