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DrX Whiz Niraj
Cathartics & Adsorbents: GI Agents
Comprehensive exam notes on Gastrointestinal agents. Understand the mechanism of Saline Cathartics (MgSO4, Sodium orthophosphate) and Adsorbents (Kaolin, Bentonite).
Cathartics are drugs or chemical agents used to relieve constipation by promoting the evacuation of the bowels (defecation). They accelerate the passage of feces through the large intestine.
Intensity of Action:
- Laxatives: Mild action. Produce soft, formed stool.
- Purgatives: Moderate to strong action. Produce semi-fluid/fluid stool.
- Drastics: Extremely strong and irritating action. Produce watery stool with severe cramps.
Saline (Osmotic) Cathartics
Saline cathartics are highly water-soluble inorganic salts that are poorly absorbed from the gastrointestinal (GI) tract. Because they stay in the intestine, they exert high osmotic pressure.
Mechanism of Action: Since these ions are not absorbed, they draw water from the surrounding tissues into the intestinal lumen via osmosis. This increases the bulk and fluid volume of the stool. The increased volume stretches the intestinal wall, stimulating peristalsis (muscle contractions), which eventually expels the stool.
Magnesium Sulphate
Formula: MgSO₄·7H₂O
Synonym: Epsom Salt.
- It is a highly effective saline cathartic.
- Action: It acts very rapidly (within 1 to 2 hours) to clear the bowel.
- Other uses: Used as an anticonvulsant in eclampsia, and as a CNS depressant in severe conditions.
Sodium Orthophosphate
Formula: Na₂HPO₄
Synonym: Disodium hydrogen phosphate.
- It acts as a mild saline cathartic and is less irritating than magnesium salts.
- It is widely used in the preparation of Fleet enemas for bowel cleansing before surgery.
Clinical Alert: Magnesium Toxicity
Saline cathartics containing Magnesium must NOT be given to patients with Impaired Renal Function (Kidney failure). Usually, the tiny amount of Mg²⁺ absorbed is easily excreted by healthy kidneys. But in renal failure, Mg²⁺ accumulates in the blood causing Hypermagnesemia, leading to severe CNS depression and cardiac arrest.
Note: While cathartics treat constipation, Adsorbents and Protectants are used to treat exactly the opposite condition: Diarrhea.
These are chemically inert, highly porous inorganic powders. When swallowed, they coat the gastrointestinal mucosa, providing physical protection.
Mechanism of Action: They have an extremely large surface area. They adsorb (trap on their surface) toxic substances, gases, and bacteria that cause diarrhea. Furthermore, they form a soothing protective coating over the inflamed intestinal wall.
Kaolin (Light & Heavy)
Chemical Name: Hydrated Aluminum Silicate.
- It is a purified native hydrated aluminum silicate, freed from gritty particles.
- Light Kaolin: Used internally for treating diarrhea and dysentery because it adsorbs toxins.
- Heavy Kaolin: Used externally for the preparation of Kaolin poultice.
Bentonite
Chemical Name: Colloidal Hydrated Aluminum Silicate (Montmorillonite type).
- Special Property: It has a massive swelling capacity. It absorbs water and swells to 12 times its original volume, forming a gel.
- Uses: It is largely used in pharmacy as an excellent Suspending Agent for preparing calamine lotion and other suspensions. It is also an adsorbent.