Which statement best defines an agonist?

• A. A drug that binds to and activates a receptor
• B. A drug that binds to a receptor but does not produce a response
• C. A drug that blocks a receptor without activating it
• D. A drug with no effect on a receptor

Answer: (A)

An agonist has both affinity for a receptor and the ability to activate it, triggering the receptor’s signaling cascade and producing a cellular and physiological response. This is why the best choice is a drug that binds to and activates a receptor—the defining feature of an agonist is that binding leads to activation and effect.

In practice, this means a true agonist not only attaches to the receptor but also induces a change that starts downstream signaling. This contrasts with a drug that binds but does not activate the receptor (an antagonist) and with a drug that has no effect on the receptor at all (inert or non-binding). Partial agonists also bind and activate but produce only a submaximal response compared with full agonists. For example, an opioid agonist activates opioid receptors to produce analgesia, whereas an antagonist would block those effects by occupying the receptor without activating it.