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DRUG ANTAGONISM. DR. SHABANA ALI. DRUG ANTAGONISM. One drug or inhibits action of another drug Types of Antagonism Physical antagonism Chemical antagonism Physiological/functional antagonism Pharmacological antagonism. A) Physical Antagonism.

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drug antagonism

One drug or inhibits action of anotherdrug

Types of Antagonism

  • Physical antagonism
  • Chemical antagonism
  • Physiological/functional antagonism
  • Pharmacological antagonism
a physical antagonism
A) Physical Antagonism
  • Based on physical property of drugs, e.g. charcoal (adsorb alkaloid) in alkaloidal poisoning

B) Chemical Antagonism

  • Chemical reaction between two drugs
  • e.g., NaHCO3 HCl
C) Physiological/Functional Antagonism

Opposite effects of two drugs on same function

  • Two drugs act on two diff. types of receptors & antagonize action of each other, e.g. histamine & adrenaline (adrenaline for treatment of anaphylactic shock); Ad & insulin on blood sugar level
d pharmacological antagonism
D) Pharmacological Antagonism
  • Opposite effect of two drugs binding to same receptors
  • Receptor antagonism is specific,e.g. atropine  spam of intestine by acetylcholine not by Hist.or 5-HT


Competitive Non-competitive

Equilibrium Non-equilibrium

(Reversible) (Irreversible)

i competitive antagonism equilibrium or reversible
i) Competitive Antagonism (equilibrium or reversible)

Competition between agonist & antagonist for specific site or receptor

  • Action of agonist is blocked if conc. of antagonist is 
  • Antagonism can be overcome (surmount) by  conc. of agonist


Agonist can produce max.response in higher conc.
  • Competitive antagonist shifts log Dose-response curve of agonist to right
  • EC50 of agonist  in presence of antagonist, e.g., Ach & atropine; Ad & Prop.; Morphine & naloxone
ii non equilibrium irreversible antagonist
II) Non-Equilibrium(irreversible) Antagonist
  • Antagonist binds to receptor with covalent bond
  • Irreversible blocking
  • Antagonist shifts DRC to right+  max. response, e.g. adrenaline & phenoxybenzamine
iii) Non-competitive (non-surmountable Antagonist
  • Antagonist binds to another site of receptor
  • DRC is flattened + max. response is 
  • e.g., verapamil (noradrenaline)