What is the effect of adding a competitive antagonist on the dose–response curve of the agonist?

• A. Shifts the curve left, increasing potency.
• B. Shifts the curve to the right, requiring more agonist for the same response.
• C. Maximal response increases.
• D. Maximal response decreases.

Answer: (B)

Competitive antagonists reduce the potency of an agonist by competing for the same receptor site. Because they occupy the receptor reversibly, you must increase the agonist concentration to achieve the same level of response as when no antagonist is present. On the dose–response curve, this shows up as a parallel shift to the right—the same maximal effect can be reached, but at higher agonist doses. The maximal response stays the same because, with enough agonist, receptors can still be fully activated despite the antagonist occupying some sites. This contrasts with noncompetitive antagonism, where the maximal effect is reduced because the antagonist blocks receptors in a way that cannot be overcome by simply increasing agonist concentration.