Metformin versus Mounjaro: what's the difference for weight loss?

How do metformin and Mounjaro actually work?

Metformin has been prescribed for type 2 diabetes since the 1950s. It works mainly by reducing the amount of glucose the liver releases into the blood and by improving insulin sensitivity in muscle tissue. It does not directly target appetite or satiety, and any weight effect it produces is modest and inconsistent, typically a few kilograms at most, often partly explained by the nausea that can accompany it at the start of treatment.

Mounjaro (tirzepatide) takes a completely different route. It is a dual GIP and GLP-1 receptor agonist (the only medicine of its kind licensed in the UK) and it works by activating two gut-hormone pathways simultaneously. The result is a marked reduction in appetite, a slowing of gastric emptying, and improvements in how the body handles blood sugar. In the SURMOUNT-1 trial, adults taking tirzepatide at the highest dose lost an average of around 20–21% of their body weight over 72 weeks, a result NICE reviewed when recommending tirzepatide for weight management in December 2024. You can read the full NICE appraisal at NICE TA1026. The mechanism and evidence base are substantially different from metformin's.

For a broader look at what Mounjaro is and how it fits into weight management, the Mounjaro overview page covers the full picture.

Are they licensed for the same things?

No, and this matters more than most comparisons acknowledge. Metformin is licensed in the UK for type 2 diabetes and, in some cases, polycystic ovary syndrome (PCOS). It is not licensed for weight management. Any weight effect is a secondary observation, not the treatment goal, and prescribing it specifically to lose weight sits outside its authorised indication.

Mounjaro holds a UK licence for weight management in adults with a BMI of 30 or above, or 27 and above where at least one weight-related condition is present, conditions including high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. It is also licensed for type 2 diabetes separately. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. The NHS medicine information page for tirzepatide on NHS.co.uk gives a plain-English overview of both licensed uses.

People who have been prescribed metformin for diabetes and are also trying to manage their weight are sometimes curious whether Mounjaro could serve both purposes. That is a clinical question, both medicines can be used in type 2 diabetes, and a prescriber can work through the options with you. The consultation at our treatment page is the right starting point.

What does the evidence say about weight loss specifically?

The table below sets out a factual side-by-side. Figures are from published sources; always discuss what is realistic for your situation with a prescriber.

The gap in weight-loss outcomes is substantial. That said, for someone managing type 2 diabetes, metformin remains a first-line medicine in UK guidelines, sometimes used alongside a GLP-1 medicine rather than instead of one. If you are already taking metformin and wondering whether Mounjaro could work alongside it, our prescribers can assess that at consultation.

For context on how Mounjaro compares against other newer medicines in the same class, the tirzepatide versus Mounjaro page addresses a common source of naming confusion, and the retatrutide versus Mounjaro page looks at what is coming next in the pipeline.

Cost, access and what to weigh up

Metformin is inexpensive and widely available on the NHS for its licensed conditions. Mounjaro, as a newer prescription medicine for weight management, is available through the NHS under phased criteria (currently BMI 40-plus with multiple weight-related conditions) or privately through a regulated online pharmacy like ours. Understanding the full cost of private treatment, not just the headline price, is worth doing, our Mounjaro cost page explains what drives the price and what a legitimate quote should include.

A note on timing: if you are planning ahead (around a bank holiday, say, or the start of a new month when budgets reset) our prescribers are available Monday to Friday, and orders placed before 12pm go out the same day once approved. It is a small practical detail, but it saves surprises.

What you eat alongside either medicine matters too. The meals for Mounjaro page covers practical food choices for people on tirzepatide, and if you are thinking about nutritional support during treatment, choosing a multivitamin alongside Mounjaro is worth a read. For a wider look at weight-loss approaches, the weight management overview sets out the landscape.

Which medicine suits your situation is a clinical decision made with a prescriber who has reviewed your full health picture. If Mounjaro sounds like the right direction, speak to our prescribers to find out whether you are eligible and what the process involves.