Cathartics and laxatives increase the motility of the intestine or increase the bulk of feces. The dosages for all of these drugs are highly empirical and usually extracted from human dosages (see Systemic Pharmacotherapeutics of the Digestive System: Cathartic and Laxative Drugs). Clinically, these drugs are administered to increase passage of gut contents associated with intestinal impaction, to cleanse the bowel before radiography or endoscopy, to eliminate toxins from the GI tract, and to soften feces after intestinal or anal surgery.
Stimulant (irritant) cathartics appear to stimulate intestinal motility via an irritant effect on the mucosa or stimulation of intramural nerve plexi. They also activate secretory mechanisms, provoking fluid accumulation in the GI lumen. These drugs can have potent effects, and excessive fluid and electrolyte loss can result. They act directly or indirectly (if a metabolic conversion is necessary before the compound is active).
Emodin is an irritant glycoside that is an active ingredient in several products. Its action is limited to the large intestine, and it may take 4–6 hr for an effect to be seen. Repeat doses should be avoided in horses because of the long latent period and risk of severe superpurgation. The naturally occurring emodins (eg, senna) are found in human formulations.
Vegetable oils are indirect-acting cathartics. They are hydrolyzed by pancreatic lipase in the small intestine to irritating fatty acids. Castor oil is a potent cathartic. It is hydrolyzed to release ricinoleic acid, which causes increased water secretion in the small intestine. It is used mainly in nonruminants and preruminant calves. Raw linseed oil (cooked linseed oil is toxic) is hydrolyzed to release linoleates, which are less irritating than ricinoleic acid. In smaller daily doses, linseed oil is a mild lubricant laxative and a source of fatty acids for horses.
Phenolphthalein and bisacodyl are diphenylmethane compounds that affect the large intestine and are found in many over-the-counter human laxative formulations. Phenolphthalein is effective only in primates and pigs. Bisacodyl appears to inhibit glucose absorption and Na+/K+-ATPase activity and to alter the motor activity of visceral smooth muscle. It is administered by mouth or by enema, and only 5% of any dose is absorbed.
These drugs are poorly absorbed from the GI tract and draw fluid into the intestine by osmosis. The fluid content of the feces increases, which causes intestinal distention and promotes peristalsis. Although hyperosmotic cathartics are relatively safe, overdoses can cause excessive fluid loss and dehydration, so adequate water intake must be assured. Examples of hyperosmotic cathartics include magnesium salts, sodium salts, and sugar alcohols.
Magnesium salts are frequently used PO as saline purgatives. Normally, only 20% of the magnesium is systemically absorbed and eliminated by the kidneys. If absorption is excessive or renal elimination is impaired, then severe hypermagnesemia and metabolic alkalosis may develop.
Sodium salts can be given PO as saline cathartics but are more commonly administered as sodium biphosphate or sodium phosphate enemas. These should not be used in cats because fatal hyperphosphatemia, hypocalcemia, and hypernatremia may result.
Sugar alcohols, such as mannitol and sorbitol, are poorly absorbed and fermented in the terminal ileum and large intestine. Lactulose is a synthetic disaccharide that is fermented in the large intestine to produce acetic, lactic, and other organic acids that have an osmotic effect. Lactulose is used to treat chronic constipation in cats with megacolon. It is also used in the management of hepatic encephalopathy, in which acidification of the large intestine promotes formation of nonabsorbable ammonium ions and quaternary amines, thereby reducing the need for detoxification by the liver.
Hydrophilic Colloids (“Bulk Laxatives”)
These are composed of nonabsorbed synthetic or natural polysaccharide cellulose derivatives. These compounds imbibe water and increase the mass of nondigestible material in the intestine. Examples include methylcellulose, psyllium, prunes, wheat bran, and canned pumpkin.
These act by coating the surface of the feces with a water-immiscible film and by increasing the water content of the feces to provide a lubricant action. Lubricant laxatives usually contain mineral oil or white petroleum. Chronic use may reduce intestinal absorption of fat-soluble vitamins and cause a granulomatous enteritis. Mineral oil is very commonly used in horses and cattle, and commercial products are available to promote passage of hairballs in cats.
Fecal Softeners (Surfactants)
Docusate sodium, docusate calcium, and docusate potassium are salts that decrease surface tension and allow water to accumulate in the feces. Docusate also increases cAMP in colonic mucosal cells, which increases ion secretion and fluid permeability. When docusate is used concurrently with mineral oil, soaps are formed and mineral oil absorption is increased.
Last full review/revision March 2012 by Patricia M. Dowling, DVM,MSc, DACVIM, DACVCP; Johann (Hans) Coetzee, BVSc, CertCHP, PhD, DACVCP