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Summary of Cathartics

In medicine, a cathartic is a substance that accelerates defecation. This is similar to a laxative, which is a substance that eases defecation, usually by softening feces. It is possible for a substance to be both a laxative and a cathartic. However, agents such as psyllium seed husks increase the bulk of the feces.

Cathartics such as sorbitol, magnesium citrate, magnesium sulfate, or sodium sulfate were previously used as a form of gastrointestinal decontamination following poisoning via ingestion. They are no longer routinely recommended for poisonings. High-dose cathartics may be an effective means of ridding the lower gastrointestinal tract of toxins; however, they carry a risk of electrolyte imbalances and dehydration. Catharsis can be an effect of pesticide poisonings, such as with elemental sulfur.

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 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 Cathartics :-
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 super purgation. 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 ricin oleic 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 ricin oleic 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.

Hyperosmotic Cathartics
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.