If you’ve been reading about Mysimba (or seen it called ‘Contrave’ in the US) and trying to work out whether it’s the right weight-loss option for you, this guide covers what it does, who it’s for, who absolutely shouldn’t take it, and how it compares clinically to other UK-licensed weight management options.
In this article:
What is Mysimba?
How much weight can I lose on Mysimba?
Why isn’t Mysimba available on the NHS?
Who shouldn’t take Mysimba? The contraindications checklist
How do I take Mysimba? The 4-week titration schedule
What are the side effects of Mysimba?
How Mysimba compares clinically to other UK-licensed weight management options
When should I stop Mysimba?
Is Contrave the same as Mysimba?
Frequently asked questions about Mysimba
What is Mysimba?
Mysimba is a weight loss pill available in the UK, but you'll need a prescription to get it. It contains two medications, Naltrexone and Bupropion, and it works by reducing appetite and certain food cravings. You can only get it after a doctor or other independent prescriber has performed a clinical assessment.
Mysimba is a prescription-only UK weight-loss tablet that combines two active ingredients: Naltrexone (32mg per day at the maintenance dose) and Bupropion (360mg per day at maintenance dose). These two ingredients affect the parts of your brain and hypothalamus that control your appetite, and the brain’s reward system, so you feel less hungry and are less drawn to foods that are high in calories (2)(3).
Naltrexone is best known for use in people with problems with alcohol or opioids. Bupropion is an antidepressant and is also prescribed to help people to stop smoking. In combination for weight loss, they work to suppress your appetite and reward system, not your mood. Mysimba is taken as a tablet, not an injection, and it doesn't work like Orlistat (a lipase inhibitor) which is another approved weight management treatment in the UK.
Adults with a BMI of 30 or higher, or 27 or higher who also have at least one weight-related health problem (like type 2 diabetes, high cholesterol, or high blood pressure) can be prescribed it, alongside a reduced-calorie diet and increased physical activity (3). If Mysimba sounds like it might be a good choice for you, the next step is a clinical consultation with an independent prescriber, who will confirm eligibility, contraindications and whether Mysimba is the right fit compared to other UK-licensed weight management options.
How much weight can I lose on Mysimba?
With Mysimba, people typically lose between 3.7 and 5.7% of their original body weight (2). That isn't as much as people may typically lose with GLP-1 injections, but it is a meaningful amount for something you take as a pill.
Three large clinical trials showed an average weight loss on Mysimba of 3.7 to 5.7% of their starting weight, while those on a placebo lost only 1.3 to 1.9% (2). Between 13 and 22% of people taking Mysimba managed to lose 10% or more of their weight, compared to 5 to 6% of those on the placebo (2). A thorough review and analysis by Dr Onakpoya and colleagues showed that Naltrexone-Bupropion leads to roughly 2.5kg more weight loss than placebo when looking at all the trials together (4).
To put this into context, this is a smaller change than with GLP-1 injections, which clinical trials have shown higher percentages of weight loss on average (6,7). However, it's a meaningful result, and for individuals who either are unable or don't wish to have injections. For most people who benefit, weight comes off at a fairly consistent rate in the first 16 weeks. The official product information advises you to stop taking it if you haven't lost at least 5% of your initial weight within those 16 weeks (3).
Why isn’t Mysimba available on the NHS?
Mysimba isn’t on the NHS because NICE have not recommended it based on its cost-effectiveness (1), therefore access in the UK is by private prescription only.
In the UK, you can only get Mysimba with a private prescription. NICE’s Technology Appraisal TA494 (which came out in 2017 and was reviewed in 2020) says that Naltrexone-Bupropion isn't to be funded by the NHS in England (1). That means that your GP can't prescribe it for you on the NHS. The NICE committee said they weren’t sure if it’s cost-effective compared to other weight loss medications, and that the potentially eligible NHS population was too broad with long treatment durations; therefore, it would be too costly to provide to patients on the NHS. You can read the NICE TA494 recommendations directly.
So, if you want to get Mysimba in the UK, you’ll need a prescription from a qualified doctor or pharmacist who is independently registered in the UK. Reputable pharmacies in the UK will require you to have a consultation and will issue a proper prescription. If you've never had a weight-loss consultation before, Oxford Online Pharmacy’s guide to what happens during a weight-loss consultation walks through the steps, and OOP’s guide to who can prescribe weight-loss medications in the UK explains the regulatory framework.
You should never buy from places that aren't properly regulated. Websites calling themselves ‘Contrave UK’ that offer the drug without a consultation with a prescriber, are nearly always based abroad and don't follow the correct rules in the UK. The General Pharmaceutical Council and the Medicines and Healthcare products Regulatory Agency have cautioned against buying weight loss medications from unregulated sources. The Advertising Standards Authority, the MHRA and the GPhC have strengthened this warning in a joint statement about prescription-only weight management medicines (September 2025).
Who shouldn’t take Mysimba? The contraindications checklist
Don’t take Mysimba if you use any opioid (including codeine), have uncontrolled hypertension or a history of seizures (3).
Mysimba has a relatively long contraindication list. Many of these are non-negotiable safety rules that you must follow (3):
Uncontrolled hypertension: Mysimba can raise blood pressure. Your blood pressure must be well-controlled before starting.
Current or past seizures, or a history of a seizure disorder: Bupropion lowers the threshold for having a seizure.
Current or prior bulimia or anorexia: increased seizure risk and clinical concerns.
Chronic opioid use, including prescription codeine, co-codamol, tramadol, methadone, buprenorphine and morphine. Naltrexone blocks opioid receptors and can exacerbate withdrawal. Even occasional post-surgical codeine means delaying Mysimba until opioids have been cleared from your system (typically 7 to 10 days).
Pregnancy and breastfeeding: not recommended for use. Discuss contraception with your prescriber.
Severe hepatic (liver) impairment: Mysimba is metabolised by the liver.
End-stage renal (kidney) disease: dose adjustment or avoidance following advice from a specialist.
Bipolar disorder: Bupropion can trigger mood episodes.
Concurrent MAOI antidepressant use: dangerous interaction. Allow at least 14 days off an MAOI before starting.
Abrupt alcohol or benzodiazepine withdrawal: seizure risk.
When you meet with your doctor, be sure to take a list of all your medications with you. This means your usual prescriptions, over the counter painkillers, inhalers, vitamins and any other supplements. That is where any problems with combining medications will be discovered.
How do I take Mysimba? The 4-week titration schedule
You generally increase your dose of Mysimba over a period of 4 weeks, going from 1 tablet each day to 2 tablets twice a day (this is the maintenance dose), and you should take it with food, but it’s best to avoid food that’s very high in fat (3).
Week | Dose | What to expect |
|---|---|---|
Week 1 | 1 tablet in the morning | Nausea may begin. Take with food (not a high-fat meal). |
Week 2 | 1 tablet in the morning + 1 tablet in the evening | Nausea often peaks. Headache and constipation may appear. |
Week 3 | 2 tablets in the morning + 1 tablet in the evening | Headache and constipation may peak. Insomnia and dry mouth may start. |
Week 4 | 2 tablets in the morning + 2 tablets in the evening (maintenance dose) | Most side effects settle by week 6 to 8 if you stay on the drug. |
When you take this medication, make sure to eat something before taking it, but avoid really fatty foods. Fatty foods will quickly increase the amount of the medicine in your system and may worsen any side effects. Should the side effects become too much to handle at any point during treatment, your prescriber might decide to keep you on your current dosage instead of increasing it, or they might have you stop the medication altogether if the side effects don't go away.
Don’t split or crush tablets as they are made in a special way to release the medicine slowly, so it’s important to keep them intact. And if you forget to take a dose, simply leave it and take your next dose at the usual time, don't take two doses to catch up.
What are the side effects of Mysimba?
Most common Mysimba side effects are nausea, constipation, headache and vomiting, usually peaking during the 4-week period where you gradually increase the dose (3).
The most common side effects of Mysimba are nausea (affecting around 1 in 3 people), constipation, headache, vomiting and dizziness. They are usually worse during the first 4 weeks of gradually increasing the dose but settle down after (3).
Very common (more than 1 in 10 users): nausea, constipation, headache, vomiting.
Common (1 in 10 to 1 in 100): dizziness, dry mouth, insomnia, abdominal pain, tinnitus, tremor, hot flush, anxiety, palpitations, hypertension, rash, fatigue, irritability.
Rare but serious: seizures, suicidal thoughts (rare, reported with Bupropion in younger adults), severe allergic reactions.
You should definitely get in touch with your doctor if you have pain in your chest, show signs of severe allergic reaction, have suicidal thoughts or are experiencing a seizure. To reduce problems with the medicine, take it with food, don't drink alcohol for the first few weeks, drink lots of fluids, and if you get constipation, get more fibre and fluids, plus stick to the plan for slowly increasing the dose.
You can report any side effect you experience through the MHRA Yellow Card scheme, which helps monitor the medicine’s safety in the UK once it’s on the market (5).
How Mysimba compares clinically to other UK-licensed weight management options
When taken as a pill, Mysimba helps people lose between 3.7% and 5.7% of their weight (8). The table below is for medical information and to give a broader understanding, it isn't for comparing treatments directly. All of the drugs listed in the table need a prescription from a doctor or are available from the pharmacy after a medical evaluation.
The table below gives a summary of how each of the main weight loss medicines approved for use in the UK work, how much weight people typically lose, how they are given, and if you can get them on the NHS. It is included to help you understand where Mysimba may fit medically; it isn't to say one is better than another. Which medication (if any) will be right for you depends on your BMI, any other health conditions you have, contraindications, tolerance, pregnancy status, concurrent medications and prescriber judgement. A clinical assessment is the only appropriate route to a decision.
Medicine | Mechanism | Average weight loss | Administration | NHS Availability |
|---|---|---|---|---|
Mysimba (Naltrexone-Bupropion) | Appetite and reward pathways | 3.7 to 5.7% (8) | Daily tablets | Private only |
Orlistat | Lipase inhibitor, blocks fat absorption | Around 3% | Tablets with meals | NHS and private |
Wegovy GLP-1 receptor agonist (Semaglutide) | GLP-1 receptor agonist | Reported around 19-21% in clinical trials (6) | Weekly injections | NHS specialist weight-management services; private availability if eligible |
Mounjaro GLP-1 and GIP receptor agonist (Tirzepatide) | Dual GLP-1 and GIP receptor agonist | Reported around 20-22.5% in clinical trials (7) | Weekly injections | Phased NHS rollout; private availability if eligible |
When should I stop Mysimba?
It is recommended to stop Mysimba if you haven’t lost at least 5% of starting weight by the 16-week mark as you are unlikely to benefit from keep using it (3).
The official product information says that Mysimba should be discontinued after 16 weeks if you haven’t lost at least 5% of your initial body weight, as you are unlikely to benefit from keep using it (3).
Keep having your progress checked regularly if you reach the 5% weight loss mark after 16 weeks and your doctor should monitor you every few months. However, if you haven’t lost 5% of your weight, talk to your doctor about what to do instead. You could move to another weight loss medication approved for use in the UK (assuming you fit the guidelines from NICE or the medication’s official eligibility criteria), or go back to getting support for diet and exercise from a specialist programme. And remember, any medication switch should be a decision made in conjunction with a prescriber, not solely by yourself.
You don't have to slowly decrease your dose of Mysimba when you stop taking it, although your prescriber might suggest reducing the dose within a short period. As with all medicines that help you lose weight, you might put some weight back on after stopping. Most research suggests you’ll regain some weight by the 12-month mark unless you continue with your new healthy lifestyle habits.
After you’ve stopped, don't immediately go back to your full dose of Mysimba. Build your dose back up gradually to lessen any side effects. If you want a prescriber to review whether Mysimba is still the right fit, start your free consultation.
Frequently asked questions about Mysimba
What is Mysimba and how does it work for weight loss?
Mysimba is a UK prescription-only tablet combining Naltrexone (32mg per day) and Bupropion (360mg per day). Together these two ingredients act on two appetite-regulation pathways, the hypothalamus and the brain’s reward system, to reduce hunger and food cravings. Mysimba is licensed for adults with BMI 30 or above, or BMI 27 or above with a weight-related health condition, alongside a reduced-calorie diet and increased physical activity.
How much weight can I lose on Mysimba?
In clinical trials, the average weight loss on Mysimba was 3.7 to 5.7% of starting body weight (8). That’s a rather modest loss compared with some GLP-1 injections, but still a meaningful difference for a tablet (6,7). Most responders lose weight steadily during the first 16 weeks. If you haven’t lost at least 5% of your starting body weight by week 16, the official product information recommends stopping it.
Is Mysimba available on the NHS?
No. The NICE appraisal (from 2017, and reviewed in 2020) doesn’t recommend Naltrexone-Bupropion for NHS use in England. In the UK, Mysimba is only available via a private prescription from a regulated prescriber. Reputable pharmacies in the UK will want you to have a consultation with a prescriber who has checked your medical history, either online or in person.
Who shouldn’t take Mysimba?
Don’t take Mysimba if you have uncontrolled hypertension, a history of seizures, current or past bulimia or anorexia, current chronic or occasional opioid use (including codeine, tramadol, methadone), pregnancy or breastfeeding, severe liver impairment, end-stage kidney disease, bipolar disorder, concurrent MAOI use, or are in acute alcohol or benzodiazepine withdrawal. An EMA safety review of Mysimba in 2025 reviewed long-term cardiovascular safety concerns, however, the outcome was that benefits continue to outweigh risks.
Is Mysimba the same as Contrave?
Yes. Contrave and Mysimba are the exact same medication, Naltrexone-Bupropion, with the same dose, clinical trial data and safety profile. Contrave is the US brand name (FDA-approved 2014). Mysimba is the UK and EU brand name (EMA-approved 2015). If you’ve read about Contrave on US blogs, all of the information applies to Mysimba in the UK. You can’t legally import Contrave, but you can get the UK-licensed equivalent as Mysimba via a UK prescriber.
Can I buy Contrave online in the UK?
No. Contrave is a US brand and can’t be legally imported into the UK. The UK-licensed equivalent is Mysimba and is available on private prescription only via a GPhC-registered UK pharmacy after a prescriber-reviewed consultation. Unregulated overseas ‘Contrave UK’ websites are unsafe and have been warned about by the MHRA.
What are the most common side effects of Mysimba?
The most common side effects are nausea (around 1 in 3 people), constipation, headache and vomiting (all very common), usually peaking during the initial 4-week period of increasing the dose. Common effects include dizziness, dry mouth, insomnia, tinnitus, tremor, hot flush, anxiety, palpitations, hypertension and rash. Rare but serious effects include seizures (approximately 0.1% of people) and suicidal thoughts (9). You should report any side effects via the MHRA Yellow Card scheme.
How long does it take for Mysimba to start working?
Appetite reduction often begins during the initial 4-week period of increasing the dose. Measurable weight loss typically becomes apparent by weeks 4 to 8 in people who respond to the treatment. The official product information specifies having an assessment at the 16-week mark. If you haven’t lost at least 5% of starting body weight by week 16, continuing on Mysimba is unlikely to help with weight loss.
How does Mysimba compare clinically to other UK-licensed weight management options?
Mysimba tends to lead to around 3.7 to 5.7% of weight loss, and is taken as a tablet. GLP-1 injections have reported higher average percentage weight loss but are injections and carry their own eligibility criteria, contraindications and monitoring requirements. Mysimba may suit patients who can’t or don’t like to inject, don’t use opioids and can tolerate the side-effect profile. Any prescription weight loss medicines should only be taken after a consultation with a qualified prescriber.
Can I take Mysimba with my antidepressant?
It depends on the antidepressant. Mysimba can’t be taken with MAOIs (allow at least 14 days off an MAOI before taking Mysimba). SSRIs (e.g. sertraline, fluoxetine, citalopram) can sometimes be taken alongside Mysimba but require a prescriber’s review because Bupropion affects the same neurotransmitter systems. You should always disclose every antidepressant, including any taken in the past year, to the medical professional at the consultation. Don’t start Mysimba on top of an antidepressant without talking to a medical professional first.
Is it safe to drink alcohol on Mysimba?
Alcohol should be kept to a minimum on Mysimba, especially during the initial dose escalation period. Bupropion can lower the threshold of having a seizure, and abrupt alcohol reduction in a previously heavy drinker can trigger seizures. Occasional light drinking may be acceptable for some patients, but should be discussed with your prescriber. Don’t start Mysimba if you are in acute alcohol withdrawal.
References
National Institute for Health and Care Excellence. Naltrexone–bupropion for managing overweight and obesity. Technology appraisal guidance TA494. Published 2017 Dec 13; reviewed 2020.
European Medicines Agency. Mysimba: European Public Assessment Report (EPAR). EMA. Marketing authorisation granted 2015 Mar 26.
Mysimba 8 mg/90 mg prolonged-release tablets. Summary of Product Characteristics (SmPC). Orexigen Therapeutics Ireland Ltd. electronic Medicines Compendium.
Onakpoya IJ, Lee JJ, Mahtani KR, Aronson JK, Heneghan CJ. Naltrexone–bupropion (Mysimba) in management of obesity: a systematic review and meta-analysis of unpublished clinical study reports. Br J Clin Pharmacol. 2020;86(4):646-667. PMID 31918448; PMCID PMC7098870.
Yellow Card: making medicines and medical devices safer. Medicines and Healthcare products Regulatory Agency. [Accessed 2026 Apr 17].
Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989-1002.
Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al.; SURMOUNT-1 Investigators. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205–216. doi: 10.1056/NEJMoa2206038.
European Medicines Agency. Mysimba: EPAR – summary for the public. EMA/83596/2015, EMEA/H/C/003687. 2015 [Internet].
Nalpharma UK Ltd. Mysimba 8 mg/90 mg prolonged-release tablets – Summary of Product Characteristics (SmPC). electronic medicines compendium (emc); 2025 [Internet].