How Does Mysimba Work?

Although weight loss injections such as Mounjaro and Wegovy have dominated the headlines in recent years, they are far from the only treatments available to help you reach your goals. Mysimba tablets, for example, are an effective alternative, especially for individuals who don’t like the idea of using needles.

Originally licensed to tackle substance addiction, Mysimba’s mechanisms of action have also been clinically proven to support weight loss when taken alongside a healthy diet and regular exercise. The question is, exactly how does Mysimba work?

In this article, we’ll explore Mysimba’s dual active ingredients and examine how they support weight loss. We also outline the outcomes of clinical trials and discuss how long it takes the treatment to work. Let’s get started.

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Mysimba: How Does it Work?

Mysimba is a combination of two medicines (Naltrexone and Bupropion) that work together like a well-oiled team, targeting different parts of the brain. This tag-team approach means you feel less hungry overall and can therefore be satisfied with smaller meals. It also helps you to use up more energy, potentially burning additional calories throughout the day.

Now, let’s take a closer look at Mysimba’s two active ingredients and outline how they work to support weight loss. 

Naltrexone for Weight Loss and Bupropion for Weight Loss: How Do They Help?

The exact mechanisms by which naltrexone and bupropion work for weight loss are not well understood. Separately, naltrexone’s primary function is to manage addiction. It binds to opioid receptors in the brain and blocks the effects of opioids. Your opioid receptors are involved in numerous bodily functions, so blocking them can significantly affect other physiological processes. Because these receptors also regulate hunger, mood, and reward, this could potentially explain the apparent efficacy of Naltrexone for weight loss.

Bupropion, on the other hand, influences neurotransmitters in the brain – masses of chemicals which send messages all over your body, almost like an internal messenger system. Two of these neurotransmitters, dopamine and noradrenaline, are affected by bupropion, which can lead to a loss of appetite, though the effect varies from person to person.

When combined, naltrexone and buproprion act on specific brain pathways to increase energy expenditure, lower food intake, and keep you feeling fuller for longer. These pathways are also associated with the control of pleasure, including the pleasurable feelings we get from food, particularly high-fat or sugary foods. By controlling the pleasurable feelings you get from food, Mysimba can help you lose weight when combined with healthy lifestyle changes.

Does Mysimba Work for Weight Loss?

Clinical trials provide important data to understand how Mysimba can help with weight loss.

Mysimba’s evidence comes from 4 studies, totalling 4,500 patients classed as obese or overweight compared to a placebo drug. The studies were part of a weight loss programme alongside diet and exercise advice. Treatment lasted over 28 or 56 weeks.

The main results (which include patients who dropped out) from the European Medicines Agency show:

• Average weight loss: Compared with placebo across three studies, Mysimba led to an average weight loss of 3.7% to 5.7%, whereas the placebo group experienced a weight loss of 1.3% to 1.9%.
• Achieving weight-loss goals: A considerably higher proportion of patients on Mysimba (28-42%) lost at least 5% of body weight. The success rate increased further with additional counselling. The placebo percentage was 12-14%.
• Double-digit reduction: For greater weight loss of 10%, this was reported in 13-22% of Mysimba users, but in only 5-6% of those on placebo. Again, the success rate improved with additional counselling.

The specific details of the clinical trials are available in the Summary of Product Characteristics for Mysimba.

Does Mysimba Work Immediately?

Yes, but it can take several weeks before you notice the full effects. It takes a while for the active ingredients to build up in your system and for you to see visible weight loss results, but you may notice positive changes in your appetite and food intake within a few days.

Research has shown that at the maximum doses of 32mg of naltrexone hydrochloride and 360 mg of bupropion hydrochloride, after 4 weeks, you can lose around 3% of your starting body weight, and after 56 weeks, 8%. 

How Long Does Mysimba Stay in Your System?

Mysimba is usually cleared from the body within about 4 days after your last dose. How long Mysimba stays in your system depends on the two active ingredients, naltrexone and bupropion, because they have slightly different half-lives. This is the time it takes for your body to reduce the dose of Mysimba by half.

Naltrexone takes approximately 24 to 28 hours to leave your body completely, while bupropion takes around 4 days. That means it should be cleared after 4 days. If you have experienced Mysimba side effects, you should notice that they improve after this time.

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How Do I Know if Mysimba Has Worked?

Within a few days of starting Mysimba, you may notice your appetite lessens, and you crave food less. These can be good signs that naltrexone and bupropion are affecting your body. However, it can take a few weeks before you may begin to lose weight or notice changes in your appearance.

There are two main stages of noticing weight loss:

• Rapid weight loss: Weight loss usually happens quickly in the first 4 to 6 weeks. Most of the weight you lose is water, rather than body fat.
• Slow weight loss: Weight loss tends to slow in the second stage, and you may experience a plateau. You may need to adjust your approach to kick-start your progress again.

More than half of people who take Mysimba lose at least 5% of their initial body weight in the first 16 weeks of treatment. If you haven’t, speak to your clinician, as Mysimba may not be the best option for you. Our clinicians can help you choose an alternative weight loss medication that may better suit your needs. 

Frequently Asked Questions

Choosing the right weight loss medication for you depends on several factors, including how it works in the body. You’ll likely also want to know the impact it can have on your current lifestyle, such as whether you can drink alcohol with it, how you take it, and what results you can expect to achieve. We answer those questions below. 

Does Mysimba Work with Alcohol?

No, ideally you should avoid or limit your alcohol intake while taking Mysimba because it can increase the risk of seizures, mental disorder events, or reduce your alcohol tolerance. Your clinician may suggest that you avoid alcohol completely or drink as little as possible.

If you drink a lot of alcohol before starting Mysimba treatment, discuss this with your clinician first. It is essential that you don’t just stop drinking alcohol, as this can also increase the chance of having a seizure.

How Do You Take Mysimba?

Mysimba is an oral tablet that you swallow, preferably with food. They should be swallowed whole and not crushed or chewed. You start on a lower dose of Mysimba, which gradually increases over four weeks until you reach the maximum dose.

The Mysimba dosage schedule looks like this:

• Week 1: One tablet once a day in the morning.
• Week 2: One tablet in the morning and one tablet in the evening every day.
• Week 3: Two tablets in the morning and one in the evening daily.
• Week 4: Two tablets in the morning and two tablets in the evening every day.  

How Much Weight Can You Lose on Mysimba?

The average weight loss on Mysimba is 8.1% over 56 weeks at the maximum dose. More than 50% of people lost at least 5% of their starting body weight after 16 weeks of taking Mysimba tablets, compared with 19% given a placebo.  

Remember, weight loss is a journey, not a race, so don’t worry if you don't see the same results as those reported in clinical trials. If you haven’t lost at least 5% of your weight by week 16, speak to your clinician, who may recommend an alternative weight loss medication. 

Begin Your Mysimba Weight Loss Journey with Prescription Doctor

If you think Mysimba tablets could be the right treatment to help you lose weight, your next decision is to determine the best place to obtain them. Since 2013, Prescription Doctor has built a strong reputation for helping people like you achieve their weight loss ambitions.

Our mission is to make healthcare more accessible, which is why we enable you to order the medication you need at any time from your own home. Just complete our brief online questionnaire, and our specialist clinicians will review your information to determine whether your chosen treatment is suitable for your circumstances. If approved, your items will be dispatched from our General Pharmaceutical Council-registered dispensing pharmacy in plain packaging for total discretion.

We’re here to support you at every stage of your weight loss journey. Our clinicians will regularly check in with you to assess your progress and ensure your treatment programme delivers the best possible results. You can also contact our team with any questions or for additional guidance, or check out our library of Mysimba articles. We are here for you.

Ready to get started? Begin your Mysimba consultation with Prescription Doctor today and let us support you throughout your journey. 

Authored By

Leanne Edermaniger

Medical Content Writer

Leanne is a science and health content writer focusing on human health and biology while utilising her solid academic background.

Published on: 31/05/2024 Re-Published on: 15/01/2026

Reviewed By

Mohamed Imran Lakhi

MPharm - Lead Pharmacist

Imran Lakhi is the superintendent pharmacist and founder at Prescription Doctor. He has been at the core of our team.

Reviewed on: 15/01/2026