Laxatives and cathartics

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Laxatives and cathartics

Gastrointestinal system


Laxatives and cathartics


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Laxatives and cathartics

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magnesium hydroxide p. 408


magnesium hydroxide and p. 408


magnesium hydroxide and p. 408

Magnesium hydroxide p. 408


Laxatives and cathartics are medications that increase the passage of stool.

Laxatives ease the passage of fully formed fecal matter from the rectum; while cathartics have a stronger effect, and cause the evacuation of the entire colon, usually in the form of watery, unformed stool.

Some medications can function both as a laxative and a cathartic, depending on the dosage.

Both types of medications are used to treat constipation, which is partly characterized by a decrease in stool passage frequency; small, hard stools; or difficulty with initiating bowel movements.

Normal stool frequency is usually at least 3 times per week for someone on a typical Western diet.

Now, the small and large intestines are where most of the absorption happens in the GI tract.

The small intestine contains smooth muscles that perform peristalsis, which is a series of coordinated wave-like muscle contractions that help push the food bolus through the GI tract.

Lining the luminal surface of the intestine is a layer called the mucosa, which absorbs nutrients or secretes different molecules, like ions and water, into the lumen.

The undigested component of the food bolus eventually reaches the large intestine and becomes feces or stool.

The large intestine mainly absorbs excess water from the stool and that helps condense it into a more solid form.

However, stool should still be 70-80% water by weight, so if the feces becomes too dry, it could condense into a large, hard mass that’s difficult to pass.

So instead of peristalsis, which only pushes the food bolus in one direction, the colon undergoes segmental contraction, which pushes the feces in both directions within the haustra to constantly mix it with water; kind of like how a cement truck keeps churning to keep the cement from drying.

Now, constipation can occur due to a poor diet or a malfunction within the GI tract itself, although, up to 60 percent of chronic constipation does not have a clear cause.

One of the most important dietary factors is the lack of fiber, which is the part of food that resists digestion and reaches the colon relatively unchanged.


  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
  4. "Physiology" Elsevier (2017)
  5. "How Useful Is Docusate in Patients at Risk for Constipation? A Systematic Review of the Evidence in the Chronically Ill" Journal of Pain and Symptom Management (2000)
  6. "Systematic Review of Stimulant and Nonstimulant Laxatives for the Treatment of Functional constipation" Canadian Journal of Gastroenterology and Hepatology (2014)

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