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. 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.
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.