Which drug is a pure opioid antagonist used as a reversal agent for morphine and fentanyl?

• A. Naltrexone
• B. Naloxone
• C. Buprenorphine
• D. Methadone

Answer: (B)

Reversal of opioid overdose relies on a drug that competes with opioid agonists at the same receptors without producing its own opioid effect. The best choice is naloxone because it is a pure opioid antagonist with very high affinity for mu receptors and no intrinsic analgesic activity. When given, it quickly displaces morphine or fentanyl from the receptors, reversing respiratory depression and coma from opioids while providing a short, predictable duration of action.

Naltrexone, while also an opioid antagonist, has a much longer duration and is typically used for addiction treatment rather than acute overdose reversal, and it can be less practical in emergency settings. Buprenorphine is a partial agonist at the mu receptor, not an antagonist, so it doesn’t reliably reverse opioid effects and can even precipitate withdrawal in dependent individuals. Methadone is a long-acting opioid agonist, not an antagonist, so it would worsen, not reverse, opioid toxicity.

Naloxone’s rapid onset, short duration, and pure antagonism at mu receptors make it the standard reversal agent for opioids like morphine and fentanyl.