False
Naltrexone is an opioid antagonist, not an agonist. 1
Mechanism of Action
Naltrexone functions as a pure competitive opioid receptor antagonist that blocks opioid receptors rather than activating them 1. Specifically:
- Naltrexone competitively binds to mu, kappa, and delta opioid receptors, preventing opioid agonists from binding and producing their effects 2
- The drug markedly attenuates or completely blocks the subjective effects of intravenously administered opioids through reversible competitive binding 1
- This antagonist action is analogous to competitive enzyme inhibition, where naltrexone occupies receptor sites without activating them 1
Clinical Implications of Antagonist Properties
The antagonist nature of naltrexone has critical clinical consequences:
- Naltrexone blocks the euphoric effects of opioids by preventing opioid agonists from activating reward pathways 3
- In patients physically dependent on opioids, naltrexone will precipitate withdrawal symptoms because it displaces opioid agonists from receptors 1
- The medication cannot be used in patients requiring opioid analgesics because it blocks pain relief from opioid agonist medications 3
- 50 mg of naltrexone blocks the pharmacologic effects of 25 mg of intravenously administered heroin for up to 24 hours, demonstrating potent antagonist activity 1
Common Pitfall to Avoid
Do not confuse naltrexone with opioid agonists like methadone or partial agonists like buprenorphine—naltrexone has no agonist properties and produces no opioid-like effects 1. The drug has few, if any, intrinsic actions besides its opioid blocking properties and is not associated with the development of tolerance or dependence 1.