Mysimba vs Mounjaro: clearing up the comparison people get wrong

The misconception worth correcting first: Mysimba and Mounjaro are not interchangeable alternatives

A common assumption is that Mysimba and Mounjaro sit in the same drawer, doing the same job via a slightly different route. They do not. Mysimba (naltrexone/bupropion) works centrally, it acts on opioid and dopamine receptors in the brain's reward and hunger pathways, reducing the drive to eat and dulling the reward signal that reinforces overeating. Mounjaro works peripherally and centrally as a dual GIP and GLP-1 receptor agonist: it slows how quickly food leaves the stomach, raises post-meal fullness signals, and reduces appetite through gut-hormone pathways. These are meaningfully different pharmacologies, not cosmetic ones.

That difference matters for side-effect profiles, for who can take each medicine, and for how much weight the evidence suggests each produces. Mysimba carries neuropsychiatric warnings; Mounjaro's effects are predominantly gastrointestinal. Certain cardiac conditions, seizure histories and opioid use can rule Mysimba out entirely; thyroid or pancreatitis histories require careful assessment before Mounjaro. Treating them as like-for-like options, and picking on price alone, is the wrong approach, and, for a prescription-only medicine, price is the wrong test anyway.

The prescriber's job is to match pharmacology to the patient, not to offer a menu.

What the clinical evidence shows for each

Trial data tells a clear story, though direct head-to-head evidence between these two medicines does not currently exist. Mounjaro's SURMOUNT-1 phase-3 trial, published in the New England Journal of Medicine, randomised 2,539 adults with obesity and found average body-weight reductions of around 20–21% at the 15mg maintenance dose over 72 weeks. That figure sits markedly higher than anything Mysimba's trials produced. Mysimba's pivotal programme reported average weight losses in the range of 5–9% above placebo, clinically meaningful, and sufficient for NICE's original appraisal, but a different order of magnitude.

NICE appraised tirzepatide in technology appraisal TA1026, published December 2024, recommending it for adults with a BMI of 35 or above plus at least one weight-related condition (with lower thresholds for some ethnic backgrounds under UK guidance). Mysimba has a separate, older NICE appraisal and is more narrowly recommended within specialist NHS weight management services. Neither appraisal compares the two medicines directly.

One practical note on the GLP-1 landscape: if you want to see how Mounjaro sits against other injectable approaches, our guide to GLP-1 versus Mounjaro covers that ground in more detail.

Eligibility, prescribing rules and what private access looks like

Both medicines are licensed for adults with a BMI of 30 or above, or 27 and above alongside a qualifying weight-related condition such as high blood pressure, type 2 diabetes or dyslipidaemia. Beyond that shared floor, the lists diverge. Mysimba is contraindicated in people with uncontrolled high blood pressure, a history of seizures, eating disorders, or who are taking opioid medicines, a significant list that rules it out for a meaningful proportion of patients. Mounjaro's contraindication list is shorter but includes personal or family history of medullary thyroid carcinoma or MEN2, and requires careful assessment for anyone with a history of pancreatitis.

On the NHS, access to either medicine through the phased tirzepatide rollout or specialist pathways involves waiting lists and strict eligibility gates. Private prescribing removes the waiting list but not the clinical assessment. At nume, a prescriber personally reviews every consultation, our clinical team reads your answers the same day you submit them. If Mounjaro is not clinically suitable, the prescriber will say so. For a broader view of what private weight-loss treatment actually involves, our treatment overview explains the landscape.

Women taking oral contraceptives who start Mounjaro should add a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because absorption may be affected, something Mysimba does not carry the same warning for. It is worth raising this at consultation, particularly if you are also thinking about HRT.

Side effects and cost: what a fair comparison looks like

On cost: Mysimba has historically been cheaper per month than Mounjaro at maintenance doses. Since Eli Lilly raised Mounjaro's UK list price in September 2025, private Mounjaro prices have typically ranged from around £149 to £375 per month depending on dose and provider, as widely reported. Mysimba tends to sit below that range at equivalent private rates. But cost comparison between a medicine producing 6% weight loss and one producing 20% is not straightforward, the clinical value delivered per pound spent does not track the headline figure in a simple way.

At nume we are not competing on price. One transparent charge covers everything, consultation, prescription, treatment and aftercare. If you want to understand what that covers in practice, our FAQs set it out plainly. And if you are curious how Mounjaro compares with other injectable options before your consultation, the Saxenda vs Mounjaro page, the tirzepatide comparison, and our guide to how retatrutide compares with Mounjaro are all useful reading. Which medicine suits you is a clinical decision our prescribers make with you, not one to settle on a comparison page.