Naltrexone is a mu opioid receptor BLANK.

• A. Partial agonist
• B. Antagonist
• C. Agonist
• D. Inverse agonist

Answer: (B)

Naltrexone acts as a mu opioid receptor antagonist. It binds with high affinity to the mu receptor but has little to no intrinsic activity, so it blocks activation by opioids without producing opioid effects itself. This competitive antagonism prevents opioids from producing euphoria or analgesia and can precipitate withdrawal in someone opioid-dependent.

In contrast, a partial agonist would activate the receptor to some degree, an agonist would fully activate it, and an inverse agonist would reduce any baseline receptor activity. Naltrexone’s ability to block effects without activating the receptor is why the best choice is antagonist.