14. CATHARTICS

Introduction to Cathartics

Cathartics are pharmaceutical agents used to relieve constipation and promote bowel evacuation. They are often administered to cleanse the intestinal tract before surgery or diagnostic procedures. Cathartics stimulate intestinal movement or soften the fecal mass, aiding in the removal of bowel contents.

Purgatives are similar in function but stronger, while laxatives are milder purgatives that act gently to facilitate defecation.

Mechanism of Action

Cathartics act by one or more of the following mechanisms:

  1. Stimulant Cathartics: These irritate the intestinal mucosa, stimulating peristaltic movement and promoting defecation. Examples: Senna, Rhubarb.
  2. Bulk Purgatives: These increase intestinal bulk, stretch the intestinal wall, and stimulate peristalsis. Examples: Sodium carboxymethyl cellulose, Methyl cellulose, Isabgol (psyllium husk).
  3. Lubricant Cathartics: These lubricate the fecal mass, facilitating smooth passage through the intestine. Examples: Paraffin oil, Glycerine.
  4. Osmotic Cathartics: These increase the osmotic pressure in the intestinal lumen, drawing water into the bowel and softening stools. Examples: Sodium phosphate, Magnesium hydroxide, Magnesium sulphate.

Important Inorganic Cathartics

1. Sodium Phosphate (Na2HPO4)

  • Limits: Contains not less than 98.5% and not more than 101% NaHPO4.
  • Properties: Colorless, transparent crystals with saline taste; soluble in water, insoluble in alcohol.
  • Preparation:
    1. Mixing hot solutions of phosphoric acid and sodium carbonate.
    2. Alternatively, reacting calcium phosphate with sulfuric acid, filtering, and treating the filtrate with sodium carbonate to yield sodium phosphate.
  • Assay: Acid-base titration — 6 g dissolved in 100 ml water and titrated with 0.5 N H₂SO₄ using bromocresol green indicator.
  • Storage: Store in tightly closed containers.
  • Uses: Used as cathartic, buffering agent, and antacid.

2. Magnesium Hydroxide (Mg(OH)2)

  • Limits: Contains not less than 95% and not more than 100.5% of Mg(OH)₂.
  • Properties: White, amorphous powder; slightly alkaline; insoluble in water but soluble in acids.
  • Preparation:
    1. By treating seawater with calcium hydroxide to precipitate magnesium hydroxide.
    2. By drying magnesium hydroxide obtained during the preparation of milk of magnesia.
  • Assay: Acid-base back titration — about 1 g of Mg(OH)₂ is treated with 25 ml of 1 N H₂SO₄, and excess acid is back titrated with 1 N NaOH using methyl red indicator.
  • Uses: Used as antacid and mild osmotic laxative.

3. Magnesium Sulphate (MgSO₄·H₂O)

  • Synonym: Epsom Salt
  • Limits: 99–100%
  • Properties: Colorless crystals with bitter saline taste; soluble in water.
  • Preparation:
    1. By reacting dilute sulfuric acid with magnesium carbonate or magnesium oxide.
    2. By treating dolomite with sulfuric acid and filtering out calcium sulphate.
    3. By reacting magnesium hydroxide (from brine) with sulfur dioxide and air.
  • Assay: Direct complexometric titration — 0.3 g dissolved in 50 ml water, 10 ml pH 10 buffer added, titrated with 0.05 M EDTA using mordant black-II indicator.
  • Storage: Store in airtight containers.
  • Uses: Used as laxative, cathartic, and anticonvulsant (in eclampsia).

Detailed Notes:

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