Mysimba vs Semaglutide: Two Very Different Approaches to Weight Loss

Step 1: Understand what each medicine actually does

Mysimba combines naltrexone (an opioid-antagonist used in addiction medicine) with bupropion (an antidepressant also used in smoking cessation). Together they act on the hypothalamus and the brain's reward circuitry, reducing appetite and blunting the drive to eat. It comes as a modified-release tablet taken twice daily, titrated over four weeks to the full maintenance dose of two tablets twice daily. There is no injection involved, which matters to some people.

Semaglutide works downstream of the brain too, but its primary mechanism is hormonal. As a GLP-1 receptor agonist, it mimics a gut hormone released after eating, slowing gastric emptying, increasing feelings of fullness, and reducing appetite signals. Wegovy is injected once weekly via a pre-filled pen; the dose is titrated gradually by the prescriber, starting low to allow the body to adjust. The GLP-1 class is the dominant pharmacological approach to weight management in the UK right now, largely because of the scale of trial evidence behind it.

Neither medicine replaces a reduced-calorie diet and increased physical activity, both licences are explicit on that point, and clinical guidance expects both alongside any pharmacological treatment.

Step 2: Compare the trial evidence side by side

The honest starting point is that these two medicines have not been compared head-to-head in a randomised trial. The figures below come from separate placebo-controlled studies with different populations and durations, so direct comparison has limits.

The STEP 1 trial figures are published in the New England Journal of Medicine and formed the basis of NICE's appraisal of semaglutide. Mysimba's trial data, while demonstrating statistically significant weight loss, consistently produced more modest average reductions, and if you want to explore how the two medicines stack up in more detail, our Mysimba vs Wegovy comparison page walks through the key differences. For many patients and clinicians, that gap is clinically meaningful.

Step 3: Check which you're likely to be eligible for, and where

Both medicines share similar headline eligibility thresholds on their UK licences (BMI ≥30, or ≥27 with a qualifying condition), but the practical access picture differs considerably. Mysimba is available privately; it does not currently have a NICE technology appraisal recommending it for routine NHS commissioning for weight management, so NHS access is limited. Semaglutide as Wegovy sits within NICE TA875, which recommends it only within specialist weight management services, for a maximum of two years, and with BMI and comorbidity criteria that are relatively strict, meaning NHS waiting lists tend to be long.

Privately, a prescriber can assess suitability for either medicine based on your individual clinical picture. Certain contraindications are specific to each: Mysimba is not suitable for people with a history of seizures, eating disorders, opioid dependence, or uncontrolled hypertension, among others. Semaglutide's contraindications include a personal or family history of medullary thyroid carcinoma or MEN2. A prescriber works through all of this before any treatment is issued. Our Wegovy pricing page covers what a private Wegovy prescription typically involves financially, and our FAQs address common questions about the consultation process.

It is also worth noting that Mysimba contains bupropion, which has interactions with a range of medicines, including some antidepressants and drugs that lower the seizure threshold. Medication review is part of any responsible prescribing process for either option.

Step 4: Factor in side-effect profiles and practical day-to-day use

Mysimba's most common side effects are nausea, constipation, headache, dizziness, insomnia and dry mouth. Nausea is particularly common in the titration phase and is the main reason the dose is built up slowly. Some people also experience blood pressure changes, which is why blood pressure monitoring is part of the prescribing protocol.

Semaglutide's side-effect profile is gastrointestinal-led: nausea, vomiting, diarrhoea and constipation are the most frequently reported, typically peaking after a dose increase before settling. The NHS semaglutide medicines page sets out what to watch for and when to seek help; the MHRA's Yellow Card scheme is the route for reporting any suspected reactions. A prescriber can advise on managing both profiles; it is not something to navigate from a webpage alone.

On the practicalities: Mysimba is taken orally twice a day, which suits people who prefer tablets. Wegovy is injected once weekly, which some find easier to build into a routine. If the injection aspect concerns you, it is worth reading about injection site options, for many people the practical reality is much simpler than anticipated. Comparing Wegovy with liraglutide, another injectable GLP-1, is covered separately on our liraglutide vs semaglutide page if that comparison is also relevant to you.

Which of these medicines is the right choice depends on your medical history, current medications, BMI, qualifying conditions and personal preferences. That is a clinical decision made with a prescriber, not a conclusion a comparison table can reach. Speak to our prescribers through a free consultation to talk through what fits your situation.