Starting naltrexone usually comes with side effects, but most are mild, predictable, and gone within two weeks. The hard part is knowing which feelings are part of the normal adjustment and which mean something’s actually wrong. This article walks through both, so you don’t spend the first month wondering if every twinge means you should stop.
This is part of our Naltrexone hub, the complete guide to using naltrexone for alcohol, including dosing, the Sinclair Method, and how to talk to your doctor about a prescription.
# What you’ll probably feel in the first two weeks
The body of clinical trial data on oral naltrexone is large and consistent. About one in three people taking 50mg daily report some side effect, and the same handful of effects show up over and over again. According to a PubMed Central review of naltrexone trials, most people who experience side effects describe them as mild, and they tend to clear up on their own within a couple of weeks.
The most common ones, ordered by how often they come up in trials:
- Nausea: by far the most common. Around a third of patients in trials report it. Usually worst in the first few days.
- Headache: roughly a quarter of people.
- Dizziness or feeling lightheaded: about one in eight.
- Trouble sleeping: also about one in seven.
- Anxiety: around one in eight, often described as feeling “wired.”
- Fatigue or low energy: varies but common.
- Loss of appetite: common, and usually not unwelcome.
- Vomiting, abdominal discomfort, diarrhoea: less common than nausea on its own, but related.
If you’re feeling several of these at once during week one, you’re having a textbook reaction to naltrexone. Annoying, but expected.
# How long they actually last
This is the question most people want answered, and the data is reassuring. The PMC review notes that side effects “usually occur only during initial therapy” and most are “self-limiting”, meaning your body adjusts and they go away without needing to do anything.
A useful rough timeline:
- Days 1–4: peak nausea and headache, low energy
- Week 1: still noticeable but milder
- Week 2: most people are through the worst of it
- Week 3+: side effects have faded; if anything is still bothering you at this point, talk to your doctor
If side effects are still as bad in week three as they were on day three, that’s not normal adaptation, and your doctor may want to review the dose or rule out something else.
# Less common but worth knowing about
A second tier of side effects shows up in a smaller percentage of patients. None of these are usually serious, but they’re worth recognising:
- Dry mouth
- Muscle or joint aches
- Decreased libido
- Vivid or unusual dreams
- Mild irritability
- Skin rash
If any of these appear and stick around, mention them at your next appointment. They don’t usually mean anything dangerous, but a doctor should know.
# The serious side effects: when to call straight away
Most people on naltrexone won’t see any of what follows. But you should know what to look for, because the difference between “wait it out” and “go to A&E” matters.
# Liver problems
Naltrexone has an FDA black-box warning about liver injury, which sounds scarier than it is. The warning came from older studies using very high doses (100–300mg daily) in obese patients. At the standard 50mg dose for alcohol, serious liver damage is rare. Cleveland Clinic flags these warning signs:
- Pain in the upper right side of your belly
- Yellowing of the skin or whites of the eyes (jaundice)
- Dark, tea-coloured urine
- Pale or light-coloured stools
- Severe and persistent nausea or fatigue
If any of these appear, stop taking the medication and call your doctor the same day. A simple blood test for liver enzymes will tell them whether there’s an issue. Most prescribers will run baseline liver function tests before starting and again about a month in, just to be safe.
# Mood changes: depression or suicidal thoughts
A small percentage of patients on naltrexone report worsening mood or, rarely, suicidal thoughts. This is listed as a serious side effect on every official drug information sheet. If you notice your mood dropping significantly, especially with thoughts of harming yourself, please reach out to a doctor or a crisis line straight away. In the UK, the Samaritans are on 116 123 any time. In the US, the 988 Suicide & Crisis Lifeline.
# Severe allergic reaction
Rare but possible. Hives, swelling of the face or throat, difficulty breathing. Call emergency services. Don’t take another dose.
# Accidental opioid interaction
This one isn’t a side effect of naltrexone itself. It’s a consequence of how it works. Naltrexone blocks opioid receptors. If you take an opioid (prescribed or otherwise) while on naltrexone, two things can happen: it won’t work the way you’d expect, and you might be tempted to take more, which raises the overdose risk significantly. Tell every doctor and dentist you see that you’re on naltrexone before any procedure.
# How to make the first two weeks easier
Side effects are mostly unavoidable, but a few small things can take the edge off.
Take it with food. Splitting the 50mg tablet across breakfast and dinner, or taking it after a substantial meal, cuts the nausea sharply for most people. The tablet works the same whether your stomach is empty or full. There’s no clinical reason to fast.
Hydrate properly. Headaches and dizziness in the first week respond well to drinking more water than you think you need. Aim for the equivalent of two large glasses on top of your normal intake.
Don’t stack it with new things. If you can avoid starting other new medications, supplements, or significant lifestyle changes the same week you start naltrexone, do. It makes it easier to figure out what’s causing what if something feels off.
Sleep matters. Insomnia is one of the more common side effects, and a bad night makes everything else feel worse. Take the dose earlier in the day if sleep becomes an issue, and tell your doctor if it lasts more than a week.
Track how you feel. Day-by-day notes for the first month, even a one-line entry, make it much easier to tell whether something is improving or getting worse. This is also useful when you talk to your doctor at follow-up.
# If side effects are bad enough to make you want to stop
Some people get through week one fine. Others find the nausea or anxiety harder to ride out, and start wondering whether the medication is worth it. Before stopping, talk to your prescriber. There are usually three options worth trying first: dropping to a half dose for a week and ramping up, switching from oral to the long-acting injection (Vivitrol), or trying a different alcohol medication entirely (acamprosate or nalmefene have very different side-effect profiles).
Stopping cold-turkey isn’t dangerous with naltrexone. There’s no withdrawal because it isn’t a substance your body becomes dependent on. But quitting without a plan often means you’ve spent the worst week of side effects for nothing. A short conversation with your doctor before you stop usually pays off.
# How AlcoLog helps with this
AlcoLog logs each naltrexone dose with a timestamp, so you can see exactly when you took it and how often. The redose timer makes it easy to space doses correctly during the first month, when side effects are at their worst.
The app doesn’t track subjective side-effect data itself. For that, a separate paper or digital journal works better: rate your nausea, fatigue, headache, and sleep quality day by day for the first two weeks. Pair the timestamps from AlcoLog’s medication card with your subjective notes, and you have a useful pattern to discuss with your doctor at the one-month follow-up.
