Naltrexone ExtendedRelease Injectable Suspension

Naltrexone is an opioid antagonist that blocks the effects of opioids and reduces cravings for alcohol. The extendedrelease injectable suspension (often marketed as Vivitrol) delivers a steady dose of the drug for up to one month after a single intramuscular injection.

Key Features

• Formulation: 380mg of naltrexone in a biodegradable polymer matrix.
• Administration: Deep intramuscular injection (gluteus medius or deltoid).
• Duration of Action: Approximately 28days; a new dose is required each month.
• Indications: Treatment of alcohol use disorder (AUD) and prevention of relapse to opioid dependence after detoxification.
• Advantages over oral naltrexone:
  • Improved adherence no daily pill.
  • Steady plasma concentrations, avoiding peaks and troughs.
  • Reduced risk of accidental overdose due to missed doses.

How It Works

Naltrexone binds competitively to opioid receptors in the brain. By occupying these receptors, it prevents opioid agonists (e.g., heroin, prescription opioids) from producing euphoria, sedation, or respiratory depression. In alcohol dependence, naltrexone appears to attenuate the rewarding effects of alcohol and diminish craving through modulation of the endogenous opioid system.

Pharmacokinetics

Indications & Clinical Use

The medication is indicated for two main populations:

1. Alcohol Use Disorder (AUD): Adults who have voluntarily abstained from alcohol for at least 7days.
2. Opioid Use Disorder (OUD): Patients who have completed opioid detoxification and are opioidfree for a minimum of 710days.

Clinicians typically combine the injection with counseling, psychosocial therapies, and relapseprevention programs to maximize outcomes.

Before Starting Treatment

Screening & Contraindications

• Active opioid use must be cleared with a urine drug screen.
• Severe hepatic impairment (ChildPugh C) not recommended.
• Pregnancy or breastfeeding avoid unless benefits outweigh risks.
• Known hypersensitivity to naltrexone or any component of the suspension.

Baseline Assessments

• Comprehensive medical and psychiatric evaluation.
• Liver function tests (ALT, AST, bilirubin).
• Urine drug screen for opioids.
• Assessment of cardiovascular status if patient has a history of hypertension or arrhythmia.

Administration Procedure

1. Verify the patient has been opioidfree for 7days.
2. Position the patient appropriately; use a 2inch, 21gauge needle for gluteal injection.
3. Rotate injection site each month to minimize local tissue reactions.
4. Observe the patient for at least 30minutes after injection for signs of an acute reaction.

Common Side Effects

Most adverse events are mild to moderate and transient.

• Injectionsite reactions pain, swelling, induration.
• Systemic nausea, headache, fatigue, dizziness.
• Psychiatric anxiety, irritability, insomnia.

Severe reactions such as anaphylaxis or hepatotoxicity are rare but require immediate medical attention.

Managing Missed Doses

If a dose is delayed beyond 35days, a new injection can be given after confirming the patient remains opioidfree. Reinduction with oral naltrexone for 35days before the next injection may be considered to reestablish receptor blockade.

Drug Interactions

• Opioid agonists: May precipitate withdrawal if administered before the injections effect is established.
• Hepatically cleared drugs: Caution with medications that share metabolic pathways (e.g., certain antiretrovirals, benzodiazepines).
• No clinically significant interactions with most antihypertensives, antidepressants, or antipsychotics.

Special Populations

Elderly

Start with standard dosing but monitor liver function closely; dose reduction is not routinely required.

Pediatric

Not approved for patients under 18years of age.

Monitoring & Followup

Regular visits (monthly, coinciding with injections) should include:

• Assessment of cravings and substance use.
• Liver function tests every 34months.
• Evaluation of injectionsite tolerance.
• Psychosocial support and counseling.

Clinical Evidence

Multiple randomized controlled trials have demonstrated that monthly naltrexone reduces heavy drinking days by 3040% compared with placebo and improves abstinence rates. In opioiddependent populations, extendedrelease naltrexone reduces the risk of relapse by roughly 50% relative to treatmentasusual, especially when combined with behavioral therapy.

Patient Education Points

• Do not take any opioid medication (including prescription painkillers) without consulting the prescriber.
• Report any persistent injectionsite pain, swelling, or redness.
• Maintain regular followup appointments for monitoring and counseling.
• Alcohol consumption should be avoided during the initial 7day abstinence period.

Resources

For additional information, please refer to:

• U.S. Food & Drug Administration Naltrexone
• SAMHSA Substance Use Treatment
• PubMed Naltrexone clinical studies