Agonist drugs activate neurotransmitter receptors to produce a certain response. Antagonist drugs work to prevent the binding of other chemicals to neurotransmitters to block a certain response, according to the Merck Manual.
Addictive drugs give the desired effect by interacting with neurotransmitters and altering brain chemistry, says the Merck Manual. The brain naturally produces a balance of chemicals to bind and block these receptors, and there are two types of agonist drugs that bind the receptors. Most agonist drugs are conventional agonists, which increase the number of active neurotransmitters. Inverse agonists work similarly to antagonist drugs and keep the receptors stabilized in an inactive conformation so that compounds cannot activate them.
Antagonist drugs are introduced chemicals that block receptor activation to produce the desired effect, the Merck Manual describes. These drugs can either directly block neurotransmitters or bind to the chemicals meant to attach to those receptors and prevent them from binding. There are two types of antagonist drugs: reversible and irreversible. Irreversible antagonist drugs bind the receptor and remain tightly associated to prevent the binding of other chemicals, while reversible antagonists readily dissociate from their receptor. Agonist and antagonist drugs can both be bound to the same receptor, but the antagonist drug binding reduces or prevents the unction of the agonist drug.