A patient overdoses on a full agonist. The physician administers a competitive antagonist. Which change is observed on the dose-response curve compared to the agonist alone?

• A Rightward shift with decreased maximal response
• B Downward shift with same EC50
• C Rightward shift with same maximal response ✓
• D Downward shift with decreased EC50

Explanation

Competitive antagonists bind reversibly to the same active site as the agonist, competing for receptor occupancy. This shifts the dose-response curve to the right, meaning a higher agonist concentration is needed to achieve the same effect. The maximal response remains unchanged because a sufficient concentration of the agonist can outcompete the antagonist. Option A describes a non-competitive antagonist, which binds irreversibly or at an allosteric site.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

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