Cathartics and laxatives increase the motility of the intestine or increase the bulk of feces. Clinically, these drugs are administered to increase the passage of gut contents associated with intestinal impaction, to cleanse the bowel before radiographic or endoscopic examination, to eliminate toxins from the GI tract, and to soften feces after intestinal or anal surgery.
Stimulant Cathartics Used in Monogastric Animals
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 hours for an effect to be observed. Repeated doses should be avoided in horses because of the long latent period and risk of severe superpurgation. The naturally occurring emodins (eg, senna) are commonly found in human laxative 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 increases water secretion in the small intestine.
Raw linseed oil is hydrolyzed to release linoleates, which are less irritating than ricinoleic acid. In small daily doses (60 mL), linseed oil is a mild laxative and a source of fatty acids for horses. Boiled linseed oil contains metallic driers (catalysts to accelerate drying for furniture finishing); it is toxic and should not be fed to horses.
Bisacodyl stimulates enteric nerves to bring about colonic mass movements. It also increases fluid and NaCl secretion. It is found in human over-the-counter formulations.
Hyperosmotic Cathartics Used in Monogastric Animals
Hyperosmotic cathartics are poorly absorbed from the GI tract and draw fluid into the intestine by osmosis. The fluid content of the feces increases, causing intestinal distention and promoting peristalsis. Although these drugs are relatively safe, overdoses can lead to excessive fluid loss and dehydration, so adequate water intake must be ensured. Examples of hyperosmotic cathartics include magnesium salts, sodium salts, and sugar alcohols.
Magnesium salts are frequently administered orally as saline purgatives. In normal circumstances, 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 administered orally as saline cathartics; however, they 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 the formation of nonabsorbable ammonium ions and quaternary amines, thereby decreasing the need for detoxification by the liver.
Polyethylene glycol 3350 is a water-soluble polymer with a high molecular weight (3350 g/mol) used widely in humans as a bulking and softening agent to treat constipation. It is not metabolized by intestinal microbiota and is minimally absorbed by the intestines. Each molecule forms hydrogen bonds with 100 molecules of water, creating high osmotic pressures within the bowel lumen. The osmotic pressure prevents the absorption of water out of the lumen. Use of polyethylene glycol 3350 is generally free of adverse effects. It is readily available in a powder form, which can be added to a dog's or cat’s regular food. It can also be administered as a solution via nasogastric tube. Unlike fiber laxatives, polyethylene glycol 3350 does not produce bloating or gas.
Hydrophilic Colloids (Bulk Laxatives) Used in Monogastric Animals
Bulk laxatives are composed of nonabsorbed synthetic or natural polysaccharide cellulose derivatives. These products imbibe water and increase the mass of nondigestible material in the bowel. Examples of bulk laxatives include methylcellulose, psyllium, and prunes. Fiber is made up of several 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 by-products, soybean hulls, beet pulp, bran, peanut hulls, and pectin.
Adding fiber to a diet improves colon health, helps with weight management, and ameliorates diarrhea, constipation, and diabetes mellitus. Many commercial brands of pet food are available in a high-fiber formula.
Contrary to popular belief, bran mashes do not have a laxative effect in horses.
With megacolon in cats, high-fiber diets are used initially to help manage constipation when there is still some normal colonic motility. Once the colonic innervation has deteriorated, however, the diet may need to be switched to a low-residue diet with aggressive laxative treatment. End-stage disease may necessitate subtotal colectomy.
Lubricant Laxatives Used in Monogastric Animals
Lubricant laxatives coat the surface of the feces with a water-immiscible film and increase the water content of the feces to provide a lubricant action. They usually contain mineral oil or white petroleum.
Chronic use of lubricant laxatives may decrease intestinal absorption of fat-soluble vitamins and lead to a granulomatous enteritis. Mineral oil is commonly used in horses and cattle.
Commercial products containing white petroleum jelly are available to promote the passage of hairballs in cats; owners should be advised to use such products only with veterinary approval and supervision.
Fecal Softeners (Surfactants) Used in Monogastric Animals
Docusate sodium, docusate calcium, and docusate potassium are salts that decrease surface tension and enable water to accumulate in the feces. Docusates also increase cyclic adenosine monophosphate (cAMP) in colonic mucosal cells, which increases ion secretion and fluid permeability. Usually considered safe, dioctyl sodium sulfosuccinate (DSS), when administered at 3–5 times the recommended dosage, has been reported to produce severe diarrhea, rapid dehydration, and death in horses. DSS should not be administered concurrently with mineral oil because the combination results in the formation of soaps and an increase in oil absorption.