NalBuphine's action on opioid receptors is best described as?

• A. competitive agonist
• B. noncompetitive antagonist
• C. competitive antagonist
• D. agonist

Answer: (C)

Nalibuphine shows a mixed action at opioid receptors: it acts as a kappa receptor agonist and as a mu receptor antagonist. The mu antagonism works by competing with other mu agonists for the same receptor sites, a reversible process known as competitive antagonism. Because it competes for the same binding site, higher levels of mu agonists can displace it and restore mu-mediated activity, which is why this description fits best. It’s not noncompetitive antagonism, which would involve irreversible binding or a change that can’t be overcome by more agonist. While it does activate kappa receptors to provide analgesia, the mu-receptor action—being a competitive antagonist—best characterizes its overall receptor pharmacology.