Naltrexone

Introduction Overview

Generic Name

Naltrexone

Brand Name

Vivitrol, Naltrexone Tablets

Dosage Form

intramuscular injection (380 mg), oral tablet (50 mg)

Drug Class

Antidotes, Drugs used in alcohol dependence

Pharmacokinetics

Absorption:
Naltrexone is well absorbed orally, reaching peak plasma concentrations within about 1 hour after administration. When administered intramuscularly as the extended-release injection (Vivitrol), the drug exhibits a biphasic absorption pattern with an initial peak around 2 hours post-injection followed by a secondary peak occurring 2 to 3 days later. This extended-release formulation maintains therapeutic plasma levels for approximately 28 days, allowing once-monthly dosing. Oral bioavailability is reduced due to significant first-pass hepatic metabolism. Food does not significantly affect absorption of the oral formulation.

Distribution:
Once absorbed, naltrexone is extensively distributed into body tissues, with a volume of distribution approximately 14 liters/kg. It crosses the blood-brain barrier, exerting its effects centrally at opioid receptors. It binds moderately to plasma proteins (~21%). The intramuscular formulation consists of polylactide-co-glycolide microspheres that slowly release naltrexone over time, resulting in sustained systemic distribution. The drug achieves steady therapeutic levels in cerebrospinal fluid correlating with plasma concentrations.

Metabolism:
Naltrexone undergoes extensive hepatic metabolism predominantly via dihydrodiol dehydrogenase enzymes to form the major active metabolite 6-beta-naltrexol, which also exhibits opioid antagonist properties but with lower potency. Both naltrexone and 6-beta-naltrexol contribute to the clinical effects. Metabolic pathways include reduction and conjugation. The metabolism exhibits some variability based on hepatic function; elevated liver enzymes or hepatic impairment can reduce clearance.

Elimination:
Elimination occurs primarily through renal excretion of metabolites, with only a small proportion of unchanged drug recovered in urine. The plasma elimination half-life of oral naltrexone is approximately 4 hours, whereas the active metabolite 6-beta-naltrexol has a half-life of about 13 hours. The extended-release IM formulation maintains detectable plasma levels for up to a month. Renal impairment can alter excretion, but dosage adjustments are generally not required except in severe impairment. Recognizing elimination parameters is vital for avoiding toxicity especially in hepatic disease.

Dosage Forms

Available Dosage Forms:
- Oral tablet: 50 mg (generic formulations available; branded tablets such as Revia and Depade have been discontinued but generics are widely used).
- Intramuscular injection: Naltrexone extended-release injection (Vivitrol) 380 mg suspension in prefilled syringe for gluteal injection, administered every 4 weeks.

Dosages for Adults and Adolescents:
- Alcohol dependence oral dosing: 50 mg once daily.
- Opioid use disorder oral dosing: Initiate with 25 mg once daily to assess opioid withdrawal symptoms, then increase to 50 mg daily if tolerated.
- Alcohol dependence and opioid use disorder IM dosing: 380 mg intramuscular injection every 4 weeks.
Notes: Therapy requires patients to be opioid-free for at least 7 to 14 days prior to initiation to avoid precipitated withdrawal.

Specific Patient Populations:
- Not approved for use in children and adolescents under 18 years.
- Use with caution in patients with hepatic impairment; liver function monitoring is recommended.
- No specific dose adjustment needed for renal impairment unless severe.
- Pregnant or breastfeeding women: use only if benefits outweigh risks; limited data regarding safety.
- Patients transitioning from opioid maintenance therapies (methadone or buprenorphine) may require careful monitoring for withdrawal risk; no established protocol for switching.

Clinical Guidelines:
- Dosage should be individualized within comprehensive addiction treatment programs including counseling.
- Missed doses: For oral, take as soon as remembered unless near next dose; for injection, reschedule promptly. Injection therapy must be administered by healthcare professionals only.

Mechanism Of Action

Naltrexone is a competitive antagonist primarily acting at the mu-opioid receptor (MOR), with additional antagonistic activity at kappa- and delta-opioid receptors. By binding to these receptors without activating them, naltrexone effectively blocks the euphoric and reinforcing effects of exogenous opioids such as heroin, morphine, and codeine, preventing opioid-induced activation of the central nervous system's reward pathways. This blockade reduces the physiological and psychological reinforcement linked with opioid intake, thereby supporting abstinence.

In alcohol use disorder, the exact mechanisms are multifaceted and involve modulation of the hypothalamic-pituitary-adrenal (HPA) axis and opioid receptor-mediated pathways. Naltrexone reduces dopamine release in the nucleus accumbens by blocking opioid receptors, thereby attenuating the rewarding effects of alcohol consumption. The interaction with the HPA axis affects neuroendocrine responses, contributing to decreased alcohol craving and consumption. These effects collectively reduce relapse risk and promote sustained abstinence.

Cellular pathways modulated include inhibition of opioid receptor-mediated G-protein coupled receptor (GPCR) signaling, leading to decreased cyclic AMP (cAMP) modulation and reduction of intracellular calcium influx in neurons related to reward processing. Naltrexone’s blockade of opioid receptors also prevents downstream activation of MAP kinase pathways involved in addiction reinforcement.

Clinical studies demonstrate that naltrexone reduces heavy drinking days, craving intensity, and relapse rates in alcohol use disorder and prevents opioid-induced euphoria in opioid dependence. Compared to other anti-craving agents such as acamprosate, naltrexone exerts a direct opioid receptor blockade, whereas acamprosate modulates glutamatergic neurotransmission. Research suggests combining naltrexone with behavioral therapies enhances treatment outcomes. Novel uses of low-dose naltrexone exploit its immunomodulatory properties via Toll-like receptor 4 antagonism, providing anti-inflammatory effects in certain chronic pain and autoimmune conditions.

Pharmacological Actions

Competitive antagonist at mu-opioid receptors, blocking opioid effects.
Blocks kappa and delta opioid receptors with lower affinity, contributing to broad opioid antagonism.
Modulates hypothalamic-pituitary-adrenal axis to reduce alcohol consumption and craving.
Decreases dopaminergic neurotransmission in reward pathways, attenuating reinforcement.
Inhibits opioid receptor-mediated GPCR signaling and downstream intracellular pathways.
Reduces physiological and psychological dependence on opioids and alcohol.
Suppresses euphoria associated with opioid use and alcohol intoxication effects.
Active metabolite 6-beta-naltrexol contributes to overall opioid antagonism.
Extended-release formulations provide sustained opioid receptor blockade.
Low-dose naltrexone exhibits anti-inflammatory actions through Toll-like receptor 4 antagonism (off-label).
Reduces heavy drinking and relapse rates in alcohol use disorder.
Precipitates opioid withdrawal if administered before opioid abstinence is achieved.

Therapeutic Or Clinical Uses

Treatment of alcohol use disorder to reduce cravings and relapse.
Prevention of relapse to opioid dependence post detoxification.
Blockade of the euphoric and sedative effects of exogenous opioids.
Adjunct in comprehensive addiction treatment programs with counseling and psychosocial support.
Off-label low-dose naltrexone use in fibromyalgia, Crohn’s disease, multiple sclerosis, and complex regional pain syndrome for anti-inflammatory effects.
Management of opioid overdose risk by reducing the likelihood of misuse during recovery.
Facilitating opioid abstinence maintenance therapy.
Support in patients with dual diagnosis of alcohol and opioid dependence (with caution).
Reduction of heavy drinking days as supported by clinical trials.
Improvement of treatment adherence owing to monthly injectable formulation.
Utility in opioid overdose emergency protocol as a receptor antagonist (distinct from naloxone).

Side Or Adverse Effects

Nausea and vomiting, particularly at treatment initiation.
Loss of appetite and weight changes including possible anorexia.
Headache and dizziness, sometimes leading to drowsiness or fatigue.
Musculoskeletal complaints including joint pain and muscle cramps.
Sleep disturbances such as insomnia.
Injection site reactions: pain, swelling, redness, bruising, hard lumps; potential severe reactions including injection site necrosis.
Tooth pain and upper respiratory symptoms (stuffy nose, sneezing, sore throat).
Anxiety, nervousness, or mood changes.
Signs of hypersensitivity or allergic reactions: hives, facial swelling, breathing difficulties.
Severe liver injury manifesting as abdominal pain, jaundice, dark urine, and elevated liver enzymes.
Precipitated opioid withdrawal symptoms if used in presence of opioids: sweating, fever, chills, diarrhea, muscle aches, restlessness.
Rare neurological or psychiatric symptoms including depressive mood and suicidal ideation.
Potential respiratory depression risk if opioids overcome antagonist effects post-treatment.
Gastrointestinal disturbances including severe diarrhea and abdominal pain.
Fatigue and malaise often reported in extended treatment courses.

Precautions

Must achieve opioid-free status for a minimum of 7 to 14 days prior to initiation to avoid precipitated withdrawal.
Use caution in patients with hepatic impairment; monitor liver function tests regularly.
Avoid concomitant opioid use during treatment to prevent withdrawal or overdose risks.
Monitor appropriately for injection site reactions, especially with extended-release formulation.
Patients with bleeding disorders or history of hemophilia should be closely observed due to injection risks.
Inform all healthcare providers about naltrexone use prior to procedures requiring anesthesia; use non-opiate anesthetics when possible.
Pregnancy and breastfeeding should be evaluated carefully; benefits must outweigh risks.
Use in adolescents under 18 years is not recommended as safety and efficacy are not established.
Patients should carry medical alert identification detailing naltrexone use.
Be vigilant for depressive symptoms or suicidal ideation during treatment.
Adjust treatment programs to address potential interactions with other medications, including over-the-counter and herbal supplements.
Counsel on avoiding alcohol abuse despite reduced euphoric effects; naltrexone does not prevent intoxication.
Carefully assess patients transitioning from methadone or buprenorphine due to potential higher risk of withdrawal symptoms.
Educate patients on risk of overdose due to increased opioid sensitivity after discontinuation of naltrexone.
Advise against driving or operating machinery until individual response to medication is established.

Warnings

Risk of precipitated opioid withdrawal if initiated before complete opioid detoxification.
Potential for severe injection site reactions including tissue necrosis requiring surgical intervention.
Risk of hepatotoxicity; monitor liver enzymes before and during therapy; discontinue if significant liver damage occurs.
Increased risk of opioid overdose and death after discontinuing naltrexone due to loss of tolerance and increased sensitivity.
Naltrexone blocks effects of opioid analgesics, potentially complicating pain management in emergency or surgical settings.
Do not use in patients currently dependent on opioids or undergoing opioid withdrawal.
Risk of severe allergic reactions including anaphylaxis.
Risk of depression, suicidal thoughts, and unusual changes in mood or behavior during treatment.
Use caution in patients with bleeding disorders or compromised immune systems.
Injection must be administered by healthcare professionals; improper administration can cause serious harm.
Avoid abrupt discontinuation without medical supervision.
Potential interaction with opioid-containing cough, cold, or diarrhea medications leading to unexpected adverse effects.
Naltrexone is not a cure for addiction and should be used in conjunction with comprehensive treatment strategies.
Patients should be informed of the risk of decreased opioid effect and need to inform all healthcare providers.
Missed doses can reduce efficacy and increase risk of relapse.

Contraindication

Active opioid use or opioid dependence.
Current opioid intoxication or experiencing opioid withdrawal symptoms.
Known hypersensitivity or allergy to naltrexone or any component of the formulation.
Patients who have failed naloxone challenge test or urine screening positive for opioids within the required opioid-free window.
Use of opioid analgesics, including fentanyl, methadone, buprenorphine, tramadol, or combination products, within 7 to 14 days prior to initiation.
Severe hepatic impairment or acute hepatitis.
Pregnancy unless the potential benefits justify the risks.
Patients with bleeding disorders where intramuscular injection poses significant risk.
Children and adolescents under 18 years (not established safe or effective).
Concomitant use with opioid-based anesthetics without careful monitoring and dosage adjustments.
Patients with severe renal impairment where safety has not been established.
Patients unwilling or unable to adhere to comprehensive addiction management programs.

Drug Interactions

Opioid analgesics and opioid-containing medications – naltrexone blocks their effects and may precipitate withdrawal.
Buprenorphine and methadone – risk of precipitated withdrawal if switching; no established safe protocol for overlap.
Opioid-containing cough, cold, or anti-diarrheal agents – risk of acute withdrawal or reduced efficacy.
Naloxone – used diagnostically for opioid dependence; interaction relates to precipitating withdrawal.
Alcohol – no direct pharmacokinetic interaction but concomitant use requires caution; naltrexone reduces euphoria but does not prevent intoxication.
Liver enzyme inducers or inhibitors – may affect naltrexone metabolism; clinical significance unclear but liver function monitoring advised.
Non-opioid anesthetics – preferred in patients on naltrexone due to risk of opioid antagonist effects.
Medications affecting CNS depression – additive sedation may occur with other CNS depressants like benzodiazepines or alcohol.
Potential interaction with immunomodulatory drugs when used off-label as low-dose naltrexone requires further study.
Drugs causing hepatotoxicity – concurrent use may increase risk of liver damage.
Psychotropic medications – monitor for increased adverse CNS effects.
Warfarin – no significant interaction but monitor due to altered metabolism in liver disease.
Herbal supplements – St John’s Wort may induce metabolism enzymes impacting naltrexone levels.
Monoamine oxidase inhibitors (MAOIs) – not established but caution advised.
Antidepressants and antipsychotics – observe for mood changes or altered therapeutic effect.

Preparations Of The Drug

Extended-release intramuscular injection (Vivitrol): 380 mg suspension in prefilled syringe for once-monthly gluteal administration, contains polylactide-co-glycolide microspheres for sustained release.
Oral tablets (generic): Naltrexone hydrochloride 50 mg tablets, various excipients including lactose monohydrate, hypromellose, magnesium stearate, polyethylene glycol, titanium dioxide, colloidal silicon dioxide, and ferric oxides.
Branded oral tablets (Revia, Depade) have been discontinued but generics are available with variable inactive ingredients.
Injection diluent: carboxymethylcellulose sodium, polysorbate 20, sodium chloride, pH adjusters, in sterile water for injection.
Low-dose formulations (off-label use) prepared as compounded capsules typically around 4.5 mg/day for immunomodulatory purposes.
Preparation storage for oral tablets recommended at room temperature (20° to 25°C) in dry conditions away from light.
Injection product requires refrigeration between 2° to 8°C; may remain at room temperature (<25°C) for up to 7 days prior to administration; do not freeze.
Sterile, single-dose administration devices provided for intramuscular injection with specific needle type for deep gluteal injection.
Generic oral formulations display variable inactive ingredients; patients should check product labels for excipient sensitivities.
Preparation integrity is critical — improper storage reduces efficacy, particularly for the injectable formulation.