Butorphanol is classified as what type of opioid receptor activity?

• A. Mu antagonist
• B. Mu agonist
• C. Kappa agonist
• D. Delta agonist

Answer: (A)

Butorphanol behaves as a mixed opioid ligand, with different actions at different receptors. Its analgesic effect mainly comes from activating the kappa receptors, but its action at mu receptors is antagonistic, meaning it blocks mu receptor activity. That mu antagonism is why it’s classified as a mu receptor–antagonist in this context. This antagonism also explains why butorphanol can precipitate withdrawal in someone dependent on mu-opioid agonists, even though it provides pain relief mainly through kappa activation. The other activities aren’t the primary driver of its mu-receptor profile, and while it does act at kappa receptors, the question specifically targets mu receptor activity, making mu antagonism the best answer.