Mounjaro is the brand name for tirzepatide, a once-weekly subcutaneous injection developed and manufactured by Eli Lilly and Company. During development it carried the internal code LY3298176, and it was the first and only medicine of its class — a dual GIP and GLP-1 receptor agonist — at the time it was first approved.[14] The science is the natural starting point, because it shaped the trials and the licence.

The science: tirzepatide, a dual GIP and GLP-1 receptor agonist

Most of the well-known weight-loss and diabetes injections act on a single gut hormone pathway, GLP-1 (glucagon-like peptide-1); semaglutide is the familiar example. Tirzepatide is different: it is a single peptide engineered to switch on two incretin receptors at once — the glucose-dependent insulinotropic polypeptide (GIP) receptor as well as the GLP-1 receptor. That dual action is why it is nicknamed a “twincretin”, and it set it apart from the pure GLP-1 agonists that came before it.[1]

Activating those two receptors together enhances glucose-dependent insulin secretion, slows the rate at which the stomach empties, and reduces appetite and food intake — the combined result is better blood-sugar control and weight loss.[7] Practically, tirzepatide is a peptide injectable given under the skin once a week, not an oral tablet, which is why it comes in a pre-filled pen.[6] Our Mounjaro doses and strengths page sets out the strengths and the step-up schedule.

Who developed Mounjaro? Eli Lilly and one molecule, two brands

Tirzepatide was developed in-house by Eli Lilly rather than licensed in from another company, and the same molecule now reaches patients under two brand names depending on country and licensed use.[14] In the United States, Lilly sells it as Mounjaro for type 2 diabetes and as Zepbound for weight management — the identical drug, made in the same facilities, the two names reflecting different indications rather than different medicines.[14]

No “Zepbound” in the UK

The US “Mounjaro versus Zepbound” framing does not apply here: in the UK and the EU there is one brand, Mounjaro, covering both type 2 diabetes and weight management, and Zepbound is not a UK product. Treat any UK advert comparing the two with caution.[5]

The trial programmes: SURPASS and SURMOUNT

Two large phase 3 programmes provided the evidence. The SURPASS programme tested tirzepatide in type 2 diabetes and underpinned the diabetes approval; the SURMOUNT programme tested it for weight management and underpinned the weight-loss indication.[1][7] Three of those trials do most of the work in explaining why Mounjaro is prescribed today.

SURPASS-2 — head-to-head against semaglutide in diabetes

SURPASS-2 was a 40-week phase 3 trial comparing tirzepatide (5mg, 10mg and 15mg once weekly) directly against semaglutide 1mg once weekly in adults with type 2 diabetes, published in the New England Journal of Medicine.[9] It matters because it is a genuine head-to-head: tirzepatide reduced HbA1c (a measure of average blood sugar) by 2.01% at 5mg, 2.24% at 10mg and 2.30% at 15mg, versus 1.86% for semaglutide 1mg, and cut weight by 7.6kg, 9.3kg and 11.2kg against 5.7kg on semaglutide.[10] In other words, tirzepatide out-performed injectable semaglutide on both blood sugar and weight in this population.[9]

SURMOUNT-1 — weight loss in people without diabetes

SURMOUNT-1 is the headline obesity trial: a 72-week, randomised, double-blind, placebo-controlled study of 2,539 adults with obesity — or overweight plus at least one weight-related health problem — and without type 2 diabetes, testing tirzepatide 5mg, 10mg and 15mg against placebo alongside diet and activity, in the New England Journal of Medicine.[7] Mean weight loss on treatment at 72 weeks was 16.0% at 5mg, 21.4% at 10mg and 22.5% at 15mg, against just 2.4% on placebo; in absolute terms the 15mg group lost about 24kg, versus roughly 2kg on placebo.[7][8]

The response rates give a sense of scale: 96% of the 15mg group lost at least 5% of their body weight, and 39.7% lost at least 25%, against 0.3% on placebo.[8] The weight lost was also disproportionately fat — the trial reported roughly three times greater reduction in fat mass than lean mass (about 33.9% versus 10.9%).[8]

SURMOUNT-2 — weight loss in people who also have type 2 diabetes

SURMOUNT-2 asked the same question in a harder population: 938 adults with obesity or overweight plus type 2 diabetes, over 72 weeks, testing tirzepatide 10mg and 15mg against placebo.[5] Mean weight loss was 13.4% at 10mg and 15.7% at 15mg, against 3.3% on placebo, and between 81.6% and 86.4% on treatment lost at least 5% of their weight, versus 30.6% on placebo.[5] The figures are lower than in SURMOUNT-1, which is expected: weight loss in people with type 2 diabetes is consistently smaller than in people without it across this whole class, so the two trials should be read side by side rather than as a contradiction.[7] The primary results were reported as published in The Lancet.[5]

Is Mounjaro safe? What the trials showed

“Is Mounjaro safe?” is not a question a development page can answer for any individual — that is what the prescriber consultation is for — but the trials describe the side effects seen. In SURMOUNT-1 the most common were gastrointestinal, mostly mild to moderate, dose-related and typical of the class: nausea, for example, affected 24.6% on 5mg, 33.3% on 10mg and 31.0% on 15mg, against 9.5% on placebo, with diarrhoea, constipation and vomiting following broadly similar patterns.[7] The slow, four-week-per-step dose escalation used with Mounjaro exists precisely to reduce these dose-related effects.[7]

The development programme also identified more serious risks that shape who should not take the medicine — including a boxed warning in the US label about thyroid C-cell tumours seen in rats, and contraindications in people with a personal or family history of medullary thyroid carcinoma or with Multiple Endocrine Neoplasia syndrome type 2.[2] Our Mounjaro side effects page covers the full frequency data, the warnings and who the medicine is not suitable for.

How Mounjaro is made and supplied

Because tirzepatide is a peptide, it is manufactured as an injectable rather than a tablet, and the diabetes brand (Mounjaro) and the US weight-loss brand (Zepbound) are the same tirzepatide made in the same Lilly facilities — the dual branding reflects different licensed uses, not different production.[14] In the UK it is supplied as the Mounjaro KwikPen, a disposable, multi-dose, single-patient pre-filled pen holding four fixed weekly doses of 0.6mL each, one pen per strength.[6] Our how to inject Mounjaro page covers pen handling, storage and technique.

Regulatory milestones: FDA, EMA and MHRA

Tirzepatide was licensed for diabetes first and for weight management later, and each regulator acted separately. The confirmed, dated milestones are:

Confirmed tirzepatide regulatory milestones by regulator and indication.
RegulatorIndicationDate
FDA (United States)Type 2 diabetes (Mounjaro, NDA 215866)13 May 2022[1]
EMA (European Union)Type 2 diabetes (Mounjaro)15 September 2022[3]
MHRA (United Kingdom)Weight loss and weight management, adults 18+8 November 2023[5]
FDA (United States)Chronic weight management (branded Zepbound, NDA 217806)8 November 2023[14]

The UK weight-management licence covers adults with a BMI of 30 kg/m² or more, or 27 to 30 kg/m² with at least one weight-related health problem such as prediabetes, high blood pressure, abnormal blood fats or cardiovascular disease, alongside a reduced-calorie diet and increased physical activity.[5] One detail worth getting right: since Brexit the EMA authorisation of 15 September 2022 covers the EU and EEA, while Great Britain is licensed separately by the MHRA, so the EU date is not the UK date.[3][4] The firmly dated MHRA milestone for Great Britain is the weight-management authorisation on 8 November 2023.[5]

Can I take Mounjaro? UK access today

“Can I take Mounjaro?” is not answered by the licence criteria alone — it is answered by a prescriber. The MHRA authorisation makes Mounjaro legally available for weight loss in the UK on prescription, but access runs entirely through a clinician.[5] On the NHS the route is deliberately narrow: NICE recommended tirzepatide for managing overweight and obesity in technology appraisal TA1026, with final guidance on 23 December 2024,[11] and NHS England set out a phased primary-care rollout from 23 June 2025 for an eligible cohort of about 220,000 people over the first three years, prioritising the highest clinical need first, with diet and physical-activity support alongside the medicine.[12]

Because NHS access is phased and restricted, most UK patients who currently obtain Mounjaro for weight loss are reported to do so through regulated private prescribers rather than the NHS.[15] Either way, it is one option a prescriber may discuss — never something to seek out yourself.

Buying Mounjaro safely

Mounjaro is a prescription-only medicine: it should only ever be supplied by a GPhC-registered pharmacy after a consultation with a prescriber. Websites offering to sell Mounjaro online with no prescription, or “no consultation” sellers, are not operating legally, and what they supply may be falsified and unsafe. You can learn how to spot fake medicines and unregulated sellers at fakemeds.campaign.gov.uk. This site does not sell, supply or link to any provider of any medicine.

Frequently asked questions

Is Mounjaro FDA approved?

Yes. The FDA approved Mounjaro for type 2 diabetes on 13 May 2022.[1] In the US the same molecule is approved for weight management under a different brand, Zepbound, from 8 November 2023.[14] There is no Zepbound in the UK.[5]

When was Mounjaro approved in the UK?

The MHRA authorised Mounjaro for weight loss and weight management in adults aged 18 and over on 8 November 2023. Great Britain is licensed separately from the EU, whose EMA diabetes authorisation was granted on 15 September 2022.[5][3]

How much weight did people lose in the Mounjaro trials?

In the 72-week SURMOUNT-1 trial (no diabetes), mean on-treatment weight loss was 16.0% at 5mg, 21.4% at 10mg and 22.5% at 15mg, versus 2.4% on placebo. In SURMOUNT-2 (with type 2 diabetes) it was 13.4% at 10mg and 15.7% at 15mg, versus 3.3% on placebo.[7][5]

Reporting side effects

If you experience side effects from any medicine, you can report them through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk[13], and speak to your GP or pharmacist.

References

  1. Eli Lilly and Company. FDA approves Lilly's Mounjaro (tirzepatide) injection, the first and only dual GIP and GLP-1 receptor agonist. Investor news release, 2022. investor.lilly.com
  2. US Food and Drug Administration. Mounjaro (tirzepatide) US prescribing information, NDA 215866, 2022. accessdata.fda.gov
  3. European Medicines Agency. Mounjaro — EPAR (marketing authorisation 15 September 2022). ema.europa.eu
  4. Eli Lilly and Company. European Commission grants marketing authorisation for Lilly's Mounjaro. lilly.com
  5. MHRA / GOV.UK. Medicines regulator authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss. 2023. gov.uk
  6. electronic medicines compendium (emc). Mounjaro KwikPen — Summary of Product Characteristics. medicines.org.uk
  7. Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). New England Journal of Medicine, 2022. nejm.org
  8. Eli Lilly and Company. SURMOUNT-1 results published in the New England Journal of Medicine. Investor news release, 2022. investor.lilly.com
  9. Frías JP et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2). New England Journal of Medicine, 2021. nejm.org
  10. Eli Lilly and Company. SURPASS-2 results published in the New England Journal of Medicine. Investor news release, 2021. investor.lilly.com
  11. National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity (TA1026). nice.org.uk
  12. NHS England. Interim commissioning guidance: NICE TA1026 tirzepatide. england.nhs.uk
  13. MHRA. Yellow Card scheme — report a side effect. yellowcard.mhra.gov.uk
  14. Drugs.com. Zepbound vs Mounjaro: complete comparison guide (same tirzepatide molecule; US brand/indication split; Zepbound NDA 217806, weight management 8 November 2023). drugs.com
  15. House of Commons Library. Weight loss medicines in England. Research briefing CBP-10171. commonslibrary.parliament.uk