What is the action of a drug classified as an antagonist?

• A. Prevents chemicals from reaching the receptor sites
• B. Binds to a receptor and causes a specific response
• C. Binds to a receptor and prevents a response
• D. Opposes the receptor function

Answer: (C)

Antagonists work by binding to a receptor without activating it, preventing the receptor from producing a response to natural signals or other drugs. In other words, they occupy the receptor and block its signaling, so the usual effect you’d expect from an agonist doesn’t happen. This is why the action is described as binding to a receptor and preventing a response.

This concept is clear in practice: naloxone blocks opioid receptors to reverse opioid effects, while beta-blockers occupy beta-adrenergic receptors to blunt sympathetic stimulation. If a drug were to prevent chemicals from reaching the receptor, that would be acting at a different level than receptor signaling; if it bound and caused a response, that would be an agonist; and if it merely opposed receptor function without specifying receptor interaction, it wouldn’t be a precise description of receptor antagonism.