An antagonist is defined as which?

• A. A drug that binds to a receptor and activates it
• B. A drug that binds to a receptor but does not produce a response
• C. A drug that enhances a receptor's response
• D. A drug that competes with oxygen for binding sites

Answer: (B)

An antagonist binds to a receptor without producing a response of its own, which blocks or reduces the effect of any agonist trying to activate that receptor. This is why antagonists can reverse or blunt drug effects: they occupy the binding site and prevent activation. For example, naloxone binds opioid receptors but does not activate them, displacing opioids and reversing their effects. Antagonists can be competitive, meaning their effect can be overcome by more agonist, or non-competitive, where the receptor is inhibited regardless of agonist amount. This contrasts with an agonist, which activates the receptor, or a drug that enhances the receptor’s response, which increases activity. The idea of a compound competing with oxygen for binding sites lies outside receptor antagonism and relates to other biochemical interactions.