Naltrexone Side Effects in the First Week: What's Normal and What's Not

Most people who experience side effects on naltrexone feel them during the first week. That window is also when side effects are most manageable, if you know what to expect.

What You'll Discover:

• Why the first week of naltrexone feels different from weeks two and beyond.

• The most common side effects and exactly when they tend to appear.

• Practical strategies to minimize nausea and other early discomfort.

• What side effects are NOT normal and when to contact your prescriber.

• What to expect once the first week is behind you.

Starting a new medication can feel uncertain, especially when it is one designed to change something as deeply ingrained as your relationship with alcohol. Knowing exactly what your body might go through in the first week, and why, makes a real difference in whether you stick with naltrexone long enough to see it work.

The good news is that most side effects from naltrexone are mild, appear in the first few days, and ease considerably by the end of the first week. The difficult part is that many people stop the medication in those early days before the body has had time to adjust. Understanding the timeline and having a plan helps you get past the hardest part.

Why the First Week of Naltrexone Is Different

Naltrexone works by blocking opioid receptors in the brain. When it does this, it interrupts the reward signals that alcohol normally triggers. For most people, this is a gradual recalibration, and like many recalibrations, the body notices it.

Your brain has become accustomed to the dopamine release that follows drinking. When naltrexone blocks that pathway, there is a brief period of neurochemical adjustment. This is not withdrawal. It is not harmful. It is simply your brain adapting to the medication, and it typically takes a few days.

The standard starting dose of naltrexone for alcohol use disorder is 50mg daily. Some prescribers begin patients at 25mg for the first few days to reduce the likelihood of early side effects, stepping up to 50mg once the body has adjusted. If your prescriber took this approach, you may notice fewer first-week symptoms than someone who started at the full dose right away.

According to the National Institutes of Health StatPearls database, the most commonly reported adverse effects with naltrexone are gastrointestinal in nature and tend to be dose-dependent and time-limited. That means they show up when the dose is new and go away as the body adjusts.

The Most Common Side Effects in Days 1 Through 3

The first three days are when the majority of people notice any side effects at all. Here is what the research and patient experience show most consistently.

Nausea

This is the most frequently reported side effect of naltrexone, and it is also the most manageable. Taking naltrexone with food significantly reduces the likelihood of nausea. Even a small snack, a piece of toast, a banana, or crackers, provides enough of a buffer to make a difference.

A 2023 meta-analysis published in JAMA involving 118 clinical trials and over 20,000 participants found that naltrexone users had a nausea risk ratio of 1.73 compared to placebo. For the majority of people, naltrexone-related nausea is mild and resolves within the first week. Our guide on minimizing and avoiding naltrexone nausea covers practical strategies in more detail.

Headache

Headaches during the first few days of naltrexone are reported by a portion of patients. These are typically mild, tension-type headaches that come on within the first 24 to 48 hours and resolve on their own. Staying well hydrated helps. If you are also reducing your alcohol intake at the same time as starting naltrexone, some of the headache you experience may be related to alcohol reduction rather than the medication itself.

Fatigue and Low Energy

Some people report feeling more tired than usual during the first few days of naltrexone. This tends to present as a general heaviness or reduced motivation rather than a specific physical symptom. It usually passes by day four or five. Getting adequate sleep and maintaining regular meal times both help support your body during this adjustment period.

Dizziness or Light-Headedness

A smaller percentage of people experience mild dizziness when they first start naltrexone, particularly in the first 24 to 48 hours. This typically occurs when standing up quickly and usually resolves without any intervention. Staying hydrated and standing up slowly are the most practical responses.

What Usually Clears Up on Its Own by Days 4 Through 7

By the midpoint of the first week, most people who experienced early side effects notice them beginning to ease. Nausea tends to drop off significantly after day three. Headaches, if they appeared, are typically gone. Fatigue normalizes.

Days four through seven often feel much more stable. The body has begun adjusting to the opioid receptor blockade, and the initial gastrointestinal sensitivity has reduced. Many people report that by day seven, they notice very little physical difference from before they started the medication, except for a meaningful reduction in the urge to drink.

A clinical reference from the NCBI Bookshelf notes that most adverse effects from naltrexone are transient and mild, with the body typically achieving tolerance to gastrointestinal side effects within the first two weeks of treatment.

Practical Tips for Getting Through the First Week

A few straightforward strategies can make the first week significantly more comfortable.

Take naltrexone with food. This is the most effective way to reduce nausea. If your current habit is to take medications on an empty stomach, changing that for naltrexone makes a real difference. Even a light meal or snack is enough.

Take it at the same time each day. Consistency helps your body anticipate and adjust to the medication. Many people find that morning works well, taken with breakfast. For details on timing, our guide on the best time of day to take naltrexone walks through the options.

Stay well hydrated. Water helps your body process the medication and supports the liver, which metabolizes naltrexone. This is especially relevant during the first week when your system is adjusting.

Do not stop and restart without talking to your prescriber. If side effects feel uncomfortable in the first few days, contact your care team before stopping on your own. Your prescriber may recommend reducing to 25mg temporarily or adjusting the timing of your dose. Stopping and restarting unpredictably can make the adjustment period feel longer than it needs to be.

Give it the full first week. The most common reason people stop naltrexone too early is side effects that would have resolved on their own by day five or six. The medication needs time to work, and the uncomfortable early period is temporary.

Side Effects That Are NOT Normal: When to Contact Your Prescriber

The vast majority of first-week side effects from naltrexone are mild and temporary. However, there are symptoms that should prompt a call to your doctor or care team.

Significant pain in the upper right side of your abdomen. This can occasionally indicate liver irritation. Naltrexone is processed by the liver, and it is contraindicated in people with severe hepatic disease. If you have a history of liver problems, your prescriber should be aware before you start.

Severe nausea or vomiting that prevents eating or drinking. Mild nausea is common and expected. Severe nausea that makes it impossible to keep food or water down is not expected and should be reported promptly.

Significant mood changes, depression, or thoughts of self-harm. Rare cases of mood changes have been reported with naltrexone. If you notice a meaningful shift in mood in the first week, contact your prescriber. This is especially important to monitor if you have a history of depression.

Skin rash, itching, or difficulty breathing. These are signs of a possible allergic reaction and require immediate medical attention.

The NIAAA notes that medications for alcohol use disorder are generally safe and well-tolerated when prescribed appropriately. The side effects that require attention are uncommon and typically distinguishable from the mild discomfort most people experience in the first few days.

Side Effects People Do Not Always Expect

A few things catch people off guard in the first week that are worth knowing about in advance.

Vivid dreams or sleep changes. Some people report more vivid dreams or slightly disrupted sleep in the first few days of naltrexone. This appears to be related to the medication's effect on the brain's opioid system and typically normalizes within the first week. It is not harmful, and most people find it passes quickly.

A temporary feeling of emotional flatness. Some people describe a brief period in the first week where they feel slightly less emotionally reactive than usual, things feeling muted or grey. It typically resolves as the body adjusts and is distinct from naltrexone's long-term profile. Our article on whether naltrexone stops all pleasure addresses that question specifically and is worth reading if you have concerns.

Reduced energy or motivation for the first few days. Some people interpret this as a sign that the medication is not working. More often, it reflects the body adjusting to the absence of the dopamine signals associated with alcohol. As naltrexone begins to work, most people find their baseline energy and motivation returning and, over time, improving.

What Happens After the First Week

Once the first week is complete, most people experience a significant settling of any side effects they had. By week two, the vast majority of naltrexone users report that the medication has become unremarkable in terms of how they feel day to day.

What they do notice, increasingly, is a shift in their relationship with alcohol. The medication's effect on craving becomes more apparent. The desire to drink starts to feel less urgent. Many people find that they can be in situations where they would previously have had several drinks and stop at one, or not drink at all.

Clinical evidence from the JAMA meta-analysis shows that the effects of naltrexone on drinking behavior become measurable within the first few weeks of treatment. The first week is a bridge to that change. Getting through it intact gives the medication the chance to do what it is designed to do. Our guide on how naltrexone works covers the pharmacology in plain language if you want a fuller picture.

A Note on Treatment Completion Rates

Across clinical trials, 93% of people who start naltrexone complete their treatment program. That figure is unusually high for a medication used in addiction medicine. It reflects the fact that naltrexone is well-tolerated and that side effects, while real for some people in the first week, are manageable.

The first week asks you to be patient with your body while it adjusts. The vast majority of people who get through that week go on to find the medication effective, easy to take, and worth the early discomfort.

If you are thinking about starting naltrexone or are in the early days of treatment, you can take an online Alcohol Use Assessment to see if naltrexone could be a good fit for you. It is completely discreet, takes just a few minutes, and gives you a clear picture of where to go from there.