Which statement about the opioid activity of butorphanol is most accurate?

• A. Mu antagonist
• B. Mu full agonist
• C. Mu partial agonist
• D. Mu antagonist and partial agonist

Answer: (A)

Opioid drugs can affect different receptor types in distinct ways. Butorphanol is a mixed opioid analgesic that produces pain relief mainly by activating kappa receptors, while its influence at mu receptors is opposite to typical mu agonists. It acts as a mu antagonist (or at least a very weak mu agonist), not as a mu full agonist or a true mu partial agonist. This combination explains why it’s described as a mu antagonist: it blocks mu receptor activity while still providing analgesia through the kappa pathway. Because it blocks mu receptors, it can precipitate withdrawal in people who are dependent on mu agonists, and its overall mu-mediated side effects (like strong euphoria and significant respiratory depression) are limited compared to classic mu agonists.