Naltrexone Side Effects: Why They Occur & Medically Approved Next Steps

Naltrexone is a prescription medication commonly used to treat alcohol use disorder (AUD) and opioid use disorder (OUD). It works by blocking opioid receptors in the brain, which reduces cravings and the rewarding effects of alcohol and opioids.

Like all medications, naltrexone side effects can occur. Most are mild and temporary, but some can be serious. Understanding why these side effects happen—and what to do about them—can help you use this medication safely and confidently.

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How Naltrexone Works (And Why Side Effects Happen)

Naltrexone blocks opioid receptors, particularly the mu-opioid receptor. These receptors play a role in:

• Pain regulation
• Pleasure and reward pathways
• Stress response
• Gastrointestinal function

When these receptors are blocked:

• Alcohol feels less rewarding
• Opioids cannot produce their usual effects
• Cravings may decrease

However, because these receptors also influence other body systems, blocking them can lead to side effects.

There are two forms of naltrexone:

• Oral tablets (daily)
• Extended-release injectable (monthly, brand name Vivitrol)

The side effect profile is similar for both, though injection site reactions are specific to the injectable form.

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Common Naltrexone Side Effects

Most naltrexone side effects are mild and often improve within days to weeks.

1. Nausea (Most Common)

• Affects up to 10–30% of users
• Often occurs during the first few days
• Usually improves within 1–2 weeks

Why it happens: Opioid receptors are involved in gut function. Blocking them can temporarily disrupt digestion.

What helps:

• Take oral naltrexone with food
• Stay hydrated
• Start at a lower dose if your doctor recommends it

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2. Headache

• Common during early treatment
• Usually mild to moderate

Why it happens: Changes in neurotransmitter signaling as the brain adjusts.

What helps:

• Hydration
• Rest
• Over-the-counter pain relievers (if approved by your doctor)

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3. Fatigue or Low Energy

Some people report:

• Feeling tired
• Mild brain fog
• Reduced motivation (early on)

Why it happens: The brain's reward system is adjusting. Reduced dopamine activity may temporarily affect energy levels.

This typically improves as the body adapts.

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4. Insomnia or Sleep Changes

• Trouble falling asleep
• Vivid dreams

Sleep disturbances are usually temporary and improve over time.

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5. Anxiety or Irritability

Some patients feel:

• Slightly restless
• More emotionally sensitive

It's important to distinguish between:

• Medication effects
• Underlying withdrawal symptoms
• Stress related to recovery

If symptoms persist beyond a few weeks, discuss them with your doctor.

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6. Decreased Appetite

Mild appetite changes may occur. Some people lose a small amount of weight.

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Injectable Naltrexone Side Effects (Vivitrol)

In addition to the side effects above:

Injection Site Reactions

• Pain
• Swelling
• Redness
• Firm lump

Most reactions are mild. However, in rare cases, severe injection site reactions can occur and may require medical treatment.

Seek care if you notice:

• Severe pain
• Open wounds
• Darkening skin
• Increasing swelling

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Less Common but Serious Naltrexone Side Effects

While uncommon, these require medical attention.

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1. Liver Problems (Hepatotoxicity)

Naltrexone is processed in the liver. High doses or pre-existing liver disease increase risk.

Watch for:

• Yellowing of skin or eyes (jaundice)
• Dark urine
• Severe abdominal pain
• Persistent nausea
• Extreme fatigue

Liver enzyme elevations are usually mild and reversible when monitored properly.

Next step:
Your doctor should check liver function before starting treatment and periodically afterward.

If you experience symptoms of liver trouble, speak to a doctor immediately.

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2. Precipitated Opioid Withdrawal

This is a serious reaction that occurs if someone takes naltrexone while opioids are still in their system.

Symptoms can include:

• Severe vomiting
• Muscle aches
• Sweating
• Agitation
• Rapid heart rate

This is why patients must be opioid-free for 7–10 days before starting naltrexone.

Always be honest with your doctor about recent opioid use.

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3. Depression or Suicidal Thoughts

Rare but reported.

If you notice:

• Worsening mood
• Hopelessness
• Thoughts of self-harm

Seek medical help immediately. These symptoms are not common, but they are serious.

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Why Naltrexone Side Effects Are Usually Temporary

Your brain and body are adjusting to:

• Reduced dopamine spikes
• Blocked opioid signaling
• Changes in reward pathways

For many people, early discomfort fades within 1–3 weeks.

Research from major clinical trials shows that most patients tolerate naltrexone well, and discontinuation due to side effects is relatively low.

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Medically Approved Next Steps If You Have Side Effects

If you're experiencing naltrexone side effects, here's what to do:

✅ Mild Symptoms

• Continue medication if tolerable
• Take with food
• Track symptoms
• Discuss at your next appointment

✅ Persistent Symptoms

• Ask your doctor about dose adjustments
• Consider switching from oral to injectable (or vice versa)
• Evaluate other medications that may interact

✅ Severe Symptoms

• Stop medication only if instructed
• Seek urgent medical care for:
  • Liver symptoms
  • Severe allergic reaction
  • Intense withdrawal
  • Suicidal thoughts

Never stop abruptly without medical guidance, especially if you're being treated for substance use disorder.

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When to Speak to a Doctor Immediately

Call a healthcare provider right away if you experience:

• Chest pain
• Severe abdominal pain
• Yellowing skin or eyes
• Severe depression
• Thoughts of harming yourself
• Signs of opioid withdrawal
• Serious injection site reactions

If anything feels life-threatening, seek emergency care.

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Balancing Risks vs. Benefits

It's important to keep perspective.

Untreated alcohol or opioid use disorder carries significant risks, including:

• Liver disease
• Overdose
• Heart disease
• Mental health decline
• Accidents and injuries

For many people, the benefits of naltrexone—reduced cravings and relapse prevention—outweigh the manageable side effects.

If you're unsure whether what you're experiencing is related to naltrexone or something else entirely, using a free AI-powered tool to assess alcohol cravings can help you understand your symptoms better and prepare for more informed conversations with your healthcare provider.

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Who Should Not Take Naltrexone?

Naltrexone may not be appropriate if you:

• Are currently using opioids
• Have acute hepatitis
• Have severe liver failure
• Are in active opioid withdrawal
• Have a known allergy to naltrexone

Always provide your full medical history before starting.

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Final Thoughts on Naltrexone Side Effects

Most naltrexone side effects are mild, temporary, and manageable. Nausea, headache, and fatigue are the most common. Serious reactions are rare but require prompt medical attention.

The key points to remember:

• Many side effects improve within weeks
• Liver monitoring is important
• Opioid-free status is essential before starting
• Severe mood changes should never be ignored

Recovery is a medical process, not a willpower test. If naltrexone is part of your treatment plan, ongoing communication with your healthcare provider is critical.

If you experience anything concerning—especially symptoms that could be serious or life-threatening—speak to a doctor immediately.

With proper medical supervision, naltrexone can be a safe and effective tool in long-term recovery.