Correct Answer: A. A
Potency is defined as the amount of drug required to produce a given effect, expressed as the ED₅₀ (effective dose producing 50% of maximum response). On a log dose-response curve, the drug requiring the smallest dose to achieve the same effect has the highest potency. Drug A reaches 50% of its maximum response at a lower log dose (further to the left on the x-axis) compared to drugs B and C, meaning it requires a smaller absolute dose to produce the same therapeutic effect. This leftward shift of the dose-response curve is the hallmark of increased potency. Potency is independent of efficacy (maximum response achievable); a drug can be highly potent but have low efficacy, or vice versa. In clinical practice, higher potency means lower pill burden and potentially better compliance—for example, fentanyl (high potency opioid) requires micrograms versus morphine (lower potency) requiring milligrams for equivalent analgesia. The key discriminator is the horizontal position of the curve on the log dose axis, not the slope or the maximum response plateau.
Why the other options are wrong
B. B — Drug B requires a higher log dose to achieve 50% maximum response compared to drug A, indicating lower potency. While B may have similar efficacy (maximum response), its ED₅₀ is rightward-shifted, meaning more drug is needed for the same effect. This is the classic NBE trap: confusing efficacy with potency by looking only at the plateau height rather than the dose required. C. Both A and B — This option incorrectly assumes A and B have equal potency. The dose-response curves clearly show different ED₅₀ values; A's curve is left-shifted relative to B's. Selecting this reflects failure to distinguish between the two drugs' positions on the log dose axis. This trap exploits students who memorize 'potency = efficacy' and see similar maximum responses. D. C — Drug C requires the highest log dose to reach 50% response, making it the least potent of the three. Selecting C represents a fundamental misreading of the graph or confusion between potency and efficacy. If C had a higher maximum response, students might mistakenly think 'higher response = higher potency,' but potency is strictly about dose requirement, not response magnitude.
High-Yield Facts
- Potency is defined by ED₅₀ (dose producing 50% max response); lower ED₅₀ = higher potency.
- On a log dose-response curve, leftward shift = higher potency; rightward shift = lower potency.
- Potency ≠ efficacy: a drug can be highly potent (small dose needed) but have low efficacy (low max response), or vice versa.
- ED₅₀ ratio between two drugs quantifies relative potency; e.g., if drug A's ED₅₀ is 1 mg and drug B's is 10 mg, A is 10× more potent.
- Clinical example: Fentanyl vs morphine—fentanyl is ~100× more potent (micrograms vs milligrams) but both have similar efficacy for pain relief.
Mnemonics
LEFT = LESS needed = POTENT On a log dose-response curve, the drug whose curve is furthest to the LEFT requires the LEAST dose → highest POTENCY. Rightward = more dose needed = lower potency. ED₅₀ Rule: Smaller dose, stronger drug Compare ED₅₀ values directly. The drug with the smallest ED₅₀ is the most potent. Use this when comparing multiple drugs on the same graph.
NBE Trap
NBE pairs dose-response curves with efficacy-potency confusion: students see similar maximum responses across drugs and incorrectly assume equal potency, or they confuse the steepness of the curve (which relates to selectivity/cooperativity) with potency (which is purely about horizontal position/ED₅₀).
Clinical Pearl
In Indian clinical practice, understanding potency is critical for dosing decisions: a highly potent antihypertensive like amlodipine (5–10 mg daily) versus a less potent agent means fewer tablets, better adherence, and lower cost—especially important in resource-limited settings where pill burden directly impacts compliance in chronic diseases like hypertension and diabetes.
_Reference: KD Tripathi Pharmacology Ch. 1 (General Pharmacology); Harrison Principles of Internal Medicine Ch. 5 (Pharmacokinetics & Pharmacodynamics)_