Drug X has an EC50 of 100 nmol/L and produces a maximum effect of 100% of the reference maximum. Drug Y acting on the same receptor system has an EC50 of 5 nmol/L and a maximum effect of 55%. Which conclusion is correct?

• A Drug Y is both more potent and more efficacious than drug X
• B Drug X is more potent, while drug Y is less efficacious
• C Both drugs have equal efficacy because their curves are parallel
• D Drug Y is more potent, while drug X is more efficacious ✓

Explanation

Potency is reflected by the position of the curve on the concentration axis: the lower the EC50, the greater the potency, so drug Y (EC50 5 nmol/L) is more potent than drug X (EC50 100 nmol/L). Efficacy is reflected by the maximum achievable effect (Emax): drug X reaches 100% whereas drug Y plateaus at 55%, making drug X more efficacious. Curve slope does not determine either parameter, which eliminates option C.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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