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How Long Does Mysimba Take To Work?

The Health Survey for England 2021 estimates that 25.9% of adults in England are obese, while a further 37.9% are overweight

Weight-related health problems can impact your physical and mental health and even shorten your lifespan. Regular exercise and nutrition are key to weight-related health problems, but some patients may require additional support in the form of medication. Mysimba is an effective weight loss medication that can help patients lose up to 5% of their body weight within the first 16 weeks of treatment. Here's an overview of what Mysimba is, how it works, and, most importantly, when you can expect to start seeing real results from taking Mysimba.

What is Mysimba, and what is Mysimba used for? 

Mysimba is a prescription weight loss medication for people who are overweight or obese. It's usually prescribed to people with a Body Mass Index (BMI) of over 27 and those struggling with weight-related health complaints, including high blood pressure, diabetes, or high levels of fat detected in the blood. 

Mysimba contains two active ingredients: naltrexone and bupropion, that are both licensed within the EU.  These active ingredients reduce your appetite by acting on parts of the brain that are known to control how much we think we need to eat and energy levels. In addition, naltrexone and bupropion can lessen the feelings of pleasure you experience when eating food. This can help reduce the likelihood of you overeating or "emotional eating."

Mysimba Speed of Action

Mysimba is absorbed into your body within a few hours. However, exact times can vary depending on your metabolism and other factors, such as taking the medication on a full or empty stomach.

When taken daily, Mysimba usually takes a few weeks to work.  However, you may have to wait up to eight weeks before seeing any significant weight loss. 

The standard Mysimba absorption time is a few hours. This depends on the size and contents of your last meal, as well as your metabolism. 

Mysimba will start to work from the first tablet, but it will take a few weeks before it reaches its full effect. You may notice a gradual change in your appetite during this time.

Simply taking Mysimba and making no other lifestyle changes is likely to be ineffective for weight loss. 

Treatment Duration of Mysimba

You can take Mysimba for as long as you need it. You will need a 16-week check-up with your doctor, followed by annual check-ups.

The half-life of a medication is how long it takes for half of the dose to leave your system. Mysimba half-life is different for each active ingredient:

  • naltrexone - 5 hours
  • bupropion - 21 hours

Mysimba will stay effective until your next dose is due and works best when it has had time to build up in your system.

After 16 weeks, your doctor will check if you’ve lost at least 5% of your starting body weight. Your doctor will recommend that you stop Mysimba if you:

  • haven’t lost at least 5%
  • get intolerable side effects that are not going away
  • have high blood pressure

You must stop taking Mysimba if you find out that you’re pregnant.


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  • Helps weight loss
  • Lowers LDL-cholesterol
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Do I still have to follow a diet and exercise while taking Mysimba? 

Simply taking Mysimba and making no other lifestyle changes is likely to be ineffective for weight loss. Mysimba is a supplementary treatment that supports a wider weight-loss plan. This plan often includes exercise appropriate to your current fitness level and a calorie-controlled diet based on healthy foods. 

How effective is Mysimba? 

Data published by the European Medicines Agency highlights several studies that concluded Mysimba is an effective treatment for weight loss. In three of the studies, patients were given Mysimba or a placebo in combination with a weight loss program. The weight loss program included counselling, dietary advice, and regular exercise. 

The average weight loss for patients taking Mysimba was around 3.7 to 5.7%, compared with 1.3 to 1.9% among the placebo group. The weight loss was even greater in a fourth study, which required patients to undergo a more intense therapy and counselling program. By the end of the trial, the average weight loss for those taking Mysimba in conjunction with intense counselling sessions was 8.1%. In contrast, those taking a placebo alongside their therapy sessions lost an average of 4.9%.

How to take Mysimba

What is the standard duration for taking Mysimba? 

There is no set course of treatment for Mysimba, you can continue taking the medication for as long as needed. However, if you're taking it long-term, you will need a 16-week check-up with your doctor. If the treatment is working, then should you have lost at least 5% of your body weight by week 16 of taking the medication. If there is no weight loss by week 16, the treatment may be ineffective and your doctor may recommend another course of action. Your doctor may also decide to cease the treatment early if your blood pressure has increased or if you experience persistent side effects.

Long-term Use

Taking Mysimba for a long time may increase your risk of certain conditions, but it can also have benefits too.

For as long as you need or until you’ve hit a target weight set out by your doctor, as long as you don’t get any intolerable or dangerous side effects.

Risks and Benefits of Long-term Use

Mysimba can have some risks with long-term use, such as:

  • high blood pressure
  • a slightly increased chance of having a fit (1 in 1000)
  • mental health problems, such as depression
  • lupus

Your doctor will monitor your health while taking Mysimba and you can stop the medication at any time, as long as you speak to a healthcare professional first.

Mysimba can also have some benefits with long-term use, such as:

  • weight loss, which can improve other health conditions, such as diabetes, and increases your life expectancy
  • a decrease in appetite, which can help you make long-lasting changes to how you eat
  • improved confidence and self-esteem

Dosage and Factors Affecting Efficacy

It’s not recommended to take Mysimba after the age of 65. If you have a pre-existing health condition or take a certain medication, Mysimba may work differently. Your doctor will discuss this with you when prescribing.

Missing a dose may decrease the effectiveness of Mysimba.

What to Do If You Find the Dose Ineffective

If you find your Mysimba dose is not effective, wait a few weeks after starting treatment. Your doctor will check in after 16 weeks, but you can speak to them sooner if you don’t think Mysimba is working for you.


Mysimba takes a few weeks to work, although it’s absorbed within a few hours. You should have lost at least 5% of your starting body weight within 16 weeks. Mysimba can be taken for as long as you need it.

If you want to lose weight, speak to a healthcare professional about a treatment plan. You can also speak to one of our prescribers, learn more about Mysimba, or check out other weight loss treatment options.

Authored & 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.

Published on: 29/09/2023 Reviewed on: 01/11/2023
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