The best weight loss injections in 2025: what the evidence actually shows

The biggest myth: more injections means more options

People searching for the best weight loss injections in 2025 often expect to find a broad menu of products. In reality the UK licensed landscape is deliberately narrow. Two medicines (Mounjaro and Wegovy) hold MHRA approval for weight management in adults. Everything else you may read about online, from older hormones to unregulated peptides, either lacks that authorisation or is licensed for an entirely different condition.

That narrowness is actually reassuring. It means the two options that do exist have been through rigorous clinical programmes, monitored by the MHRA, and assessed by NICE before reaching patients. There are no shortcuts hiding behind a crowded marketplace. If a seller is offering you a third or fourth "weight loss injection" at a suspiciously low price, that is worth treating with real caution, the MHRA has seized roughly 20 million doses of illegally traded weight loss medicines worth around £45 million in street value, including counterfeit tirzepatide pens raided from a Northampton facility in October 2025.

So the practical question is not "which of many" but "which of two", and that is a more tractable decision to make with a prescriber. Our guide to weight loss injections covers the landscape in full if you want background before reading on.

How Mounjaro and Wegovy actually compare on results

Mounjaro's trial programme is where the numbers get striking. In SURMOUNT-1, adults with obesity and no diabetes lost an average of around 20–21% of their body weight over 72 weeks at the 15 mg maintenance dose, figures described by NICE in its Technology Appraisal TA1026 as among the strongest in a weight management medicine to date. Wegovy's STEP 1 trial, run over 68 weeks at the 2.4 mg dose, showed an average reduction of around 15%.

The head-to-head data came with SURMOUNT-5, published in the New England Journal of Medicine in 2025. Over 72 weeks in 751 adults with obesity and no diabetes, tirzepatide produced greater average weight loss than semaglutide 2.4 mg. That is a direct comparison, not an extrapolation across separate trial populations, it is the most relevant single piece of evidence for anyone weighing the two up.

Wegovy has since gained MHRA approval for a higher 7.2 mg dose, announced in January 2026, which closes some of the gap. Trials at 7.2 mg reported around 20.7% average weight loss over 72 weeks. Whether that dose is appropriate for any individual patient is a clinical question, not a consumer one. If you want to explore how the medicines sit alongside each other, this comparison page goes deeper on the decision factors.

What both medicines share, and where the real differences sit

The similarities matter as much as the differences. Both are once-weekly subcutaneous injections delivered via a pre-filled pen. Both start at a low tolerability dose and are increased gradually by the prescriber, usually in four-week steps; the starting pen is not the therapeutic dose, it is the period your system uses to adjust. Both require storage in the fridge, Mounjaro's pen fits neatly in the fridge door, which is where most patients keep it between weekly doses. Both carry a Black Triangle designation from the MHRA, indicating additional post-market monitoring.

The meaningful differences are in mechanism and average outcomes. Mounjaro's dual receptor action (GIP plus GLP-1) is unique among UK-licensed weight management medicines. Wegovy acts on GLP-1 alone, which is the same pathway as Ozempic, though Ozempic is licensed for type 2 diabetes, not weight management, and the two are not interchangeable for this purpose.

Side effects for both are led by gastrointestinal symptoms: nausea, loose stools, constipation, indigestion and belching are the most commonly reported, typically most noticeable in the early weeks or after a dose increase, then settling. Guidance on injection sites, storage windows and what to do if you feel unwell lives in your Patient Information Leaflet and should be your first reference, not a comparison article. The NHS tirzepatide page sets out side effects in plain language and is worth reading before you start.

Eligibility, access and what comes next

Both medicines are licensed for adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, obstructive sleep apnoea or raised cholesterol. BMI alone does not decide the outcome of a consultation, a prescriber looks at the full clinical picture, including medications, medical history and what you are hoping to achieve. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance; your prescriber will take this into account.

On the NHS, access to tirzepatide is phased by NICE criteria and expanding gradually through 2025 and 2026; Wegovy follows a separate pathway through specialist weight management services. Waiting times vary considerably. For people who do not meet the current NHS criteria or prefer not to wait, a regulated private service is the alternative, subject, always, to clinical approval. You can read more about how costs compare across providers on our weight loss injection cost page, which explains what a legitimate price should include.

If you are newer to this and want to understand what 2025 has brought to the UK market specifically, this page on new injections in 2025 covers the approvals and what changed. Some patients also arrive having read about glucose injections for weight loss, a term that circulates online and is worth understanding clearly before starting any programme. If you would like context on how today's options compare with what was available a few years ago, our summary of the best weight loss injections in 2022 shows how quickly the landscape has shifted. And for a more personalised steer on which medicine might suit your situation, our prescribers are the right people to ask.

At nume, every consultation is read by a GPhC-registered Independent Prescriber, not an automated system. If you're ready to take that step, you can start your free consultation today.