Laxatives

Laxatives 

Definition—drugs that promote defaecation are known as Laxatives or Purgatives.

 

Drugs causing constipation—

Fe, Al(OH)₃

Anticholinergic drugs

Morphine and its analogues

Anti-parkinsonism drugs

Anti-Histamine drugs

TCA, NSAID

Calcium channel blockers

Barium Sulphate

Diuretics

Ganglion blockers

MAO inhibitors

   

Classification of Laxatives with description—

Bulk forming laxatives—

Ispaghula Husk, indigestible vegetables (Agar, Bran), Kelp, Vegetables

Plant cell wall, Psyllieum preparations, Gum (Methyl Cellulose, Calcium Polycarbofil)

 

Mechanism of Action—

it will trap water and electrolyte within itself, thus increasing the bulk of the stool and stimulating the myenteric plexus causing peristalsis. Decrease the viscosity of the stool. If water intake is low then the water that is to be trapped will come from ECF.

    

*** for daily digestion 9 litres of water is required.

*** 20-60 grams of vegetables is required for proper digestion daily.

 

Adverse effects—

1.Abdominal distension, discomfort

2.Burburic effect

3.Certain drugs are not absorbed—Cardiac glycosides, TCA, Nitrofurontoin.

Indications—

Anal fissure, Haemorrhoides, Diverticular disease, IBS.

 

Osmotic laxatives—

MgSO₄, Milk of Magnesia {40% solution of Mg(OH)₂}, Lactulose, Na₃PO₄

Na-Citrate, Mg-Citrate, Na₂SO₄, Na-K-Tartarate

*** all of them are inorganic salts

   

Mechanism of Action—

these salts are not absorbed by the GIT and within the gut canal solution of these salts have a high osmotic tension which prevents absorption of water in the gut canal. This results in distension (stretching) of gut. Thus there is increased peristalsis and evacuation.

    

MgSO₄—used in Eclampsia to ↓ BP

 

Lactulose—

It is a disaccharide.

Drug of choice in hepatic encephalopathy to trap NH₃

Lactulose is converted into lactic acid which decreases the luminal pH. So, NH₃ is trapped and prevented from absorption.

 

*** Needs 3 days for effect

Adverse effects—

There may be allergic reaction, inflammation of the gut mucosa, excessive fluid loss.

Na₃PO₄—Sodium causes aggravation of HTN and CCF.

MgSO₄—causes diarrhoea; there may be water and electrolyte imbalance.

 

Stimulating laxatives—

Castor oil, Bisacodyl, Anthraquinone group (Senna), Sodium Picosulphate, Cascara, Aloes

    

*** cause injury to the mucous membrane, so usually not used. (reserve drugs)

*** but in paraplegics, castor oil is given (1 ounce)

*** used before X-ray, surgery of colorectal region

 

Mechanism of Action—

inhibits Na-K-ATPase, ↑ cAMP, ↑ PG synthesis. These three stimulates peristalsis. They also increase the permeability of the intestinal mucosa. Causes increase in secretions.

    

Trade names—

Laxenna— (5+5) 10mg at night orally/suppository.

Daltolate—Phenol Phenolphthalein and Bisacodyl. It is given orally but Bisacodyl is not absorbed when given orally. It is deacetylated the gut by the bacteria. Deacetylated products act o the colon and cause peristalsis.

    

Adverse effects—

Steven Johnson syndrome may occur.

    

Faecal Softeners—

Liquid Paraffin, Mineral Oil, Docusate Sodium.

    

Liquid Paraffin—

consists of higher hydrocarbon. It is not appreciably absorbed form the gut and acts as a laxative by the following ways,

    Causing lubrication of the stool

    Increasing the bulk of the intestinal contents as it reduces the water absorption

    Also by softening the stool

    

Disadvantages of liquid paraffin—

    Can slip out of anal sphincter and causer embarrassment.

    There may be reflux into the oesophagus and enter into the lung causing Lipid pneumonia.

    Prevents absorption of the fat soluble vitamins.

 

Indications—

Anal fissure, Haemorrhoids.

    

Other drugs—

Enema Simplex—slight warm water and soap solution.

    

Types—

Glycerine suppository—to lubricate and stimulate passage. Acts by osmotic effects in the rectum and increases the rectal contraction.

Sorbitol—it is a poly-alcohol given per-rectally.