Drug X has an EC50 of 5 ng/mL, while Drug Y (same class, same receptor) has an EC50 of 50 ng/mL. Both achieve the same maximum effect. Which statement is correct?
- A Drug Y is more potent than Drug X
- B Drug X has greater efficacy than Drug Y
- C Drug X is more potent than Drug Y ✓
- D Drug Y is a partial agonist
Correct answer: C. Drug X is more potent than Drug Y
Explanation
Potency refers to the concentration required to produce 50% of maximum effect (EC50) — a lower EC50 means greater potency. Drug X at 5 ng/mL is 10 times more potent than Drug Y at 50 ng/mL. Since both achieve the same maximum effect (Emax), their efficacy is equal. Partial agonism is defined by a submaximal Emax relative to a full agonist, which is not described here.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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