Mysimba

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Medically reviewed by
Dr Alexandra Cristina Cowell
Writer & Clinical Content Reviewer
on March 22, 2024.
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What are the side effects of Mysimba?

Side effects are listed into one of five categories, depending on their frequency. The categories include: very common, common, rare, very rare and unknown. The ones you’re most likely to experience are very common and common side effects.

These side effects affect more than 1 in 10 people. They include:

  • feeling sick;
  • vomiting; 
  • constipation; and:
  • headache.

These side effects tend to happen in the first few weeks of taking Mysimba, and are completely normal. This is because Mysimba can impact parts of your brain and the gastrointestinal system, which can affect your bowel movements and certain receptors related to food.

But once your body adjusts to taking Mysimba, they’ll generally go away or become milder. If after a few weeks, you continue to experience these side effects, you should contact your clinician for advice. They’ll either lower your dose, suggest treatment for the side effects or suggest an alternative treatment.

Experiencing side effects in the first few weeks of starting a drug can be quite normal, even with Mysimba. But if you’re worried you should speak to your clinician.

These side effects affect 1 in every 10 people:

  • anxiety;
  • dizziness;
  • feeling shaky;
  • difficulty in sleeping;
  • changes in the way food tastes
  • dry mouth;
  • difficulty concentrating;
  • fatigue;
  • sleepiness and drowsiness;
  • lack of energy;
  • ringing in the ears;
  • fast or irregular heartbeat;
  • hot flush;
  • increased blood pressure;
  • pain in or near the abdomen;
  • excessive sweating;
  • itching;
  • hair loss; and:
  • Irritability.

These side effects will affect up to 1 in every 100 people:

  • hives;
  • hypersensitivity;
  • abnormal dreams;
  • feeling nervous, tensed or agitated;
  • balance disorder;
  • loss of memory (amnesia);
  • tingling or numbness of the hands or feet;
  • motion sickness;
  • burping;
  • abdominal discomfort;
  • indigestion;
  • inflammation of the gallbladder;
  • increased creatinine levels in the blood;
  • increased liver enzymes and bilirubin levels;
  • difficulty in getting or keeping an erection;
  • weakness;
  • thirsty;
  • feeling hot;
  • chest pain;
  • increased appetite; and:
  • weight gain.

For the full list of side effects, check the patient information leaflet.

Are there any serious side effects from taking Mysimba?

While serious side effects with Mysimba are possible, they are rare, making it unlikely for most individuals to experience them. These include:

  • Feeling depressed or suicidal – some reports have found people experiencing suicidal thoughts and feeling depressed. If you experience these feelings, contact your doctor straight away.
  • Fits/seizures – a rare side effect that is more likely to occur if you take too much Mysimba. They’re also more likely to happen if you’re taking other medications, so it’s important to let your clinician know all the medicines you’re taking.
  • Erythema multiforme – a condition that causes redness and itchy spots on various parts of the body.
  • Acute generalised exanthematous pustulosis – a condition in which red, scaly patches form, causing a widespread rash and bumps.
  • Rhabdomyolysis – an abnormal breakdown of tissue, which could lead to kidney problems, muscle weakness and pain.
  • Lupus – Mysimba might trigger lupus flares and symptoms.
  • Serotonin syndrome – a condition that causes too much serotonin to accumulate in the brain, affecting the body’s temperature, mood coordination and gastrointestinal system.

While some of these side effects might sound concerning, it’s important to note that there’s a very low chance of you actually experiencing them. If you have any concerns about any of the side effects, let your prescribing clinician know and they’ll discuss them with you.

When do Mysimba side effects start?

It’s possible to experience side effects within a couple of hours, as the active drugs in Mysimba are absorbed shortly after taking it. It’s important to note that not everyone will experience side effects, and some individuals may have a different response.

How long do Mysimba side effects last?

This depends on many factors, including excretion, absorption and each individual’s response.

Mysimba has two active drugs, naltrexone hydrochloride and bupropion hydrochloride, both of which have a relatively long half-life. Half-life is the time it takes for the drug concentration to half and be eliminated from the body. So, the longer the half-life, the longer the drug will stay active in your system and so the window in which you can experience side effects is longer. Naltrexone has a half-life of approximately 5 hours and 21 hours for bupropion. This means the side effects might persist for a while after taking it.

Additionally, the amount of the active ingredient in Mysimba that your body absorbs can determine the severity and likelihood of experiencing side effects. As each body is different, not everyone will absorb the same amount of the drug. This can influence both the occurrence of side effects and their duration.

How can I avoid side effects when taking Mysimba?

As you progress through a course of taking Mysimba your body should get more used to it, making the side effects go away or become easier to handle. Nevertheless, here are some tips to make managing the symptoms much easier.

1. Gradual dosing

Starting on a lower dose and gradually increasing to a higher dose will help your body adjust to the medication. This can potentially reduce the likelihood and severity of certain side effects.

When taking Mysimba your dose will increase each week until you reach the maximum dose at week four. But if needed, you can ask your clinician to stay on a lower dose until the side effects are more manageable.

2. Take Mysimba with food

It is usually recommended to take Mysimba with a meal. Doing so may help reduce the risk of nausea and other gastrointestinal side effects.

3. Stay hydrated

Drinking plenty of water can help with various side effects, including constipation and headaches, so make sure to stay hydrated.

4. Avoid caffeine

Excessive caffeine consumption can cause dehydration and make some of your side effects worse, like agitation and mood. If you notice these side effects creeping in, then consider switching a caffeinated drink to a decaf version.

Can I take Mysimba with alcohol?

It’s best to avoid alcohol as much as you can when taking Mysimba. It can increase your risk of side effects, such as fits and mental conditions, and also your calorie intake. This would just counteract your efforts in losing weight. Additionally, some research shows alcohol can increase nausea and dizziness, so it’s best to consider these factors before taking it with Mysimba.

Are there any side effects when stopping Mysimba?

When you stop taking Mysimba there’s a possibility you’ll experience side effects for a few days after. This is because it can take a couple of days for the active drugs to be processed by the body.

There’s also a chance of regaining some of the weight you’ve lost. When using a weight loss drug you should aim to adopt sustainable lifestyle changes that you can maintain after treatment. Mysimba works by controlling your cravings and suppressing your appetite, so once you stop taking it, you may feel like you want to eat more.

Is Mysimba safe?

If you use Mysimba correctly and under the guidance of your clinician, it’s safe. Yes, you might still get side effects, but oftentimes these are mild and reduce as you continue taking it. If you do notice side effects that are troubling you or persist for too long, you should let your clinician know so they can check everything is okay.promise1-1.svg

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This page was medically reviewed by Dr Alexandra Cristina Cowell, Writer & Clinical Content Reviewer on March 22, 2024. Next review due on March 22, 2026.

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