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Ulcers (gastrointestinal)

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Antidiarrheal medications are used to treat diarrhea, a word which actually means “flow through.”

Diarrhea can be defined as stool that contains fluid weight over 200g of fluid per day.

Increased frequency of bowel movement is also common, but not always present.

It’s important to note that these medications are typically used to treat mild to moderate diarrhea; therefore, they should not be used in individuals with severe illness, bloody diarrhea, or high fever because they can mask or exacerbate the underlying condition.

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

Both regions contain smooth muscles which perform what’s called peristalsis, which is a series of coordinated wave-like muscle contractions that help squeeze the chyme or the food bolus after it leaves the stomach, in one direction.

Lining the luminal surface of the intestine is a layer called the mucosa, which secretes and absorbs different molecules to change the contents of the intestinal lumen.

The mucosa of the small intestine has a lot of tiny ridges and grooves, each of which projects little finger-like fibers called villi.

And in turn, each villus is covered in teeny tiny little microvilli.

All of this gives the small intestine plenty of surface area to absorb nutrients and ions.

The large intestine mainly absorbs excess water from the chyme, and that helps condense it into dry fecal matter, which eventually ends up in the rectum.

There are four main causes for diarrhea: osmotic, secretory, inflammatory, and diarrhea associated with deranged, or unstable, intestinal motility.

Osmotic diarrhea is caused by poor absorption of certain molecules, which leads to an excessive amount of solutes in the intestinal lumen.

The extra solutes cause fluid retention due to osmosis, which is when water moves from intestinal cells across semipermeable membranes into the lumen so that solute concentrations are equal on both sides.

One example of this is lactose intolerance, where there’s a deficiency in the brush border enzyme lactase in the small intestine, which breaks down lactose.


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  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. "Existing and emerging therapies for managing constipation and diarrhea" Current Opinion in Pharmacology (2017)
  5. "Osmotic and stimulant laxatives for the management of childhood constipation" Cochrane Database of Systematic Reviews (2016)
  6. "Osmotic and stimulant laxatives for the management of childhood constipation" Cochrane Database of Systematic Reviews (2016)

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