What is naloxone's mechanism of action?

• A. Beta-blocker
• B. Cholinesterase inhibitor
• C. Opioid receptor agonist
• D. Opiate antagonist

Answer: (D)

Naloxone reverses opioid effects by blocking opioid receptors. It is a pure opioid receptor antagonist with high affinity for mu receptors, which are mainly responsible for respiratory depression and other opioid effects. As a competitive antagonist, it binds to these receptors and displaces any opioid molecules already attached, rapidly reversing sedation, slowed respiration, and pinpoint pupils. It has little to no intrinsic activity, so it does not produce opioid-like effects on its own. Because its action is quick but shorter than many opioids, its effects can wear off while the opioid remains in the body, so ongoing monitoring and possible repeat dosing are important. The other options do not fit: a beta-blocker affects heart and blood vessels, a cholinesterase inhibitor increases acetylcholine, and an opioid receptor agonist would enhance opioid effects rather than reverse them.