In a leftward shift of a dose-response curve, what happens to potency?

• A. Potency decreases
• B. Efficacy increases
• C. Efficacy decreases
• D. Potency increases

Answer: (D)

Potency is about how much drug is needed to produce a given level of effect. A dose–response curve shows response versus dose; a leftward shift means you achieve the same response at a lower dose. That directly means the drug is more potent because less quantity is required to reach that effect. The maximal effect achievable (efficacy) is indicated by the top of the curve; shifting left or right changes potency (the dose needed for a given effect) but does not inherently change the maximal possible effect. So a leftward shift corresponds to increased potency, not increased efficacy.