- Affinity = How well a drug binds.
- Efficacy (intrinsic activity) = What happens after binding.
A drug may have:
- High affinity + High efficacy → Full agonist (e.g. morphine)
- High affinity + Low efficacy → Partial agonist (e.g. buprenorphine)
- High affinity + Zero efficacy → Competitive antagonist (e.g. naloxone)
Question: Can a drug with zero efficacy still produce a profound clinical effect?
Answer: Yes.
Competitive antagonists such as naloxone and flumazenil have zero efficacy, yet they produce dramatic clinical effects by blocking the action of endogenous ligands or agonist drugs, not by activating the receptor themselves.
