naltrekson

Naltrekson, more commonly spelled naltrexone, is an opioid receptor antagonist used in the management of opioid use disorder and, in some cases, alcohol use disorder. It binds competitively to mu, kappa, and delta opioid receptors with high affinity, blocking the effects of exogenous opioids and reducing the rewarding properties of alcohol in some patients. It is not a medication for acute opioid intoxication; naloxone is used for emergency reversal of overdose.

Medical uses and formulations

Naltrexone is indicated for preventing relapse in opioid dependence after detoxification and for reducing heavy drinking and relapse in adults with alcohol dependence. It is available as an oral tablet, typically 50 mg once daily, and as an extended-release injectable formulation (380 mg IM) administered every four weeks (often under the brand name Vivitrol). Initiation with a brief period of oral therapy may be used to assess tolerance and to ensure opioid abstinence before starting the injectable form.

Mechanism and pharmacology

As a competitive antagonist at mu receptors, naltrexone blocks opioid agonists and diminishes their subjective effects. The oral drug undergoes substantial first-pass metabolism, producing an active metabolite, 6-beta-naltrexol. The oral half-life is relatively short, while clinical blockade may last beyond the plasma half-life due to receptor occupancy. The extended-release formulation provides a prolonged once-monthly effect.

Contraindications and precautions

Naltrexone should not be started in individuals currently dependent on opioids or who have not been opioid-free for a period (to avoid precipitated withdrawal). It is used with caution in liver disease and requires monitoring of liver function, as hepatotoxicity has been reported, particularly at higher doses. Common side effects include nausea, headache, dizziness, fatigue, and injection-site reactions (for the injectable form).