Competitive antagonist

A competitive antagonist is a receptor antagonist that binds to a receptor but does not activate the receptor. The antagonist will compete with available agonist for receptor binding sites on the same receptor. Sufficient antagonist will displace the agonist from the binding sites, resulting in a lower frequency of receptor activation.

Presence of a competitive antagonist will shift an agonism dose-response curve to the right.^[1] A Schild plot for a competitive antagonist will have a slope equal to 1, and the X-intercept and Y-intercept will each equal the dissociation constant of the antagonist.^[1]

Competitive antagonists are used to prevent the activity of drugs, and to reverse the effects of drugs that have already been consumed. Naloxone (also known as Narcan) is used to reverse opioid overdose caused by drugs such as heroin or morphine. Similarly, Ro15-4513 is an antidote to alcohol and flumazenil is an antidote to benzodiazepines.

A competitive antagonist can be reversible competitive antagonist or irreversible competitive antagonist.

References

1 2 Doseresponse curves in the presence of antagonists Archived 2008-05-09 at the Wayback Machine.

Concepts in pharmacology
Pharmacokinetics	(L)ADME: (Liberation) Absorption Distribution Metabolism Excretion (Clearance) Loading dose Volume of distribution (Initial) Rate of infusion Compartment Bioequivalence Bioavailability Onset of action Biological half-life Mean residence time Plasma protein binding Therapeutic index (Median lethal dose, Effective dose)
Pharmacodynamics	Mechanism of action Toxicity (Neurotoxicology) Dose–response relationship (Efficacy, Potency) Antimicrobial pharmacodynamics: Minimum inhibitory concentration (Bacteriostatic) Minimum bactericidal concentration (Bactericide)
Agonism and antagonism	Agonist: Inverse agonist Irreversible agonist Partial agonist Superagonist Physiological agonist Antagonist: Competitive antagonist Irreversible antagonist Physiological antagonist Other: Binding Affinity Binding selectivity Functional selectivity
Other	Drug tolerance: Tachyphylaxis Drug resistance: Antibiotic resistance Multiple drug resistance
Drug discovery strategies	Classical pharmacology Reverse pharmacology
Related fields/subfields	Pharmacogenetics Pharmacogenomics Neuropsychopharmacology (Neuropharmacology, Psychopharmacology)

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