Agonist vs Antagonist

Agonist

• Binds to a receptor and produces the receptor’s expected physiological response.
• Think “super-hero”: works with the body’s normal signalling.

Antagonist

• Binds to a receptor but blocks or diminishes the expected response.
• Think “villain”: prevents the normal action at that site.

Key Drug Examples

• Epinephrine (Agonist)
  • Targets adrenergic receptors.
  • Triggers sympathetic / fight-or-flight effects: $\uparrow$ heart rate, $\uparrow$ blood pressure, bronchodilation.
• Naloxone (Narcan) (Antagonist)
  • Targets opioid receptors with higher affinity than opioids (e.g., fentanyl).
  • Displaces opioids, blocks their action, reverses respiratory depression and CNS depression.
  • Opioids such as fentanyl are agonists at the same receptors (produce analgesia, sedation, $\downarrow$ respiratory drive).

Quick Comparison

• Produces expected response: Agonist | Blocks expected response: Antagonist
• Example: Epinephrine | Example: Naloxone
• Mechanism: Stimulates receptor | Mechanism: Occupies receptor without stimulation