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Pharmacology · single question

A pharmacologic dose-response graph shows Drug A reaches its maximum effect at a lower concentration than Drug B, but both drugs achieve the same maximum effect (Emax). Based on this graph, which is the correct interpretation regarding potency and efficacy?

  1. Drug A is more potent; both drugs have equal efficacy
  2. Drug B is more potent; Drug A is more efficacious
  3. Both drugs have equal potency and equal efficacy
  4. Drug A is more potent; Drug B is more efficacious
Show answer and explanation

Correct answer: A. Drug A is more potent; both drugs have equal efficacy

Potency refers to the concentration (EC50) needed to produce 50% of the maximum effect — lower EC50 = more potent. Efficacy refers to the maximum effect a drug can produce (Emax). If Drug A has a lower EC50 but both reach the same Emax, Drug A is more potent and both have equal efficacy.
  • A. CORRECT. Drug A is more potent (lower EC50 = lower concentration needed for 50% effect). Both drugs reach the same Emax (maximum effect), so efficacy is equal between them.
  • B. Incorrect on both counts. Drug A has the lower EC50 (reaches max effect at lower concentration), making it MORE potent, not Drug B.
  • C. Incorrect — the drugs clearly differ in potency since Drug A requires a lower concentration to reach its effect.
  • D. Partially correct about potency (Drug A more potent) but incorrect about efficacy — both drugs reach the same Emax, so efficacy is equal, not different.

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