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Mechanism of Drug Action -Physical Mechanisms

When the drug does not produce any chemical reaction or change in the cells of the body and the effect is only physical, the mechanisms involved are called physical mechanisms. These include:

  1. Local Application– Emollients, Counter irritants, Massagers, pastes
  2. Physically acting antacids– Milk, Mucin
  3. Bulk forming purgatives– Bran, Ispaghula, Magnessium sulphate
  4. Mannitol
  5. Liquid paraffin
  6. Activated charcoal
  7. General Anesthetics
  8. Rubfacients
  9. Astringents
1. Local Application:

Drugs are applied topically on the external surfaces like the skin and the mucous membranes.


Emolients are the oily substances used to soften the skin especially dry skin.

Counter Irritants:

Counter irritants are the substances applied locally on intact skin to abolish deep musculoskeletal pain. They mask the pain sensations.

2. Antacids

Antacids are the drugs which neutralize the acids in the stomach. They form a coating over the surface of the stomach and adsorb HCl on their surface.

3. Bulk forming Purgatives

Purgatives are the drugs used to treat constipation. Bulk forming purgatives contain high fiber. This fiber by adsorbing water swells, increasing the bulk of stools causing distention of the colon. This leads to an increase in peristalsis and evacuation of the bowel.

4. Mannitol

Mannitol is an osmotic diuretic which increases urine formation. It is freely filtered, not reabsorbed and produces osmotic effect.

5. Liquid Paraffin

Liquid paraffin is used to relieve constipation. It lubricates and softens the stools to facilitate removal.

6. Activated Charcoal

Activated charcoal is used to neutralize various poisons. It binds with the poisons in the stomach.

7. General Anesthetics

General anesthetics act by biophysical mechanisms acting mainly on the cell membrane and its constituents (lipids, proteins, water). They cause disorganization of the molecules in the cell membranes. Their effect may be reversed by increasing the atmospheric pressure, by which the disordered molecules come into order.

8. Rubfacients irritate the surface on which they are applied causing hyperemia, so the area becomes warm.

9. Astringents cause the sloughing off of superficial skin layer.

Up Next

Chemical Mechanisms

Drug- receptor interactions

Drug- enzyme interactions

Drug channel interactions

Miscellaneous mechanisms



An Introduction






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