Mounjaro (tirzepatide) is a once-weekly injection and a dual GIP and GLP-1 receptor agonist — the first medicine to work on both receptors. In SURMOUNT-1, average weight change reached about −22.5% at the top dose over 72 weeks.[5] It is MHRA-licensed for weight management and type 2 diabetes, and NICE recommends it on the NHS for defined groups — one option a prescriber may discuss alongside diet and activity, not something you must take.
What Mounjaro is
Mounjaro is the brand name for tirzepatide, a once-weekly medicine injected under the skin, developed by Eli Lilly and Company (internal code LY3298176).[1] What sets it apart is that it is a dual GIP and GLP-1 receptor agonist — nicknamed a "twincretin" — the first and, at its approval, only medicine to activate both of those gut-hormone receptors rather than the GLP-1 receptor alone.[1] That is why it is inaccurate to call Mounjaro simply "a GLP-1": the dual GIP/GLP-1 description is the accurate one.[2]
Brand naming across countries causes confusion. In the United States the same molecule is sold as two brands — Mounjaro for type 2 diabetes and Zepbound for weight management.[14] There is no "Zepbound" in the UK: here, Mounjaro itself is the brand licensed for both, so US "Zepbound vs Mounjaro" comparisons do not apply.[3]
How Mounjaro works
Tirzepatide mimics two natural gut hormones — GIP and GLP-1 — that the body releases after eating. Acting on both receptors, it enhances glucose-dependent insulin secretion, slows stomach emptying and reduces appetite, so people tend to feel full sooner and for longer and have steadier blood sugar.[2] Engaging both receptors is why it is positioned as producing greater weight loss than single-receptor GLP-1 medicines — though, like the whole class, it works alongside a reduced-calorie diet and more activity, not on its own.[15]
The pen, the strengths and the dose ladder
Mounjaro is a once-weekly subcutaneous injection, not a tablet. In the UK it comes as the multi-dose KwikPen (four weekly doses of 0.6 mL) and as single-dose pre-filled pens, injected into the abdomen, thigh or upper arm on any day, with or without food.[2] It is made in six strengths: 2.5, 5, 7.5, 10, 12.5 and 15 mg.[2]
Nobody starts on a high dose. Like every medicine in this class, tirzepatide is titrated — started low and stepped up slowly — because the most common side effects are gastrointestinal and worst while the dose is climbing. The MHRA escalation is:
| Stage | Once-weekly dose | Notes |
|---|---|---|
| Weeks 1–4 | 2.5 mg | Non-therapeutic starting dose to build tolerance |
| From week 5 | 5 mg | First maintenance option |
| Increases | +2.5 mg steps | No more often than every 4 weeks |
| Maintenance | 5, 10 or 15 mg | Chosen for response and tolerability |
| Maximum | 15 mg | The ceiling dose |
The 2.5 mg starting dose is a stepping stone, not a treatment dose. The pace of any increase is the prescriber's decision, not the patient's, and a dose can be held for longer, or stepped back down, if side effects are troublesome.[3]
What the SURMOUNT trials showed
The headline weight-loss evidence for Mounjaro comes from the SURMOUNT programme. SURMOUNT-1 was a 72-week, randomised, double-blind, placebo-controlled trial of 2,539 adults who had obesity (or were overweight with weight-related conditions) but did not have type 2 diabetes.[5]
| Once-weekly dose | Average weight change at 72 weeks |
|---|---|
| 5 mg | −16.0% |
| 10 mg | −21.4% |
| 15 mg | −22.5% (about 24 kg) |
| Placebo | −2.4% |
At the top 15 mg dose, 96% lost at least 5% of their body weight and 39.7% lost at least a quarter of it (25% or more), against 0.3% on placebo.[6] Those figures use the efficacy estimand (the effect while staying on treatment); a second, equally valid measure that counts everyone regardless of stopping gives slightly lower averages of about −15.0%, −19.5% and −20.9% — which is why coverage sometimes quotes different numbers for the same trial.[5]
A companion trial, SURMOUNT-2, tested tirzepatide in 938 adults who also had type 2 diabetes, with mean weight change of −13.4% at 10 mg and −15.7% at 15 mg versus −3.3% on placebo.[3] Weight loss is generally somewhat lower in people who also have diabetes — a known feature across the class, not something specific to Mounjaro. Tirzepatide has also been compared head-to-head with semaglutide: in the 40-week SURPASS-2 diabetes trial it reduced HbA1c and weight more than semaglutide 1 mg at every dose, for example −11.2 kg at 15 mg against −5.7 kg.[7] Our Wegovy (semaglutide) page and GLP-1 medication comparison set the two out in more detail.
UK status and who it is licensed for
Mounjaro was originally licensed as a type 2 diabetes treatment before it gained its weight-management use; the European Medicines Agency granted its EU-wide diabetes authorisation on 15 September 2022, and post-Brexit Great Britain is licensed separately by the MHRA.[4] The MHRA authorised Mounjaro for weight loss and weight management in adults aged 18 and over on 8 November 2023, for adults with a BMI of 30 kg/m² or more (obesity), or 27 to under 30 kg/m² with at least one weight-related condition such as prediabetes, high blood pressure, abnormal blood fats or cardiovascular disease, alongside diet and activity.[3]
A licence is only the first gate: being MHRA-licensed does not mean NHS funding, which needs a separate NICE recommendation. For tirzepatide, NICE published final guidance (TA1026) on 23 December 2024, recommending it for adults with a BMI of at least 35 plus a weight-related condition, with a phased rollout through primary care beginning 23 June 2025, starting with those in greatest clinical need.[8] NICE also lowers the BMI thresholds (usually by 2.5 kg/m²) for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds.[9]
Because NHS access is narrow and phased, many people currently obtain Mounjaro privately, through a GPhC-registered pharmacy after a consultation with a prescriber. Whichever route, a prescriber decides it is appropriate first — a website cannot. We do not name, rank or link to any provider, and we carry no prices; this is an information site, not a shop.
Key safety points
As with the whole GLP-1 class, the most common side effects are gastrointestinal — the UK product information lists nausea, diarrhoea, vomiting, constipation and abdominal pain as very common (at least 1 in 10 people).[2] They are usually mild to moderate, tend to appear as the dose is increased, and often settle over time. In SURMOUNT-1 the frequencies across the dose groups were roughly:
- Nausea — about 24.6% to 33.3%, against 9.5% on placebo.[5]
- Diarrhoea — about 18.7% to 23.0%, against 7.3% on placebo.[5]
- Constipation — about 11.7% to 17.1%, against 5.8% on placebo.[5]
- Vomiting — about 8.3% to 12.2%, against 1.7% on placebo.[5]
Beyond those, the UK Summary of Product Characteristics flags warnings a prescriber weighs up first: acute pancreatitis (treatment is stopped if it is suspected), gallbladder problems, dehydration and, rarely, acute kidney injury if vomiting or diarrhoea are severe, and a higher risk of low blood sugar when tirzepatide is used with insulin or a sulfonylurea (which may then need reducing).[2] The only outright contraindication in the UK SmPC is hypersensitivity (allergy) to tirzepatide or the other ingredients.[2]
Unlike most of the class, the Mounjaro SmPC warns that reduced effectiveness of oral contraceptives cannot be excluded in women with obesity, and advises using an additional or barrier method of contraception around starting the medicine and for a period after each dose increase.[2] If this applies to you, it is worth raising specifically with your prescriber.
One caveat for anyone who has read US information: the American prescribing information for tirzepatide carries a boxed warning about thyroid C-cell tumours seen in a two-year rat study, and contraindicates a personal or family history of medullary thyroid cancer or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).[11] The UK SmPC does not list those as contraindications — it names only hypersensitivity — and handles thyroid risk as a precaution.[2] Do not assume US label wording applies here: in the UK the emc SmPC is the authority, and the warnings for your circumstances are something to go through with a prescriber.
If you take Mounjaro and have a suspected side effect, report it through the MHRA Yellow Card scheme[12] and speak to your GP or pharmacist. Because these medicines are in high demand, they are a frequent target for counterfeiters: prescription medicines bought from unregulated websites or social-media sellers may be falsified, wrongly dosed or contaminated, and buying a prescription-only medicine without a prescription is unlawful. The government's FakeMeds campaign[13] explains how to check a seller is legitimate.
Storing the pen
Before first use, unopened Mounjaro pens are kept refrigerated at 2–8 °C and must not be frozen. After first use, a pen may be kept at not above 30 °C for up to 30 days, then discarded even if medicine remains, and it should never be stored with a needle attached.[10] The leaflet in the box is the definitive guide for the exact pen you are given.
Where Mounjaro fits with the other GLP-1 medicines
Mounjaro sits alongside the other weekly injection, Wegovy (semaglutide), while the newer oral options — the Wegovy pill (oral semaglutide) and Foundayo (orforglipron) — are tablets. Our GLP-1 medication comparison and GLP-1 medication FAQ set out how they differ in molecule, form, dosing and UK licensing. For a site devoted entirely to this one medicine, there is also a separate independent resource, Mounjaro Injection UK.
Frequently asked questions
What is Mounjaro and how is it different from Wegovy?
Mounjaro is the brand name for tirzepatide, a once-weekly injection from Eli Lilly. It is a dual GIP and GLP-1 receptor agonist — it acts on two gut-hormone receptors. Wegovy is semaglutide, which acts on the GLP-1 receptor alone. Both are weekly injections for weight management, but different molecules with different dose ladders.
How much weight did people lose on Mounjaro in the trials?
In the 72-week SURMOUNT-1 trial (2,539 adults with obesity, no type 2 diabetes), average weight change (efficacy estimand) was about −16.0% at 5mg, −21.4% at 10mg and −22.5% at 15mg — roughly 24kg — versus about −2.4% on placebo. These are trial averages alongside diet and exercise, and individual results vary.
Is Mounjaro a tablet or an injection?
An injection, given once a week under the skin as a pre-filled pen, in six strengths from 2.5mg to 15mg. It is not a tablet. The oral GLP-1 options are different medicines — for example the Wegovy pill and Foundayo.
Can I get Mounjaro on the NHS?
It depends on eligibility. NICE recommends tirzepatide (TA1026) for adults with a BMI of at least 35 plus a weight-related condition, with lower thresholds for some ethnic backgrounds. NHS access is being phased in through primary care from June 2025, starting with those in greatest clinical need. A prescriber decides.
How do I get Mounjaro safely?
Mounjaro is prescription-only, so it can only be supplied lawfully after a prescriber has assessed you, through a GPhC-registered pharmacy. Buying it from unregulated websites or social-media sellers is unsafe and unlawful. The FakeMeds campaign explains how to check a seller. We name no seller and carry no prices.
References
- 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, May 2022. investor.lilly.com
- Mounjaro KwikPen — Summary of Product Characteristics (UK product information; mechanism, strengths, administration, side effects and warnings). electronic Medicines Compendium (emc), product 15481. medicines.org.uk
- MHRA / GOV.UK. "MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss." November 2023. gov.uk
- European Medicines Agency. "Mounjaro" (EPAR / EU marketing authorisation, 15 September 2022). ema.europa.eu
- New England Journal of Medicine. "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1). nejm.org
- Eli Lilly and Company. "Lilly's SURMOUNT-1 results published in the New England Journal of Medicine." Investor news release. investor.lilly.com
- New England Journal of Medicine. "Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes" (SURPASS-2). nejm.org
- NICE. "Tirzepatide for managing overweight and obesity" (Technology appraisal TA1026), final guidance 23 December 2024. nice.org.uk
- NICE. TA1026 — Recommendations (BMI thresholds, including ethnicity-adjusted thresholds). nice.org.uk
- Mounjaro KwikPen — Patient Information Leaflet (UK product information; storage section: refrigerate at 2–8 °C before first use, keep below 30 °C for up to 30 days after first use, never store with a needle attached). electronic Medicines Compendium (emc), product 15481. medicines.org.uk
- US Food and Drug Administration. Mounjaro (tirzepatide) US prescribing information, NDA 215866 (cited only for the US boxed-warning contrast). accessdata.fda.gov
- MHRA. "Yellow Card scheme — report a side effect." yellowcard.mhra.gov.uk
- MHRA. "#FakeMeds — buying medicines safely online." fakemeds.campaign.gov.uk
- Drugs.com. "Zepbound vs Mounjaro: complete comparison guide" (US brand and indication split — the same tirzepatide molecule sold as Mounjaro for type 2 diabetes and Zepbound for weight management). drugs.com
- Peer-reviewed mechanism review: tirzepatide as a dual GIP and GLP-1 receptor agonist, positioned as producing greater weight loss than single-receptor GLP-1 agonists. PMC9060465, National Library of Medicine (PubMed Central). ncbi.nlm.nih.gov (PubMed Central)