Drug A has an EC50 of 2 mg/L and a maximum effect (Emax) of 80 units. Drug B has an EC50 of 8 mg/L and an Emax of 80 units. Both are agonists at the same receptor. Which statement correctly characterizes the relationship between these drugs?

• A Drug A has greater efficacy than Drug B
• B Drug B is a partial agonist relative to Drug A
• C Drug A has greater potency than Drug B but equal efficacy ✓
• D Drug A has greater intrinsic activity, making it a preferred full agonist

Explanation

Potency refers to the drug concentration required to produce 50% of maximal effect (EC50) — a lower EC50 means greater potency. Drug A (EC50 = 2 mg/L) is 4-fold more potent than Drug B (EC50 = 8 mg/L). Efficacy refers to the maximum effect a drug can produce (Emax) — both drugs have identical Emax (80 units), so they have equal efficacy. Partial agonists have a lower Emax than full agonists; since both have the same Emax, neither is partial relative to the other.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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