A competitive antagonist is administered alongside an agonist. Which of the following best describes its effect on the dose-response curve of the agonist?

• A Decrease in both Emax and potency
• B Decrease in maximal response (Emax) with no shift in EC50
• C Parallel leftward shift with increase in maximal response
• D Parallel rightward shift with no change in maximal response (Emax) ✓

Explanation

D competitive antagonist binds reversibly at the same site as the agonist, so high agonist concentrations can fully overcome the block. This produces a parallel rightward shift (decreased potency, higher EC50) with unchanged maximal response. This is the hallmark distinguishing it from non-competitive antagonism, which reduces Emax. Phentolamine versus phenylephrine is a classic example.

Reference: Katzung and Trevor's Pharmacology: Examination and Board Review, 13th ed.

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