Naltrexone Side Effects: What to Realistically Expect When Starting Treatment

Naltrexone is an opioid antagonist that blocks the effects of opioids and reduces alcohol cravings. Its common side effects include nausea, headache, dizziness and fatigue, which are usually mild and ease within the first few weeks. It must only be started after an opioid-free interval, because taking it too soon can trigger precipitated withdrawal.

Maybe a doctor mentioned naltrexone at a discharge appointment, and it went into your bag with a leaflet you never fully read. Maybe you searched for it at two in the morning after another day you promised would be different. Or maybe you are a parent, a partner or a close friend, reading this on behalf of someone you love and trying to understand what they are being asked to take. Whoever you are, you are here because a tablet has entered the picture and you want to know, plainly, what it will do to the body before anyone commits to it.

That is a reasonable thing to want. Naltrexone is not a sedative, it will not get you through a withdrawal, and it is not a substitute you take instead of opioids. It works in the opposite direction, and understanding that difference is the first step to using it well. The side effects are real but usually manageable. The genuine risk lies in timing, and in starting it without the right clinical checks around you.

This article sets out what naltrexone is, what you should realistically expect in the first days and weeks, and why we prescribe it inside a structured residential programme rather than handing it over as a standalone fix. We will be honest about who it suits and who it does not.

What is naltrexone and how does it work?

Naltrexone is a pure opioid antagonist. It binds to the mu-opioid receptors in the brain and blocks them, so opioids can no longer produce their usual effect, and it also dampens the reward signal that alcohol triggers.Because it occupies those receptors without activating them, naltrexone reduces cravings and removes the reinforcing “high” that keeps a cycle going. According to the StatPearls clinical reference, naltrexone functions as a mu-opioid receptor antagonist with weaker activity at the kappa and delta receptors. It is approved for both alcohol use disorder and opioid use disorder, and it is not addictive, has no street value, and does not create dependence of its own.

What are the most common side effects of naltrexone?

The most frequent side effects are nausea, headache, dizziness and fatigue, and most are mild. They tend to appear early and settle as your body adjusts.The clearest figures come from the FDA prescribing information. In an open-label safety study of roughly 570 people with alcoholism, the new-onset reactions reported in 2% or more of patients were nausea (10%), headache (7%), dizziness (4%), nervousness (4%), fatigue (4%), insomnia (3%), vomiting (3%), anxiety (2%) and somnolence (2%). In studies of opioid use disorder, StatPearls notes that headache, low energy, joint and muscle pain, abdominal pain, insomnia and nausea were each reported in more than 10% of patients. Numbers vary with the formulation and the population, but the pattern is consistent: gastrointestinal upset and headache lead, and they are usually short-lived.

How long do naltrexone side effects last?

For most people the early side effects fade within the first one to two weeks as the body adapts. Persistent or worsening symptoms are the exception, not the rule, and should be reported.Nausea and headache are typically at their most noticeable in the opening days. If they do not ease, or if you notice signs that could point to a liver reaction — pain in the upper right abdomen, unusual tiredness, yellowing of the skin or eyes, dark urine — these need prompt medical attention rather than watchful waiting. This is one of the practical reasons we monitor patients closely during the first fortnight rather than leaving them to interpret their own symptoms.

Why must you be opioid-free before starting naltrexone?

Because naltrexone blocks opioid receptors, taking it while opioids are still in your system forces them off those receptors all at once. That triggers a sudden, severe withdrawal known as precipitated withdrawal.The FDA guidance is specific: patients must be opioid-free for a minimum of 7 to 10 days before the first dose. SAMHSA’s clinical guidance extends this to 10 to 14 days for longer-acting opioids such as methadone or buprenorphine, because they clear the body more slowly. This is not a formality. Skipping the window is the single most dangerous mistake a person can make with this medication, and it is why a medically supervised gap between detox and induction is non-negotiable.

What is precipitated withdrawal and how is it avoided?

Precipitated withdrawal is a withdrawal syndrome brought on abruptly when an antagonist displaces opioids from the receptors. It can be far more intense than the natural withdrawal a person expects.The FDA label records that symptoms have appeared within five minutes of ingestion and lasted up to 48 hours, and SAMHSA notes the reaction can be severe enough to require hospitalisation. It is avoided by confirming the opioid-free interval, and clinicians may use a naloxone challenge or a supervised low-dose test to check that the receptors are clear before a full dose is given. In residential care this sequencing is built into the pathway, so induction only happens after a properly completed medically supervised detox.

Why is naltrexone used within a Dual Treatment programme rather than on its own?

Naltrexone removes the reward, but it does not address why the reward was being sought. On its own, a blocked receptor is a wall a person can still walk away from by simply stopping the tablet.That is why we prescribe it inside a wider clinical structure. Many people arriving in treatment carry co-occurring behavioural health difficulties alongside their substance use, and medication works best when it is paired with therapy that addresses both. This combined approach is the basis of our Dual Treatment programme, delivered across 30, 60 and 90-day tiers so that the length of care matches the depth of what a person is working through. The evidence supports the pairing: a study published in Alcohol and Alcoholism found that people in residential treatment who took naltrexone reported a more rapid decrease in alcohol craving than those receiving usual care alone. Medication and structured therapy reinforce each other.

“Naltrexone is a genuinely useful tool, but it is a tool, not a treatment in itself. What makes it safe is the sequencing around it — the confirmed opioid-free interval, the monitoring in the first two weeks, and the therapy that runs alongside it. We never start it in isolation, because the medication holds the door shut while the real work of recovery happens behind it.” — Dr. Natalie Lindemann, Clinical Director (Global), Holina Global

Who is naltrexone appropriate for, and who should avoid it?

Naltrexone suits people who have completed detox, are medically stable, and want to reduce cravings for alcohol or opioids without an opioid-based medication. It is not right for everyone.It is not appropriate for anyone still using opioids or within the required opioid-free window, for people in acute opioid withdrawal, or for those with significant liver impairment, which requires careful assessment first. Anyone who may need opioid pain relief, for surgery or a serious injury, must tell their clinicians, because the blockade also blocks pain medication. These decisions are made individually. If you are weighing it up, the safest path is to have a clinician review your history rather than deciding alone, and our clinical team assesses each person before any medication is started.

The bottom line

Naltrexone’s side effects are, for most people, mild and temporary. The part that demands respect is the timing and the supervision around it. Used correctly, after a proper detox and inside a programme that treats the whole person, it can quietly remove the pull that has been running the show. If you want to understand whether it fits your situation, speak with our admissions team, and there is dedicated support for families trying to help someone consider treatment.

How Holina Rehab Uses Naltrexone Within Treatment

At Holina Rehab, naltrexone is never dispensed as a standalone prescription. It is introduced only within a residential stay on our beachfront campus in Koh Phangan, Thailand, where a person lives on-site throughout their care and every stage of medication is directly supervised. That setting matters: the confirmed opioid-free interval, the naloxone challenge or supervised test dose, and the close monitoring through the first fortnight all happen under one roof, with clinical staff present rather than left to a person to manage alone at home.Induction sits inside our Dual Treatment programme, delivered across 30, 60 and 90-day tiers so the length of care matches the depth of what each person is working through. Naltrexone holds the door shut on cravings while the underlying behavioural health work runs alongside it, and the tier is chosen with that timeline in mind. Our clinical team assesses each person before anything is prescribed, reviewing history, liver function and any co-occurring difficulties. Dr. Natalie Lindemann, our Clinical Director, oversees how medication and therapy are sequenced together, so that a blocked receptor is only ever one part of a wider, individually shaped pathway rather than the whole of it.

Frequently asked questions

Is naltrexone addictive?

No. Naltrexone is not an opioid and does not produce a high or dependence. It blocks receptors rather than activating them, so there is no craving for the medication itself and no street value.

How long before I can start naltrexone after using opioids?

You must be opioid-free for at least 7 to 10 days for short-acting opioids, and 10 to 14 days for longer-acting ones such as methadone or buprenorphine, to avoid precipitated withdrawal.

What happens if I take naltrexone with opioids still in my system?

It can trigger precipitated withdrawal, a sudden and severe withdrawal that has appeared within five minutes of taking the tablet and can require hospitalisation. This is why the opioid-free interval is essential.

Does naltrexone help with alcohol as well as opioids?

Yes. It is approved for both alcohol use disorder and opioid use disorder. For alcohol, it reduces cravings and the reinforcing effect of drinking.

What are the most common naltrexone side effects?

Nausea, headache, dizziness and fatigue are the most frequently reported. In an FDA safety study of people with alcoholism, nausea affected 10% and headache 7%.

How long do the side effects last?

For most people they are mild and ease within the first one to two weeks as the body adjusts. Symptoms that persist or worsen should be reported to your clinician.

Can naltrexone damage my liver?

Liver reactions are uncommon but possible, which is why liver function is assessed before and during treatment. Report any upper-right abdominal pain, unusual tiredness, dark urine or yellowing of the skin or eyes promptly.

Will naltrexone block pain medication if I have surgery?

Yes. Because it blocks opioid receptors, it can prevent opioid pain relief from working. Always tell your medical team you are taking it before any procedure or if you have a serious injury.

Do I still need therapy if I am taking naltrexone?

Yes. Naltrexone reduces cravings but does not address the underlying causes of substance use. It works best within a structured programme that combines medication with therapy.

Is naltrexone taken as a tablet or an injection?

Both forms exist. There is a daily oral tablet and a long-acting injection given roughly monthly. Which one suits you is a clinical decision based on your circumstances.

Can I drink alcohol while taking naltrexone?

Naltrexone reduces the reinforcing effect of alcohol rather than making you ill if you drink. The goal of treatment is to stop drinking, and the medication supports that by lowering cravings.

Is naltrexone started before or after detox?

After. It is started only once detox is complete and the required opioid-free interval has passed, which is why it is best begun in a medically supervised setting.

Clinically reviewed by Dr. Natalie Lindemann — Clinical Director, Holina Global · Last reviewed 20 July 2026.