Antagonist binds to the receptor but produces no effect (IA is 0). But now agonist is not able to bind to the receptor because these are already occupied by the antagonist. Thus, it decreases the action of the agonist but itself has no effect.

Types of Antagonism

These may be physical, chemical, physiological or pharmacological

• Physical antagonist binds to the drug and prevebts its absorption like charcoal binds to the alkaloids and prevents their absorption
• Chemical antagonist combines with a substance chemically like chelating agents bind with the metals.
• Physiological antagonist produces an action opposite to a substance but by binding to the different receptors. Eg: adrenaline is a physiological antagonist of histamine because adrenaline causes bronchodilation by binding to beta 2 receptors, which is opposite to bronchoconstriction caused by histamine through H1 receptors
• Pharmacological antagonists produce oppostite actions by binding to the same receptors. Eg: beta blockers

1. A) Physical antagonism
2. B) Competitive antagonism
3. C) Non competitive antagonism
4. D) Physiological antagonism

Ans. D, Physiological antagonism