Drug X has an EC50 of 2 mg and Drug Y has an EC50 of 20 mg for the same effect. What can you conclude?
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Clinical Pharmacology · Pharmacodynamics
Potency and Efficacy
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In 30 seconds
Potency and efficacy answer two different questions about a dose-response curve. Potency asks how much drug is needed for an effect, captured by EC50/ED50 and shown as the curve's left-right position. Efficacy asks how strong an effect the drug can ever produce, captured by Emax and shown as the curve's height. A more potent drug is not automatically a better drug — clinical usefulness depends on efficacy, safety, and practicality, not on needing a smaller dose.
The college version
The dose-response curve
Plotting response against the log of dose typically produces a sigmoidal curve that rises then plateaus. Two features matter most: where the curve sits horizontally (potency) and how high it climbs (efficacy).
Potency: EC50 and ED50
Potency is the amount of drug needed to produce a given effect, usually 50% of maximum. EC50 is the concentration producing 50% of maximum response in a graded assay (a continuously measurable effect, such as blood pressure change). ED50 is the analogous dose in a quantal, all-or-nothing assay — the dose at which 50% of a population shows a defined response, such as adequate analgesia. Lower EC50/ED50 means less drug is needed, so that drug is more potent. Potency is a horizontal-shift property: a more potent drug's curve sits left of a less potent one, but both can reach the same height.
Efficacy: Emax
Efficacy is the maximum effect a drug can produce, represented by Emax, the curve's plateau. It reflects the intrinsic ability of the drug-receptor complex to trigger a response, not how tightly the drug binds. Two drugs at the same receptor can share potency but differ in efficacy, or the reverse.
Why potency does not equal a "better" drug
Students often assume the more potent drug is more powerful. Clinically, what matters is whether a drug's efficacy meets the therapeutic need at a practical, safe dose. Example: a strong opioid may be far more potent than a weaker analgesic, but if both max out at only mild-pain relief, the potent one still cannot achieve high efficacy for severe pain. A less potent drug needing a larger dose is not a disadvantage if it is inexpensive, well tolerated, and reaches the needed efficacy. Potency mainly affects dosing convenience; efficacy sets the ceiling of benefit.
Graded versus quantal curves
A graded curve measures a continuously variable effect in one subject (e.g., degree of muscle relaxation), rising smoothly to Emax. A quantal curve measures the fraction of a population reaching a fixed endpoint at each dose, producing a cumulative-frequency curve used to derive ED50 and, with toxicity data, the therapeutic index.
Antagonists shift the curve
A competitive antagonist binds reversibly at the same site and can be displaced by enough agonist, shifting the curve rightward (raising apparent EC50) without lowering Emax — full efficacy stays reachable at a high dose. A noncompetitive antagonist (irreversible or allosteric) reduces receptor signaling capacity so no added agonist restores the original maximum — it pushes Emax down, flattening the curve.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine two flashlights. One turns on fully with just one battery — that's "potent," needing very little power to work. The other needs four batteries — less potent, but it might glow just as bright, or brighter, once fully powered. Potency is how much "battery" (drug) you need to flip the switch. Efficacy is how bright the flashlight can shine at its best, no matter how many batteries you add.
A blocker can mess with the flashlight two ways. One stands in front of the switch, but enough battery power shoves them aside — you still get full brightness, just with more effort (competitive antagonist). The other breaks the bulb a little — now no matter how many batteries you add, it never shines as bright as before (noncompetitive antagonist).
Check yourself
2 review questions from the chapter. Try each one, then open the answer.
Drug A needs only 5 mg to relieve moderate pain, while Drug B needs 200 mg, yet both top out at the same maximum pain relief. Is Drug A a "better" analgesic than Drug B? Explain briefly.
Show answer
Not necessarily "better" — potency alone doesn't determine clinical value.
Both drugs reach the same maximum pain relief (same efficacy), so Drug A's lower dose is mainly a potency/convenience difference; the real choice should weigh efficacy, side effects, and cost, not dose size.
A researcher adds an irreversible antagonist to a tissue preparation and finds that even very high agonist doses can no longer produce the original maximum. Is this antagonist competitive or noncompetitive, and how does it affect Emax versus EC50?
Show answer
Noncompetitive — it lowers Emax more than it shifts EC50.
Since no added agonist dose restores the original maximum, the antagonist has permanently reduced the tissue's maximal signaling capacity — the hallmark of a noncompetitive (often irreversible) antagonist, unlike a competitive one that excess agonist could overcome.
Quick check
3 questions here. Answers stay hidden until you check.
On a dose-response curve, efficacy is best represented by:
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Which best describes a competitive antagonist's effect on an agonist's dose-response curve?
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