Competitive vs noncompetitive antagonists: effect on EC50 and Emax
Two antagonists can block the same receptor, yet only one can be beaten by more agonist. The shape of the dose-response curve tells them apart.
Chapters (9)
Predict how competitive and noncompetitive antagonists shift the agonist dose-response curve, in EC50 and Emax.
The short version
- A competitive antagonist binds the agonist site reversibly, shifts the curve in parallel to the right, raises the EC50 and leaves the Emax alone.
- A noncompetitive antagonist lowers the Emax, and more agonist cannot overcome it.
- On the standard exam graph, the noncompetitive EC50 is drawn where it was.
- Irreversible binding, as with phenoxybenzamine, acts like a noncompetitive antagonist on the graph, even at the agonist site.
- Memory hook: competitive shifts the curve right; noncompetitive pushes it down.
Two blockers, one receptor: the puzzle
Picture two blockers sitting on the same receptor. Flood the tissue with agonist and the first blocker gives way. Flood it again and the second never does. The difference between competitive vs noncompetitive antagonists is the reason, and the dose-response curve shows it.
The goal of this lesson is to predict how each kind of antagonist moves the curve, in terms of EC50 and Emax.
Recap: EC50, Emax and what an antagonist does
The EC50 sets where the dose-response curve sits along the dose axis, which is potency. The Emax sets how high the curve reaches, which is efficacy.[2][5]
An antagonist binds a receptor but does not activate it. Its job is to reduce the effect of an agonist.[1][4]
What matters for the exam is how it binds.
Competitive antagonists: same site, reversible
A competitive antagonist binds the same site as the agonist, reversibly, so the two are mutually exclusive: one receptor holds one or the other.[1][4]
They compete, and the winner depends on concentration. Add more agonist and it crowds the antagonist out. Naloxone against morphine is the classic example: give enough morphine and the block is overcome.[4]
On the graph, this is a parallel shift to the right. More agonist is needed for the same response, so the EC50 rises and potency falls. But enough agonist still wins every receptor, so the Emax does not change.[1][6]
Noncompetitive and irreversible antagonists: the Emax falls
A noncompetitive antagonist binds a separate, allosteric site. Agonist and antagonist can be bound at once, but the antagonist stops the receptor responding.[1][4][6]
An irreversible antagonist, such as phenoxybenzamine, binds permanently or nearly permanently (phenoxybenzamine by a covalent bond), so it does not come off, even if it sits at the agonist site. On the exam graph it behaves the same way as a noncompetitive antagonist.[1][4][7]
More agonist cannot help, because the blocked receptors are out of play. So the ceiling comes down: the Emax falls.[1] On the standard exam graph, the EC50 is drawn where it was.
Memory hook and a worked example
The memory hook: competitive shifts the curve right; noncompetitive pushes it down.
| Feature | Competitive | Noncompetitive |
|---|---|---|
| Binding site | Same site as the agonist | Separate (allosteric) site |
| Agonist and antagonist together | Mutually exclusive | Can be bound at once |
| Overcome by more agonist | Yes | No |
| Curve | Parallel shift to the right | Lower ceiling |
| EC50 | Rises (potency falls) | Unchanged on the exam graph |
| Emax | Unchanged | Falls |
- Agonist alone: EC50 10 nM, full response.
- Add a competitive antagonist: EC50 becomes 80 nM, and 80 / 10 = 8, so eight times higher. Enough agonist still reaches the full response.
- Add a noncompetitive antagonist instead: EC50 stays at 10 nM, but the maximum drops to 30% of the full response.
Competitive: potency down, Emax unchanged. Noncompetitive: Emax down and, on the exam graph, potency unchanged.
A partial agonist can behave like a competitive antagonist
A partial agonist given with a full agonist at the same receptor competes at the same site. That is surmountable: enough full agonist restores the full maximum, so the curve shifts right rather than down.[3]
Practice question
Original · USMLE styleA researcher studies the contraction of isolated vascular smooth muscle in response to phenylephrine. In the absence of any other drug, the EC50 of phenylephrine is 12 nM and the maximal contraction is set at 100%. After the tissue is incubated with drug Q, the EC50 of phenylephrine is 12 nM and the maximal contraction is 45%.
Common questions
What is the difference between competitive and noncompetitive antagonists?
A competitive antagonist binds the agonist site reversibly and can be overcome by more agonist. A noncompetitive antagonist binds a separate allosteric site, and more agonist cannot overcome it.
How does a competitive antagonist change EC50 and Emax?
It shifts the dose-response curve in parallel to the right, so the EC50 rises and potency falls. The Emax does not change, because enough agonist still wins every receptor.
How does a noncompetitive antagonist change EC50 and Emax?
It lowers the Emax, because blocked receptors are out of play. On the standard exam graph, the EC50 is drawn where it was.
Is an irreversible antagonist competitive or noncompetitive?
It can bind the agonist site, but an irreversible antagonist such as phenoxybenzamine binds permanently or nearly permanently (phenoxybenzamine by a covalent bond), so more agonist cannot overcome it. On the exam graph it behaves like a noncompetitive antagonist.
Which classic drug pair shows competitive antagonism?
Naloxone against morphine. Give enough morphine and the block is overcome.
Transcript
Read the full narration, by chapter650 words
0:00Two blockers on one receptor: the puzzle
Two blockers sit on the same receptor. Flood the tissue with agonist, and the first blocker gives way. Flood it again, and the second never does. Why? Today: predict how competitive and noncompetitive antagonists move the dose-response curve, in EC50 and E max.
0:19Recap: potency, efficacy, and what an antagonist does
Last lesson: the EC50 sets where the curve sits, which is potency, and the E max sets how high it reaches, which is efficacy. An antagonist binds a receptor but does not activate it. Its job is to cut the effect of an agonist. What matters is how it binds.
0:39Competitive antagonists: same site, naloxone and morphine
A competitive antagonist binds the same site as the agonist, reversibly, so the two are mutually exclusive: one receptor holds one or the other. They compete, and the winner depends on concentration. Add more agonist and it crowds the antagonist out. Naloxone against morphine is the classic: give enough morphine and the block is overcome.
1:03The parallel right shift: EC50 up, Emax unchanged
On the graph, that is a parallel shift to the right. You need more agonist for the same response, so the EC50 rises and potency falls. But enough agonist still wins every receptor, so the E max does not change.
1:19Noncompetitive and irreversible antagonists: Emax falls
A noncompetitive antagonist works differently. It binds a separate, allosteric site. Agonist and antagonist can be bound at once, but the antagonist stops the receptor responding. An irreversible antagonist, such as phenoxybenzamine, forms a covalent bond and never leaves, even if it sits at the agonist site. On the exam graph it behaves the same way. More agonist cannot help, because the blocked receptors are out of play. So the ceiling comes down: the E max falls. On the standard exam graph, the EC50 is drawn where it was.
1:59Memory hook and a worked example
Memory hook: competitive shifts the curve right; noncompetitive pushes it down. An agonist alone has an EC50 of 10 nanomolar, and gives the full response. Add a competitive antagonist: the EC50 becomes 80 nanomolar, eight times higher, yet enough agonist still reaches the full response. Add a noncompetitive antagonist instead: the EC50 stays at 10 nanomolar, but the maximum drops to 30% of the full response. Competitive: potency down, E max unchanged. Noncompetitive: E max down, and on the exam graph, potency unchanged.
2:41Practice question
Your turn. A researcher studies the contraction of isolated vascular smooth muscle in response to phenylephrine. Alone, the EC50 is 12 nanomolar, and the maximal contraction is set at 100%. After incubation with drug Q, the EC50 is still 12 nanomolar, and the maximal contraction is 45%. Which of the following best describes drug Q? A, a competitive antagonist, because it reduces the potency of phenylephrine. B, a competitive antagonist, because its effect can be overcome with more phenylephrine. C, a noncompetitive antagonist, because it lowers the maximal response while the EC50 stays the same. D, a noncompetitive antagonist, because it shifts the curve to the right with the same maximal response. E, a partial agonist, because it competes with phenylephrine at the same site. Pause here and decide.
3:46Answer and explanation
A is out: reduced potency would mean a higher EC50, and here it has not moved. B is out: a competitive antagonist can be overcome, but here the maximum fell, so more phenylephrine will not restore it. D is out: a shift to the right with the same maximum is the competitive pattern, not this one. E is out: a partial agonist competing at the same site is surmountable: enough phenylephrine restores the full maximum, so the curve shifts right, not down. So the answer is C. The EC50 is unchanged and the maximum has fallen: the noncompetitive pattern.
4:28Three things to keep and the next lesson
Three things to keep. One: a competitive antagonist binds the agonist site reversibly, shifts the curve right, raises the EC50, and leaves the E max alone. Two: a noncompetitive antagonist lowers the E max, and more agonist cannot overcome it. Three: irreversible binding acts like noncompetitive on the graph, even at the agonist site. Next, the therapeutic index and the therapeutic window.
Sources
Numbers in the text, e.g. [1], point here- Deranged Physiology. Competitive and non-competitive antagonists. derangedphysiology.com/main/cicm-primary-exam/pharmacodynamics/Chapter-418/competitive-and-non-competitive-antagonists
- Deranged Physiology. Potency and efficacy. derangedphysiology.com/main/cicm-primary-exam/pharmacodynamics/Chapter-415/potency-and-efficacy
- Deranged Physiology. Full agonists, partial agonists and inverse agonists. derangedphysiology.com/main/cicm-primary-exam/pharmacodynamics/Chapter-417/full-agonists-partial-agonists-and-inverse-agonists
- MSD Manual Professional Version. Drug-Receptor Interactions. msdmanuals.com/professional/clinical-pharmacology/pharmacodynamics/drug-receptor-interactions
- MSD Manual Professional Version. Dose-Response Relationships. msdmanuals.com/professional/clinical-pharmacology/pharmacodynamics/dose-response-relationships
- Receptor antagonist. Wikipedia (secondary). en.wikipedia.org/wiki/Receptor_antagonist
- Phenoxybenzamine in complex regional pain syndrome. Europe PMC PMC3880724. europepmc.org/article/PMC/PMC3880724
The curves on screen are schematic teaching models with synthetic values. On the standard exam graph a noncompetitive antagonist is drawn with the EC50 unchanged; with spare receptors a right shift can also occur, which is not taught here.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): "Arteriole and smooth muscle cells". Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted: syringe, smooth-muscle-relaxed, smooth-muscle-contracted. Medical ampoule by DBCLS via Bioicons (bioicons.com), CC BY 4.0, adapted. Receptor protein (BioArt 430) and seven-transmembrane receptor (BioArt 651): NIAID NIH BioArt Source (bioart.niaid.nih.gov), public domain, adapted. The dose-response curves, the agonist and antagonist symbols, the covalent bond, the question card and all handwriting are drawn in code (original). No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-PHARM-001.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Potency vs efficacy: EC50, Emax and agonist types5:17
- EP 02Competitive vs noncompetitive antagonists: effect on EC50 and Emax5:01
- EP 03Therapeutic index explained: TD50 / ED505:01
- EP 04First-pass metabolism and bioavailability: calculating oral F from AUC5:18
- EP 05Volume of distribution: why Vd can be bigger than the body5:17
- EP 06Clearance and half-life: t½ = 0.693 × Vd / CL5:01
- EP 07Zero-order vs first-order elimination explained5:12
- EP 08Loading dose and maintenance dose, explained5:02
- EP 09CYP450 inducers and inhibitors: how they change drug levels5:16
- EP 10Autonomic receptors: alpha, beta and muscarinic5:33
- EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade5:24
- EP 12Vasopressors compared: receptors, pressure and heart rate5:46
- EP 13Organophosphate poisoning: muscarinic vs nicotinic signs5:36
- EP 14Receptor desensitization, downregulation and upregulation5:01
- EP 15Beta-lactams and vancomycin: enzyme versus substrate5:04
- EP 16Protein synthesis inhibitors: 30S vs 50S antibiotics5:07