This guide explains what Mysimba is, how the bupropion-naltrexone combination works, who it is licensed for, how it is taken, its side effects and its important contraindications. It draws on the UK Summary of Product Characteristics. It is general information; whether Mysimba is suitable for you is a clinical decision, and any mental-health history is especially important to discuss.
What is Mysimba?
Mysimba is a prolonged-release oral tablet that combines two active ingredients: naltrexone hydrochloride 8 mg and bupropion hydrochloride 90 mg per tablet 1. It is a prescription medicine for weight management, taken by mouth rather than injected 1.
It is licensed as an adjunct to a reduced-calorie diet and increased physical activity for weight management in adults, so, like the other weight-loss medicines, it is meant to support lifestyle change rather than replace it 1. The sections below explain how it works and who it is for 12.
Because it combines two centrally-acting medicines, Mysimba has a distinctive profile of effects and cautions that set it apart from the GLP-1-based injections, which this guide sets out so you can discuss it with a prescriber 12. Being a tablet rather than an injection appeals to some people, but the trade-off is a medicine that acts on the brain and so carries a different, broader set of cautions, which is why the assessment for it matters so much 12.
How Mysimba works
The exact appetite-suppressing mechanism is not fully understood, but the two components act on two main brain areas: the appetite centre (the arcuate nucleus of the hypothalamus) and the mesolimbic dopamine reward system2. Bupropion is a weak inhibitor of dopamine and noradrenaline reuptake, and naltrexone is an opioid-receptor antagonist 2.
In simple terms, bupropion stimulates appetite-regulating neurons, and naltrexone is thought to block a feedback loop that would otherwise dampen that effect, so the two together may reduce food intake more than either alone 2. So Mysimba works on the brain's appetite and reward pathways, a different route from the gut-hormone injections 2.
This brain-focused mechanism is why Mysimba's side effects and cautions extend into areas like mood, sleep and seizures, covered below, rather than being mainly gut-related 2.
It also helps to recognise the two ingredients in their own right 2. Bupropion is a medicine also used in other contexts, and naltrexone is an opioid-blocking medicine used elsewhere too, and combining them at these doses is what creates Mysimba's weight-management effect 2. Knowing that the components have effects of their own makes sense of why the medicine interacts with the brain, mood and seizure threshold in ways an appetite-only or gut-only medicine would not 2.
Who Mysimba is licensed for
Mysimba is licensed for adults (18 and over) with an initial BMI of at least 30 (obesity), or at least 27 with one or more weight-related comorbidities such as type 2 diabetes, dyslipidaemia or controlled high blood pressure, alongside diet and activity 1. So it targets a similar BMI group to the other weight-loss medicines 1.
Its effectiveness is reviewed at 16 weeks: if treatment is working, it continues and is re-evaluated annually 1. So, as with the other options, it is expected to be producing results to be continued 1.
There are also age limits: Mysimba should be used with caution over 65 and is not recommended over 751. Whether it is appropriate for you depends heavily on your medical and mental-health history, given its contraindications below 12.
It is also worth noting that the dose escalation at the start exists for a reason 1. Rather than starting at the full two-tablets-twice-daily dose, treatment is built up over the first weeks, which helps the body adjust and reduces the chance of side effects bad enough to stop 1. Following that schedule, rather than rushing to the full dose, is part of getting on well with the medicine, much as the gradual build matters for the injectable options too 1.
Considering treatment for weight management? You can start an assessment with a Cloud Pharmacy clinician, who will review your medical history and confirm whether treatment is appropriate.
How Mysimba is taken
Mysimba is built up gradually to reduce side effects. The maximum recommended dose is two tablets twice daily (a total of 32 mg naltrexone and 360 mg bupropion a day), reached through a step-up schedule over the first weeks 1. So it is taken by mouth several times a day rather than as a single weekly dose 1.
An important practical point: the tablets must be swallowed whole and must not be cut, crushed or chewed, because doing so alters the prolonged-release profile and increases the risk of seizures1. This is a genuine safety instruction, not just a preference 1.
As with the other weight-loss medicines, Mysimba is meant to be taken alongside diet and activity, and the 16-week review checks it is working 1. Our guide comparing Mounjaro and Mysimba sets it against the injectable option 1.
Side effects
Mysimba's very common side effects include nausea, constipation, vomiting and headache2. Common effects include dizziness, dry mouth, anxiety and insomnia, along with palpitations, an increased heart rate, hot flushes and raised blood pressure 2. So it affects sleep, mood and the cardiovascular system as well as the gut 2.
There is a recognised, if low, seizure risk, around 0.1 percent (1 in 1,000), which is part of why several of the contraindications and the swallow-whole instruction exist 2. The product information also lists reports of suicidal ideation and behaviour, so any change in mood while taking it should be taken seriously 2.
If you experience troubling mood changes, or any thoughts of harming yourself, seek help promptly, and do not stop the medicine or any mental-health medicine on your own; speak to a clinician 2. This is a sensitive area, and support is available if you are struggling 2.
The cardiovascular effects are worth a specific mention because they shape who Mysimba suits 2. Because it can raise heart rate and blood pressure, it is not appropriate for people with uncontrolled high blood pressure, and blood pressure may be monitored during treatment 12. If you have heart or blood-pressure conditions, this is an important thing to raise, since it bears directly on whether the medicine is safe for you 12.
Important contraindications and cautions
Mysimba has a notably long list of contraindications. It must not be used in people with uncontrolled high blood pressure, a current or past seizure disorder, a known central nervous system tumour, bipolar disorder, a current or previous diagnosis of bulimia or anorexia nervosa, opioid dependence, during alcohol or benzodiazepine withdrawal, or alongside MAOI antidepressants1.
Beyond the outright contraindications, caution is needed where the seizure threshold may be lowered, including with certain other medicines (such as some antidepressants, antipsychotics, tramadol and others) and in situations like low blood sugar in diabetes 1. This is why your full medical and medicine history matters so much 1.
Because so many of these relate to mental health and the nervous system, anyone with a relevant history must discuss it honestly with their prescriber, and Mysimba is not suitable for everyone 12. It is a medicine where the assessment is especially important 1.
None of this is meant to put you off if Mysimba is appropriate for you; it is meant to explain why the assessment for it is more searching than for some other medicines 1. The long contraindication list exists to keep the medicine away from the people for whom its specific risks, particularly around seizures and mood, would be a problem, so that it can be used safely by those for whom it suits 12. Honesty about your history at the assessment stage is what makes that work, and a good prescriber will ask the questions that draw it out 1.
Frequently asked questions
What is Mysimba?
Mysimba is a prolonged-release oral tablet combining naltrexone 8 mg and bupropion 90 mg, licensed for weight management alongside diet and activity in adults with a BMI of 30+, or 27+ with a weight-related condition 1. It acts on the brain's appetite and reward pathways rather than gut hormones 2.How does Mysimba work?
It combines bupropion (a weak dopamine and noradrenaline reuptake inhibitor) and naltrexone (an opioid-receptor antagonist), which act on the appetite centre of the hypothalamus and the brain's reward system to reduce food intake 2. The exact mechanism is not fully understood, but the two are thought to work better together than alone 2.How do you take Mysimba?
It is built up gradually to a maximum of two tablets twice daily, taken by mouth, and the tablets must be swallowed whole, never cut, crushed or chewed, because that increases the seizure risk 1. Effectiveness is reviewed at 16 weeks, and it is used alongside diet and activity 1.What are the side effects of Mysimba?
Very common effects include nausea, constipation, vomiting and headache; common ones include dizziness, dry mouth, anxiety, insomnia, palpitations and raised blood pressure 2. There is a low seizure risk (around 0.1 percent), and the information lists reports of suicidal ideation, so mood changes should be taken seriously 2.Who cannot take Mysimba?
It is contraindicated in uncontrolled high blood pressure, current or past seizures, a CNS tumour, bipolar disorder, current or previous bulimia or anorexia, opioid dependence, alcohol or benzodiazepine withdrawal, and with MAOIs, among others 1. It is used with caution over 65 and not recommended over 75, so the assessment is especially important 1.Is Mysimba safe if I take antidepressants or have a mental-health history?
It depends on your individual situation and is a clinical decision; some mental-health conditions (like bipolar disorder) and certain medicines are contraindications or cautions, partly because of the seizure risk 1. Disclose your full history, never stop a mental-health medicine on your own, and seek help promptly for any mood changes or thoughts of self-harm 2.Your next step
Mysimba is an oral tablet combining naltrexone and bupropion, licensed for weight management alongside diet and activity in adults with a BMI of 30+, or 27+ with a weight-related condition. It acts on the brain's appetite and reward pathways, is built up to two tablets twice daily (swallowed whole to avoid increasing seizure risk), and is reviewed at 16 weeks. Its side effects extend into mood, sleep, heart rate and blood pressure.
Mysimba has an important and fairly long list of contraindications, many relating to mental health and the nervous system, so an honest discussion of your full medical and mental-health history with a prescriber is essential, and it is not suitable for everyone. Never stop a mental-health medicine on your own, and seek help promptly for any mood changes or thoughts of self-harm. The long list of cautions is not there to put you off if Mysimba suits you, but to keep it away from those for whom its specific risks would be a problem, so that it can be used safely by those it does suit, which is why honesty at the assessment stage matters so much for this medicine in particular, more than it does for some of the other weight-loss options, and a good prescriber will ask the questions that draw your history out.
Disclaimer
This guide is for general information only and does not constitute medical advice, diagnosis or treatment. The information here describes general clinical context based on UK regulatory sources cited above; it is not a recommendation for any specific medicine or treatment, which can only be made by a prescriber following individual assessment.
If you are considering treatment, speak to your GP or pharmacist, or arrange a consultation with a Cloud Pharmacy clinician. Prescription-only medicines are issued only after clinical assessment and where appropriate.
If you experience side effects from any medicine, you can report them through the Yellow Card scheme at yellowcard.mhra.gov.uk.
References
- Mysimba SmPC 2/4.1/4.2/4.3 (composition naltrexone 8mg/bupropion 90mg PR; BMI indication; max 2 tablets twice daily; swallow whole/seizure risk; 16-week review; age limits; contraindications)
- Mysimba SmPC 4.8/5.1 (mechanism: bupropion + naltrexone on hypothalamus/reward; very common nausea/constipation/vomiting/headache; common anxiety/insomnia/dizziness/palpitations; seizure ~0.1%; suicidal ideation reports)
