Two antihypertensives are tested in healthy volunteers. Drug A reduces systolic blood pressure with an EC50 of 0.01 mg/L and a maximum reduction of 30 mmHg. Drug B achieves an EC50 of 1 mg/L with a maximum reduction of 55 mmHg. Which conclusion is correct?
- A Drug B is both more potent and more efficacious than Drug A
- B Drug A is more potent but Drug B is more efficacious ✓
- C Drug A is more efficacious because it acts at a hundred-fold lower concentration
- D Both drugs have equal efficacy since their curves eventually reach a plateau
Correct answer: B. Drug A is more potent but Drug B is more efficacious
Explanation
Potency is read from position along the concentration axis, reflected in EC50, while efficacy is read from the height of the curve, reflected in Emax. Drug A reaches half its effect at a hundred-fold lower concentration, making it far more potent, but its plateau of 30 mmHg is lower than Drug B's 55 mmHg, so Drug B has greater maximal effect. Confusing these two axes is the single commonest error in interpreting graded dose-response data.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.
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