How Mysimba works

Mysimba is not a GLP-1 medication. It works through a completely different mechanism, combining two active ingredients that act on the brain:

Naltrexone is an opioid antagonist — it blocks opioid receptors in the brain that are involved in the reward pathways associated with eating. By dampening the pleasurable response to food, it can help reduce cravings and emotional eating.

Bupropion is an aminoketone antidepressant (also used for smoking cessation under the brand name Zyban). It acts on the hypothalamus — the brain's appetite control centre — to reduce hunger and increase energy expenditure. It also affects dopamine and noradrenaline pathways.

Together, these two ingredients target both the appetite and reward aspects of eating behaviour, making Mysimba particularly relevant for people whose weight gain is linked to cravings, emotional eating, or food reward-seeking behaviour rather than purely physical hunger.

Dosing schedule

Mysimba is titrated over 4 weeks to the full maintenance dose:

• Week 1: 1 tablet in the morning
• Week 2: 1 tablet morning + 1 tablet evening
• Week 3: 2 tablets morning + 1 tablet evening
• Week 4 onwards: 2 tablets morning + 2 tablets evening (maintenance)

Each tablet contains 8mg naltrexone and 90mg bupropion (extended-release). The tablets should be swallowed whole — not crushed, cut, or chewed.¹

Who is Mysimba for?

Licensed for: Weight management in adults with BMI ≥30, or BMI ≥27 with at least one weight-related comorbidity, alongside a reduced-calorie diet and physical activity.¹

NHS: Available on NHS prescription through specialist weight management services.

Private: Available from private prescribers. Generally cheaper than GLP-1 medications.

Not suitable for: People with uncontrolled high blood pressure, seizure disorders, current or past eating disorders (bulimia or anorexia), people undergoing abrupt withdrawal from alcohol or benzodiazepines, or those taking MAO inhibitors.¹

Common side effects

• Nausea (~30% — most common, usually improves after first few weeks)
• Constipation, headache, vomiting
• Dizziness, insomnia (difficulty sleeping)
• Dry mouth, anxiety, hot flushes, tremor

Serious risks:

• Seizures — the bupropion component carries a dose-dependent seizure risk. Mysimba is contraindicated in people with seizure history.
• Increased blood pressure — blood pressure should be monitored regularly during treatment. Treatment should be stopped if clinically significant increases occur.
• Psychiatric effects — rare reports of suicidal thoughts, associated with the bupropion component. Patients with a history of depression should be monitored.

View full side effects breakdown →

Key clinical trials

COR programme: A series of trials (COR-I, COR-II, COR-BMOD, COR-Diabetes) showed naltrexone/bupropion produced approximately 5–8% weight loss over 56 weeks compared to placebo. The COR-BMOD trial, which combined the medication with intensive behavioural modification, showed the best results.²

Cost in the UK

Private prescriptions for Mysimba typically cost £70–£100 per month, making it the cheapest prescription weight loss medication after orlistat. It's significantly less expensive than GLP-1 injections, though the average weight loss is also lower.

How Mysimba compares to GLP-1 medications

Mysimba produces less weight loss on average than Mounjaro or Wegovy (5–8% vs 20%+). However, it has some advantages: it's an oral tablet (no injections), it's cheaper, and it works through different pathways — making it potentially suitable for people who can't tolerate GLP-1 medications or who find their eating is more reward-driven than hunger-driven.

Some clinicians prescribe Mysimba alongside GLP-1 medications in specific cases, though this should only be done under specialist supervision.