Noncompetitive antagonists are drugs that:

• A. Bind to the same receptor site as the agonist
• B. React with some receptor sites preventing the reaction of another chemical with a different receptor site
• C. Inhibit metabolism of drugs
• D. Activate receptors directly

Answer: (B)

Noncompetitive antagonists reduce the maximum effect an agonist can produce by binding to a site on the receptor that is different from the agonist’s binding site (an allosteric site) or by binding irreversibly. This alters the receptor’s conformation so it cannot be activated effectively, even when the agonist is present in high concentrations. Because the antagonist isn’t competing for the same binding site, simply raising the amount of agonist can’t fully overcome the inhibition, unlike competitive antagonists that can be outcompeted. So the described action fits noncompetitive antagonism.

Binding to the same site as the agonist describes competitive antagonism; inhibiting metabolism is a pharmacokinetic interaction, not a receptor-site effect; activating receptors directly describes an agonist, not an antagonist.