A drug that causes a direct action on a receptor site is called?

• A. Antagonist
• B. Agonist
• C. Inhibitor
• D. Enzyme

Answer: (B)

Understanding how drugs interact with receptors is key. A drug that directly activates a receptor does so by binding to the receptor and turning on its signaling pathway, just like the natural messenger would. That direct activation is what an agonist does—it mimics the body’s own ligand and produces a biological response.

So the best term is agonist because it describes this activating action at the receptor site. An antagonist would bind the receptor but block activation, preventing a response. An inhibitor targets enzymes, reducing their activity rather than activating receptors. An enzyme is not a receptor-targeting drug at all. For example, morphine acts as an agonist at opioid receptors, producing analgesia by activating those receptors.