Cathartic and Laxative Drugs (Monogastric)

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. 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.

Senna and bisacodyl are stimulant cathartics that affect the large intestine and are found in many over-the-counter human laxative formulations.

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 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.

Polyethylene glycol (PEG3350) is a large molecular weight, water-soluble polymer used widely in people as a bulking and softening agent for treatment of constipation. It is not metabolized by the intestinal bacterial flora and is minimally absorbed by the intestines. It forms hydrogen bonds with 100 molecules of water per molecule, creating high osmotic pressures within the bowel lumen. The osmotic pressure prevents absorption of water out of the lumen. It is relatively free of adverse effects. PEG3350 is readily available in a powder form, which can be added to a dog or cat’s regular food. It can also be administered as a solution via nasogastric tube. Unlike fiber laxatives, it does not cause bloating or gas.

The bulk laxatives use fiber to draw water in to the bowel. Fiber is made up of several different compounds, all of which are carbohydrates. The term "fiber" is used to describe the "insoluble carbohydrates" that resist enzymatic digestion in the small intestine. Found in the cell walls of plants and grains, the most common fibers are cellulose, hemicellulose, pectin, gums, and resistant starches. Almost all carbohydrate sources contain some fiber. Some of the most common sources of fiber in pet foods include rice hulls, corn and corn byproducts, soybean hulls, beet pulp, bran, peanut hulls, and pectin. Adding fiber to a diet improves colon health, helps with weight management, and helps with diarrhea, constipation, and diabetes mellitus. Many commercial brands of pet food are available in a high-fiber formula. Bulk laxatives may cause bloating and flatulence. Contrary to popular belief, bran mashes do not cause a laxative effect in horses. In cats with megacolon, high-fiber diets are used initially to help manage constipation when there is still some normal colonic motility. But once the colonic innervation has deteriorated, the diet may need to be switched to a low-residue diet with aggressive laxative treatment.

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.

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. Usually considered very safe, concentrations of dioctyl sodium sulfosuccinate (DSS) ranging from 3–5 times the recommended dosage produced severe diarrhea, rapid dehydration, and death in horses. DSS should not be administered concurrently with mineral oil; soaps are formed and oil absorption is increased.