Laxatives And Cathartics (Purgatives)

Laxatives and cathartics are drugs used orally to evacuate the bowels or to promote bowel elimination (defecation).

The term laxative implies mild effects, and eliminative of soft formed stool. The term cathartic implies strong effects and elimination of liquid or semi liquid stool. Both terms are used interchangeably because it is the dose that determines the effects rather than a particular drug.

Example:- castor oil laxative effect= 4ml

Cathartic effect = 15-60ml

Laxative and cathartics are arbitrarily classified depending on mode of action as:

• Bulk forming laxatives: are substances that are largely unabsorbed from the intestine. They include hydrophilic colloids such as psyllium, bran, methylcellulose, etc. When water is added, the substances swell and become gel-like which increases the bulk of the fecal mass that stimulates peristalsis and defecation.

Osmotic laxatives such as magnesium sulfate, magnesium hydroxide, sodium phosphate, etc. also belong to bulk – forming laxatives.

These substances are not efficiently absorbed, thus creating a stronger than usual solution in the colon which causes water to be retained. The increase in pressure and volume causes stimulation of peristalsis.

• Stimulant (irritant) laxatives (cathartics): are substances that are themselves irritant or contain an irritant substance to produce purgation. Individual drugs are castor oil, bisacodyl, phenolphthalein, cascara sagrada, glycerine, etc.

They are the strongest and most abused laxative products that act by irritating the GI mucosa and pulling water into the bowel lumen. The feces is moved too rapidly and watery stool is eliminated as a result. Glycerine can be administered rectally as suppository only.

• Fecal softners – Decrease the surface tension of the fecal mass to allow water to penetrate into the stool. They have detergent – like property e.g. docusate.
• They may also decrease water absorption through intestinal wall.
• Lubricant laxatives e.g. liquid paraffin (mineral oil). It lubricates the intestine and is thought to soften stool by retarding colonic absorption of fecal water.

Used as retention enema.

Indications for use

Laxatives and cathartics are used:

1. To relieve constipation – bulk – forming
2. To prevent straining – stool softeners
3. To empty the bowel in preparation for bowel surgery or diagnostic procedures (saline or stimulant)
4. To accelerate elimination of potentially toxic substances from the GI tract (saline or stimulant)
5. To accelerate excretion of parasite after anthelmintic drugs (saline or stimulant) have been administered.

• Constipation is a common problem in older adults and laxatives are often used or overused. Non drug measures to prevent constipation (e.g. increasing intake of fluid and high –fiber foods, exercise) are much preferred to laxatives.