Naltrexone is an opioid receptor antagonist. Because of its long-lasting effects, it’s primarily used to reduce alcohol dependence as well as opioid dependence by reducing cravings. Remember that naltrexone is long-acting so it cannot be used to reverse acute opioid intoxication.

Key Points

  • Naltrexone
    • Mechanism
      • Opioid receptor antagonist
        • Onset within 1 hour (slow-acting)
        • Contrast vs. Naloxone, which acts quickly
    • Indications
      • Alcohol dependence
        • 1st-line treatment for alcoholism
      • Opioid dependence
        • Blocks rewarding/reinforcing effects and reduces cravings
      • Effects are slow (cannot be used for acute intoxication)
        • Often given as a long-acting (1 month long) depot injection to increase compliance
    • Methylnaltrexone
      • Mechanism
        • Opioid receptor antagonist
          • Similar to naltrexone, except methyl group makes molecule too large to cross blood-brain-barrier (peripheral only)
      • Indications
        • Treats opioid-induced constipation
        • Does not cause opioid withdrawal (due to lack of CNS penetration)