Paper I — 2023 July (Supplementary) (2010 Scheme) — Q7
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Paper I
2023 July (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Drug Antagonism
Q73 marksShort Notes
Answer
Drug antagonism occurs when the combined effect of two drugs is less than expected — one drug reduces or abolishes the action of another.
Types:
Physical antagonism — based on a physical property, not receptor interaction (e.g. activated charcoal physically adsorbing a poison in the gut).
Chemical antagonism — the two drugs react chemically, neutralizing each other’s effect (e.g. protamine sulfate neutralizing heparin; chelating agents binding heavy metals).
Physiological (functional) antagonism — two agonists acting at different receptors produce opposite effects on the same physiological system (e.g. histamine and adrenaline on bronchial smooth muscle/blood pressure).
Receptor antagonism — the clinically dominant category, one drug directly blocks the receptor the other needs:
Competitive (equilibrium): the antagonist competes reversibly for the same site, shifting the agonist’s dose-response curve rightward without lowering the maximum (surmountable) — e.g. atropine versus acetylcholine, naloxone versus morphine.
Non-competitive: the antagonist binds a different (allosteric) site, lowering the achievable maximum response regardless of agonist concentration.
Clinical relevance: the basis of most antidote therapy (naloxone in opioid overdose, atropine in organophosphate poisoning, protamine in heparin overdose).
