Everything You Need To Know About Mysimba

How does Mysimba work?

Mysimba contains two active ingredients, naltrexone and bupropion, which work together by influencing the brain chemistry to reduce food cravings and help manage emotional compulsive eating.

• Bupropion is used as a smoking cessation aid. It works by increasing levels of dopamine and norepinephrine, two neurotransmitters involved in mood, motivation and appetite. This effect can help reduce food cravings and emotional eating

• Naltrexone is typically used in addiction treatments (such as opioid and alcohol) and works by blocking the effects opioids

Together, these ingredients help to reduce the reward system of emotional eating and cravings. This can make it easier to maintain healthy eating habits over time.

As foods in high fat and high sugar trigger a release of dopamine in the brain's reward system and this is what reinforces the eating behaviour. This is what can lead to compulsive over-eating, particularly a response to stress or low mood. 

Mysimba works by altering these reward pathways. Bupropion boosts dopamine and norepinephrine levels, while naltrexone blocks the opioid feedback loop that would normally amplify the pleasure response. Together, they reduce the urge to eat for pleasure or comfort rather than for physical hunger. Making it easier to develop and maintain healthier eating patterns over time.

Does Mysimba affect GLP-1?

Although Mysimba does not contain a GLP-1 receptor agonist like semaglutide (found in Wegovy and Ozempic) or tirzepatide (found in Mounjaro), it can have indirect effects on appetite similar to those of GLP-1 medications. Both influence the central nervous system to reduce hunger and food intake but through different neurochemical pathways.

Eligibility for Mysimba Treatment

Mysimba is available through a private prescription only. To determine eligibility, you need to have a consultation with a clinician and you need to meet the criteria:   

• You are over 18 years old

• Your BMI is 30+ or

• Your BMI is over 27 and you have a weight-related health condition, such as type 2 diabetes, high blood pressure or sleep apnoea

The consultation will review your medical history, current medications, BMI and whether you've tried any other weight loss methods previously. This is to ensure that the Mysimba tablets are both safe and suitable for your individual needs.

Who can’t take Mysimba?

There is a list of those who are not eligible to take Mysimba and they include if you: 

• Are allergic to any of the ingredients including the active ingredients, naltrexone and bupropion

• Taking medication that also contains naltrexone or bupropion

• Have uncontrolled high blood pressure (hypertension)

• Have seizures or had them in the past

• Have a brain tumour

• Have severe liver disease

• Have end stage renal disease (severely reduced kidney function)

• Have been diagnosed at any stage throughout your life with 

  • bipolar disorder

  • an eating disorder, such as bulimia or anorexia nervosa

• Have been taking or plan to stop taking sedatives or anxiety medication such as benzodiazepines

• Are currently dependant on opioids (e.g. methadone) or going through opioid withdrawal

• Have taken or recently taken monoamine oxidase inhibitors (MAOIs) for depression or Parkinson’s disease (within the last 14 days)

• Are a heavy drinker, are planning or have stopped drinking while taking Mysimba tablets

Does Mysimba work?

Yes, clinical trials have shown that Mysimba is effective for weight loss, particularly when it has been used in conjunction with diet and an active lifestyle.

The table below summarises outcomes from clinical trials over 56 weeks, comparing Mysimba weight loss to the placebo. All participants received lifestyle advice but the level of counselling support varied:

• Less intensive counselling involved standard weight loss advice, such as general dietary guidance and encouragement to exercise, typically delivered during routine appointments

• More intensive counselling included a structured behavioural programme with 28 sessions (weekly, bi-weekly, then monthly) led by professionals. These sessions focused on diet, exercise, motivation, coping strategies and long-term habit change

Mysimba weight loss was greater than the placebo and those who received the more intensive counselling support, they saw the best results. On average, patients taking Mysimba with basic counselling lost around 5.7% of their body weight, while those with more structured support lost 8.1%. Notably, nearly one in three people in the intensive support group lost 10% or more of their body weight.

How to take Mysimba

Mysimba is a titration medication, which means the dosage will increase gradually according to the recommended schedule. The dosage is to increase over 4 weeks until you reach the maintenance dose. Once you are on the maintenance dosage, a single pack of 112 Mysimba tablets will last 4 weeks.

Week 1 - Day: 1 tablet. Night: None

Week 2 - Day: 1 tablet. Night: 1 tablet

Week 3 - Day: 2 tablets. Night: 1 tablet

Week 4 - Day: 2 tablets. Night: 2 tablets

Each Mysimba tablet must be taken with food and swallowed whole. Do not crush or chew the tablets. The maximum dose is four tablets per day, two tablets to be taken in the morning and two in the evening.

To stay on the medication, you're recommended to lose at least 5% of your body weight by week 16. Your clinician will review your progress to determine whether you should continue with Mysimba or if an alternative would be better suited to you and your goals.

How long does Mysimba take to work?

Effects on appetite may begin within a few hours of the first Mysimba dose and gradually build over several weeks as the dosage increases. Typically patients noticed a reduction in appetite, cravings and emotional eating during the first month of treatment. Weight loss progress is usually assessed after around 16 weeks, although the timing can vary depending on factors such as individual metabolism and how your body responds to the dose adjustments.

While some early changes may be felt in the first few weeks, it can take up to 8 weeks to see more noticeable Mysimba weight loss. During this time, patients report a gradual reduction in appetite and food-related impulses, which supports more consistent progress over time.

Mysimba side effects

Like all medications, Mysimba tablets can cause side effects, although not everyone will experience them. Side effects are more common in the early stages of treatment and often settle as your body adjusts to the medication, particularly during the first 4 weeks when the dose is gradually increased.

The common Mysimba side effects include: 

• Nausea

• Headaches

• Insomnia or sleep disturbances

• Dizziness

• Dry mouth

• Constipation or stomach upset

• Anxiety or restlessness

Rare and severe Mysimba side effects include: 

• Feelings of depression or feeling suicidal - if you experience these feelings, contact your clinician immediately or emergency medical services if your clinician isn't available 

• Serotonin syndrome (serotonin toxicity) - is a serious and potentially life threatening a reaction to medication where serotonin builds up.

• Fits or seizure - More common for those who've had a history of seizures, if you take too much Mysimba or it can be a reaction from taking other medication

• Erythema multiforme - a skin condition that causes red patches, blisters or sores

• Acute generalised exanthematous pustulosis - an allergic reaction to medication, that presents with a sudden skin reaction, a fever and pustules

• Rhabdomyolysis - where muscle breakdown occurs and it can lead to kidney damage

• Lupus - an autoimmune disease that can cause joint pain, skin rashes and tiredness

If you experience any of these Mysimba side effects, please contact 999 for immediate medical attention or go to A&E if you develop seizures, symptoms of serotonin syndrome (agitation, fever, stiffness), severe allergic reaction (swelling, breathing difficulty), or suicidal thoughts. For non-urgent advice, contact your clinician or NHS 111. 

While rare side effects occur less frequently, they are more serious. If you have questions or concerns about the side effects, either review the patient leaflet or get in contact with your clinician. 

How long do Mysimba side effects last?

Mysimba side effects can vary from person to person but many are most noticeable during the first few weeks of treatment. As your dose gradually increases over the first month, your body is adjusting and this is when side effects like nausea, dizziness or headaches are most likely to occur. These usually ease as your system gets used to the medication.

Common side effects begin to settle within 2 to 4 weeks, however, some side effects may last longer. Prolonged Mysimba side effects, such as dry mouth or sleep disturbances can continue even after you’ve reached the full maintenance dose. If side effects continue for more than a few weeks or become difficult to live with, it’s important to speak to your clinician. In some cases, adjusting the dose or switching medication might be recommended.

Serious side effects are rare but if they do happen, they won't go away on their own. If you experience symptoms like seizures, significant mood changes or allergic reactions, seek immediate medical attention.

Alternatives to MySimba

Choosing the right weight loss medication is personal. Each treatment has its own benefits, side effects, and considerations. Having a consultation with our clinician will help decide which option is most suitable based on your medical history, goals and preferences.

*% of people losing more than 5% of body weight.

Mysimba vs Saxenda

Mysimba and Saxenda are both prescription weight loss medications but they differ in how they work and how they’re taken. Mysimba is a tablet combining naltrexone and bupropion, targeting areas of the brain linked to cravings and emotional eating. Saxenda is a daily injection containing liraglutide, which helps control appetite by mimicking a natural hormone that regulates hunger. Both medications require a gradual increase in dosage over several weeks to minimise side effects.

The choice between them comes down to a personal preference, your medical history and your weight loss goals. Mysimba tablets are taken twice orally and Saxenda is injected once a day. 

Mysimba may not be suitable for individuals with certain mental health or seizure-related conditions, while Saxenda may not be recommended for those with a history of thyroid issues or pancreatitis. Your clinician will help determine which option is more appropriate for your needs.

Mysimba vs Orlistat

Mysimba and Orlistat are both prescription, tablet based medications for weight loss management and they work very differently to each other. Mysimba affects the brain’s appetite and reward centres, helping to reduce cravings and emotional eating. Orlistat, by contrast, works by blocking fat absorption in the digestive system, which is passed out of the body in the stools. Unlike Mysimba, Orlistat does not affect hunger or cravings and does not require a dose titration.

Mysimba may not be suitable for those who have certain mental health or seizure related conditions. Your clinician can help determine which medication is better suited to your needs and weight loss goals.

Advice and information to support weight loss