Mechanism of action
Naltrexone is a competitive antagonist at all three major opioid receptor subtypes: mu (μ), kappa (κ), and delta (δ), with highest affinity for the mu-opioid receptor. By occupying these receptors without activating them, naltrexone blocks the binding of endogenous opioids (endorphins, enkephalins) and exogenous opioids (morphine, heroin, oxycodone).
In opioid dependence, naltrexone prevents the euphoric and reinforcing effects of opioid agonists. If a patient attempts to use opioids while on naltrexone, the opioids cannot bind effectively to their receptors, eliminating the reward and reducing the motivation for continued use. This approach is effective only after complete detoxification, as naltrexone will precipitate severe withdrawal in physically dependent individuals.
In alcohol use disorder, the precise mechanism is less clear but involves modulation of the endogenous opioid system. Alcohol consumption increases endorphin release, which contributes to its rewarding effects. By blocking opioid receptors, naltrexone reduces the pleasurable effects of alcohol and decreases craving, thereby supporting abstinence or reduced drinking.
Naltrexone has no agonist activity and does not produce physical dependence or tolerance. Its effects are purely antagonistic, distinguishing it from partial agonists like buprenorphine.
- Naltrexone is a competitive opioid receptor antagonist used for alcohol use disorder and opioid dependence maintenance.
- Precipitated withdrawal is the key risk: never give naltrexone to opioid-dependent patients without 7–10 days of abstinence.
- Naltrexone blocks the effects of opioid analgesics, complicating acute pain management.
- Hepatotoxicity can occur; check baseline liver function and avoid in acute hepatitis or liver failure.
- Available as daily oral tablets or monthly intramuscular injection (extended-release) for improved adherence.
- Naltrexone has no abuse potential and is not a controlled substance.
- After stopping naltrexone, patients have reduced opioid tolerance and are at high risk for overdose if they relapse.
- Unlike naloxone (used for acute overdose), naltrexone is for chronic relapse prevention and has a long duration of action.
Indications
Naltrexone is FDA-approved for:
- Alcohol use disorder: As part of a comprehensive treatment program including psychosocial support. Naltrexone reduces heavy drinking days and promotes abstinence. Both oral and extended-release injectable formulations are effective.
- Opioid dependence: For maintenance of opioid abstinence after successful detoxification. The patient must be opioid-free for at least 7–10 days (longer for long-acting opioids like methadone) to avoid precipitated withdrawal.
Off-label uses include:
- Impulse control disorders
- Self-injurious behavior in autism spectrum disorder
- Pruritus (itching) in cholestatic liver disease
- Binge eating disorder
Naltrexone does not treat acute opioid withdrawal or intoxication and is not a substitute for detoxification. It is used for relapse prevention, not acute management.
Pharmacokinetics
Absorption: Naltrexone is well absorbed orally, with peak plasma concentrations reached in approximately 1 hour. Bioavailability is around 5–40% due to extensive first-pass hepatic metabolism.
Distribution: Widely distributed throughout the body. Naltrexone crosses the blood-brain barrier to exert its central effects.
Metabolism: Extensively metabolized in the liver to 6-β-naltrexol, the major active metabolite, which also has opioid antagonist activity. Further metabolism produces inactive conjugates.
Excretion: Primarily renal. The elimination half-life of naltrexone is approximately 4 hours, but 6-β-naltrexol has a longer half-life of about 13 hours, contributing to sustained receptor blockade.
Formulations:
- Oral tablets: Typically taken once daily (often 50 mg). Receptor blockade lasts 24–48 hours depending on dose.
- Extended-release intramuscular injection: Administered monthly (380 mg IM), providing sustained plasma levels and improved adherence compared to daily oral dosing.
The long-acting injectable formulation reduces the burden of daily adherence and is particularly useful in patients with poor medication compliance.
Adverse effects
Common adverse effects
- Gastrointestinal: Nausea, vomiting, abdominal pain, diarrhea, constipation
- Neuropsychiatric: Headache, dizziness, anxiety, insomnia, fatigue, nervousness
- Injection site reactions: Pain, tenderness, induration, swelling (with extended-release IM formulation)
- Musculoskeletal: Arthralgia, myalgia
Serious adverse effects
- Hepatotoxicity: Dose-dependent liver injury has been reported, particularly at doses higher than recommended. Baseline and periodic monitoring of liver function tests is advised. Naltrexone is contraindicated in acute hepatitis or liver failure.
- Precipitated opioid withdrawal: If given to a patient with current opioid dependence, naltrexone can cause acute, severe withdrawal symptoms (agitation, myalgias, nausea, vomiting, diarrhea, tachycardia, hypertension). This is a critical board exam concept. Always verify opioid-free status (at least 7–10 days) before initiating naltrexone. A naloxone challenge test may be used to confirm absence of physical dependence.
- Depression and suicidality: Reports exist of depressive symptoms and suicidal ideation, though causality is unclear. Patients with alcohol or opioid use disorders have elevated baseline suicide risk.
- Opioid analgesic blockade: Naltrexone blocks the effects of opioid analgesics, complicating pain management. In emergencies requiring analgesia, higher opioid doses, regional anesthesia, or non-opioid analgesics may be necessary.
- Eosinophilic pneumonia: Rare but serious, associated with the injectable formulation.
Contraindications and warnings
Absolute contraindications
- Current physiologic dependence on opioids (risk of precipitated withdrawal)
- Acute opioid withdrawal
- Concurrent use of opioid analgesics
- Acute hepatitis or liver failure
- Hypersensitivity to naltrexone or any formulation component
Warnings and precautions
- Hepatic impairment: Use with caution and monitor liver function. Dose adjustment may be needed; avoid in severe impairment.
- Renal impairment: Caution advised, though specific dosing adjustments are not well established.
- Opioid analgesia: Patients on naltrexone may not respond to usual doses of opioid analgesics. In emergencies (trauma, surgery), higher opioid doses under close monitoring, regional techniques, or non-opioid agents should be used. Patients should carry a warning card or wear medical identification.
- Opioid overdose risk: After discontinuation of naltrexone, patients may have reduced opioid tolerance. Resumption of prior opioid doses can lead to fatal overdose.
- Pregnancy: Category C. Use only if benefit justifies potential risk. Data are limited.
- Lactation: Excreted in breast milk; use with caution.
Patients should be fully informed of risks, especially precipitated withdrawal and interference with opioid analgesia.
Drug interactions
- Opioid agonists: Naltrexone blocks the therapeutic effects of all opioid medications, including codeine, tramadol, morphine, oxycodone, hydrocodone, fentanyl, and methadone. Co-administration negates opioid efficacy and can precipitate withdrawal.
- Thioridazine: Concurrent use may increase lethargy and somnolence. Caution is advised.
- Disulfiram: When used together for alcohol use disorder, monitor for hepatotoxicity, as both agents can affect liver function.
- Other hepatotoxic drugs: Additive risk of liver injury when combined with other hepatotoxic agents (e.g., acetaminophen at high doses, certain antiretrovirals).
- Antidepressants and anxiolytics: No major pharmacokinetic interactions, but overlapping CNS effects (sedation, dizziness) should be monitored.
Naltrexone does not significantly inhibit or induce cytochrome P450 enzymes, so major pharmacokinetic drug interactions are uncommon.
Clinical pearls
- Always confirm opioid-free status before starting naltrexone. The standard recommendation is 7–10 days abstinence from short-acting opioids and up to 14 days for long-acting opioids. Urine drug screens and naloxone challenge tests help confirm safety.
- Naltrexone vs naloxone: Naltrexone is used for chronic maintenance therapy (oral or monthly injection), while naloxone is used for acute opioid overdose reversal (IV, IM, intranasal). Both are competitive antagonists, but naloxone has a shorter half-life and poor oral bioavailability.
- Injectable naltrexone improves adherence. The once-monthly intramuscular formulation (Vivitrol) eliminates daily pill-taking, which is critical in populations with adherence challenges.
- Managing pain in naltrexone patients: Non-opioid analgesics (NSAIDs, acetaminophen), regional anesthesia, or temporarily discontinuing naltrexone (if safe) are preferred strategies. In true emergencies, large opioid doses under monitoring or rapid-acting opioids may override blockade, but this carries risks.
- No abuse potential: Naltrexone is not a controlled substance. It has no euphoric or rewarding properties and does not cause dependence.
- Hepatotoxicity is dose-related: Standard doses carry low risk, but liver function should be checked at baseline and periodically. Avoid in patients with active liver disease.
- Patient selection is key: Naltrexone works best in motivated patients who have completed detoxification and are engaged in psychosocial treatment. It is not effective during active opioid use.
- Consider in alcohol use disorder even without complete abstinence: Evidence supports naltrexone for reducing heavy drinking, not only for maintaining total abstinence. The Sinclair Method involves taking naltrexone 1 hour before planned drinking to reduce reinforcement.
Practice questions
1. A 34-year-old man with a history of opioid use disorder completed detoxification 5 days ago and now presents to the clinic requesting medication to help maintain abstinence. He reports no opioid use since detoxification ended. Physical examination is unremarkable. Urine drug screen is positive for opioids. Which of the following is the most appropriate next step in management?
2. A 42-year-old woman with alcohol use disorder has been taking oral naltrexone 50 mg daily for 3 months with good adherence and reduced drinking. She is involved in a motor vehicle accident and sustains a fractured femur requiring surgical fixation. Which of the following is the most important consideration for perioperative pain management?
3. A 29-year-old man with opioid use disorder has been receiving monthly intramuscular naltrexone injections for 6 months. He reports feeling well and denies cravings. Laboratory tests show ALT 180 U/L (normal <40) and AST 150 U/L (normal <40). He denies alcohol use, and hepatitis B and C serologies are negative. Which of the following is the most likely cause of his elevated liver enzymes?
Frequently asked questions
How does naltrexone work?
Naltrexone is a competitive antagonist at opioid receptors (mu, kappa, delta), blocking the binding of both endogenous and exogenous opioids. In opioid dependence, it prevents the euphoric effects of opioids, reducing relapse risk. In alcohol use disorder, it diminishes the rewarding effects of alcohol by blocking endorphin-mediated reinforcement, thereby decreasing craving and heavy drinking.
What are the side effects of naltrexone?
Common side effects include nausea, headache, dizziness, fatigue, and injection site reactions with the IM formulation. Serious adverse effects include hepatotoxicity (dose-dependent liver injury), precipitated opioid withdrawal if given to dependent patients, and blockade of opioid analgesics complicating pain management. Depression and suicidal ideation have been reported but causality is uncertain.
Is naltrexone a controlled substance?
No, naltrexone is not a controlled substance. It has no abuse potential, does not produce euphoria, and does not cause physical dependence or tolerance. Because it is a pure antagonist without agonist activity, there is no reinforcing or rewarding effect associated with its use.
Naltrexone vs buprenorphine — what is the difference?
Naltrexone is a pure opioid antagonist used for relapse prevention after detoxification; it requires the patient to be opioid-free for 7–10 days before starting. Buprenorphine is a partial mu-opioid agonist used for both detoxification and maintenance therapy in opioid dependence, and it can be started during withdrawal. Buprenorphine has some agonist effects and is a controlled substance, while naltrexone has none.
Can you take naltrexone if you are still using opioids?
No. Taking naltrexone while physically dependent on opioids will precipitate acute, severe withdrawal symptoms including agitation, muscle aches, nausea, vomiting, diarrhea, and autonomic instability. Patients must be completely detoxified and opioid-free for at least 7–10 days (longer for long-acting opioids) before naltrexone initiation. A naloxone challenge test can confirm safety before starting treatment.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N07BB04
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.