Drug antagonism is the phenomenon where one drug (antagonist) diminishes or abolishes the action of another drug (agonist).
Types
Physical antagonism: Based on physical properties (e.g., activated charcoal adsorbing a poison)
Chemical antagonism: Antagonist chemically combines with the agonist forming an inactive compound (e.g., protamine sulphate neutralizing heparin)
Physiological (functional) antagonism: Two agonists act on different receptors/systems producing opposite effects on the same physiological function (e.g., histamine vs. adrenaline on bronchial smooth muscle)
Receptor (pharmacological) antagonism:
Competitive (reversible): Antagonist competes with agonist for the same receptor site; surmountable by increasing agonist concentration (e.g., atropine vs. acetylcholine)
Non-competitive (irreversible): Antagonist binds irreversibly or allosterically; cannot be overcome by increasing agonist concentration (e.g., phenoxybenzamine at α receptors)
