On a dose–response curve, efficacy is indicated by which feature?

• A. The height of the leftmost point.
• B. The steepness of the slope at half-maximal effect.
• C. The area under the curve.
• D. The position of the rightmost point on the curve.

Answer: (D)

Efficacy is the maximal effect a drug can produce, the height of the curve when the response has saturated. On a dose–response plot, increasing the dose eventually levels the response off at a maximum value—the plateau. The rightmost point sits on that plateau, so its height reflects the greatest achievable response, i.e., the drug’s efficacy. The height at the left end is near baseline and doesn’t tell you how large the maximum response can be. The steepness of the curve’s slope tells you how rapidly response rises with dose (related to potency and cooperativity), not the ultimate size of the effect. The area under the curve describes overall exposure across doses, not the maximal response at saturation.