Common side effects of naltrexone (and how to manage them)

Naltrexone is a well-established medication that has been prescribed since the 1990s. Most patients tolerate it well. The side effects that do occur are usually mild and limited to the first week or two.

Most common: mild nausea

This is the side effect most patients notice. It's usually mild, often described as a slightly queasy feeling for an hour or two after taking the tablet. It typically settles within 7-14 days as your body adjusts.

Practical management:

• Take the tablet with food rather than on an empty stomach

• Use the gradual titration we recommend (1/4 → 1/2 → 3/4 → full over 2-3 weeks). This significantly reduces side effects.

• Stay well hydrated, especially in the first few days

• If nausea persists past 2 weeks, message your nurse — we can adjust the schedule or consider switching to nalmefene

Headaches

Some patients get mild headaches in the first week. Usually relieved by paracetamol (avoid aspirin and ibuprofen if your stomach is sensitive). If headaches are persistent or severe, contact us.

Tiredness

A subtle reduction in energy is sometimes reported in the first week. Some patients describe feeling slightly 'flatter' for the first few days as the brain adjusts to having its opioid receptors blocked. This passes.

Mood changes

Rarely, patients report a transient low mood in the first 1-2 weeks. This is usually mild and self-limiting. If you have a history of depression, we'll have flagged this at your consultation and we'll check in with you closely. If you experience persistent low mood or any thoughts of self-harm, contact us immediately.

Liver enzymes

Naltrexone is metabolised by the liver. Very rarely, it causes a transient rise in liver enzymes (ALT). We monitor this with LFTs at baseline and at 1 and 6 months. If your LFTs rise meaningfully, we'd switch you to nalmefene, which is kinder to the liver.

More on bloods in our liver function tests article.

Rare but important: precipitated opioid withdrawal

If you take naltrexone while you have opioids in your system (prescription pain medication, methadone, recreational opioids), it will precipitate acute opioid withdrawal — sweating, vomiting, agitation, severe pain. This is why we screen carefully for opioid use at consultation. If you've ever taken any opioid in the days before starting naltrexone, you must tell us. This is the single most important safety question.

What to expect overall

In our experience, around 70-80% of patients have either no side effects or only mild ones that pass within the first two weeks. The titration schedule we use is specifically designed to minimise side effects. If yours don't settle, we have options — switching to nalmefene, slower titration, or a different approach altogether.

If you're worried, message your nurse. That's what we're here for.

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