Which term describes a drug that opposes another in producing opposite effects to cancel out the effect of another drug?

• A. Competitive antagonist
• B. Inverse agonist
• C. Partial agonist
• D. Physiological antagonist

Answer: (D)

Physiological antagonism occurs when two drugs oppose each other by acting on different receptors or physiological pathways, so their effects cancel each other out even though they don’t compete for the same receptor. This is different from blocking a receptor directly with a competitive antagonist, where the antagonism can be overcome by more agonist. It’s also not about a drug reducing the receptor’s activity at baseline (inverse agonist) or about a drug producing only a partial response at the same receptor (partial agonist). A classic example is epinephrine causing vasoconstriction via alpha receptors opposing histamine‑induced vasodilation, or acetylcholine slowing heart rate while norepinephrine speeds it up; the two effects oppose each other and can balance out.