The success of naloxone in reversing opioid overdoses led to the search for longer acting opioid antagonists. Naltrexone and nalmefene are two longer acting opioid antagonists that possess no opioid agonist activity. They are both FDA approved for a wide range of uses in both non dependent and opioid dependent individuals.
Naltrexone is available only in the oral form.
It is a modification of naloxone made by adding a carbonyl group to C-6. Once taken, absorption is rapid and almost complete. However naltrexone has an extensive first pass hepatic metabolism so that bioavailability is poor. For ethanol detoxification, naltrexone doses are given at 50 mg per day, or 100 mg every other day, or 150 mg three times per week. In opioid dependence, a 25 mg test dose is given.