In a concentration-response study, Drug X produces a maximum effect of 100 units with an EC50 of 1 mg/L. Drug Y produces a maximum effect of 40 units with an EC50 of 0.01 mg/L. Which statement is correct?
- A Drug Y is more potent but less efficacious than Drug X ✓
- B Drug X is more potent than Drug Y
- C Drug Y is more efficacious than Drug X
- D Both drugs have equal efficacy and equal potency
Correct answer: A. Drug Y is more potent but less efficacious than Drug X
Explanation
Potency reflects the position of the curve along the concentration axis and is expressed as EC50; the lower the EC50, the higher the potency. Efficacy is the ceiling of the curve, the Emax. Drug Y needs a tenfold lower concentration for half-maximal effect, so it is more potent, but its lower Emax means it is less efficacious. The most tempting distractor, option B, confuses the two axes: potency is read horizontally, efficacy vertically.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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