A competitive antagonist is introduced alongside an agonist at its receptor. Which change in the concentration-effect curve of the agonist is expected?

• A Rightward shift with unchanged Emax ✓
• B Rightward shift with decreased Emax
• C Leftward shift with increased Emax
• D Depressed Emax with no shift

Explanation

Competitive antagonists bind reversibly to the same site as the agonist. Increasing agonist concentration can overcome the antagonism, so the maximal response (Emax) is preserved, but higher agonist concentrations are needed, producing a parallel rightward shift. This distinguishes competitive from non-competitive antagonism, where Emax is depressed because the antagonist binds irreversibly or at a different site.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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