What is a physiological antagonist?

• A. Binds to the same receptor as the agonist and blocks it
• B. Directly binds to the second drug to inactivate it
• C. Produces a physiological effect opposite to the agonist via a different mechanism
• D. Binds irreversibly to receptor to block agonist

Answer: (C)

A physiological antagonist is a drug that produces an opposite effect to another drug, but by acting through a different mechanism or system rather than by blocking the same receptor. For example, two drugs can influence heart rate in opposite directions because they act on different receptors: one may raise heart rate via beta-adrenergic receptors, while another slows it via muscarinic receptors. Their opposing actions at the level of end physiology counter each other even though they don’t compete for the same receptor.

This is why the correct description is “produces a physiological effect opposite to the agonist via a different mechanism.” The other scenarios involve blocking or inactivating the same receptor or drug directly (receptor antagonism, chemical antagonism, or irreversible receptor blockade), which are not physiological antagonism.