Drugs for constipation

These are the drugs which promote evacuation of bowels.

Laxatives :- milder action, elimates soft but formed stools.

Purgatives :- stronger action, more fluid evacuation.

 

Mechanism of action :

All purgatives increase the water content of faeces by :

  1. An osmotic action, retaining water & electrolytes in the intestinal lumen. This increases volume of stools & is easily propelled.
  2. Decrease net absorption of water & electrolytes in intestinal mucosa
  3. Increasing the propulsive activity. This allows less time for salt & electrolytes to be absorbed.

 

The mechanisms by which they obtain above actions are :

  • Inhibiting Na -K- ATPase of villous cells – impairing electrolyte & water absorption.
  • Stimulating adenylyl cyclase in crypt cells–increasing water & electrolyte secretion.
  • Enhancing PG synthesis in mucosa– increases secretion.
  • Increasing NO synthesis. Enhances secretion & inhibits non propulsive contractions in colon.

 

1. Bulk purgatives :-

 

  • Dietary fibre – Bran :

-Bran absorbs water in the intestines , swells & increases water content of faeces – softens it.

-Osmotically active products are formed in colon, which tend to retain water.

-Bran supports bacterial growth –contribute to faecal mass.

-Useful when straining at stools has to be avoided.

-It is safe, but unpalatable.

-20-40 g/day needs to be ingested.

-It does not soften the faeces already present in the rectum.

-Bran is useful for prevention of constipation, but not for treating already constipated patients.

 

  • Ispaghula

-Contains natural colloidal mucilage which forms a gelatinous mass by absorbing water.

-Is largely fermented in colon – Increases bacterial mass, softens the faeces.

-Ispaghula husk is mixed (3-8 g) with cold milk, fruit juice or water.

-Should not be swallowed dry.

 

  • Methyl cellulose.

 

2. Stool softeners :
  • Docusates – It emulsifies the colonic contents & increases penetration of water into faeces.
  • Liquid paraffin – Softens stools. Lubricates hard scybala by coating them. It is bland. Difficult to swallow. Carries away fat soluble vitamins with it. Deficiency may occur on chronic use.

 

3. Stimulant purgatives :

Mechanism of action –

  • They inhibit NaK ATPase at the basolateral membrane of villous cells – transport of Na & accompanying water into interstitium is reduced.
  • So there is accumulation of water & electrolytes which increase the bulk of faeces.
  • Also , they increase intestinal motility by acting on myenteric plexuses.

 

4. Osmotic purgatives :
  • Solutes that are not absorbed in intestine retain water osmotically & distend the bowel – increase peristalsis.
  • All inorganic salts used here augment the motility & secretion .
  • Saline purgatives are not used now for constipation because they produce watery stools after constipation. So are unpleasant.
  • They are preferred for preparation of bowel before surgery & colonoscopy.

 

 

Indications of laxatives :-

 

  1. Functional constipation
  2. In Bedridden patients (To prevent constipation – give bulk forming agents, To treat constipation – Enema)
  1. To avoid straining at stools
  2. To prepare bowel for surgery, colonoscopy, abdominal X-ray.
  3. Food / drug poisoning.

 

Laxatives are contraindicated in :

  1. A patient of undiagnosed abdominal pain, colic or vomiting.
  2. Organic constipation due to strictures or obstruction, hypothyroidism, malignancies
  3. Certain drugs like sedatives, anticholinergic, clonidine, verapamil, antihistamines.

 

Contributed by –  Soumya Khot, Jay Shah.

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