Antagonists are drugs that

• A. Mimic physiological activity
• B. Activate receptors directly
• C. Increase the effect of an agonist
• D. Decrease the effect of an agonist without stimulatory action

Answer: (D)

Antagonists work by blocking receptor activation. They bind to the same receptor sites as the natural ligand or an agonist but do not produce a stimulatory response themselves. By occupying the receptor, they prevent the agonist from binding or signaling, which lowers the overall effect the agonist can produce. Some antagonists can be overcome by higher levels of agonist (competitive antagonists), while others reduce the maximal response and can’t be overcome (noncompetitive antagonists). That’s why the correct idea is that antagonists decrease the effect of an agonist without providing stimulation themselves. The other options describe agonist actions or increasing effects, which is not what antagonists do.