What Mounjaro actually is
Mounjaro is a prescription-only medicine containing tirzepatide, made by Eli Lilly. It belongs to a family of medicines most people now call "GLP-1s", although Mounjaro is slightly different from the rest: tirzepatide mimics two natural gut hormones rather than one. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). You will sometimes see it described as a "dual GIP/GLP-1 receptor agonist" or a "twincretin".
Those two hormones are released by your gut after eating. Together they tell your brain you are full, slow the speed at which food leaves your stomach, and help your body manage blood sugar. Tirzepatide copies both signals, which is why people on it typically feel full sooner, stay full longer, and think about food less.
In the UK, Mounjaro is licensed for two separate uses:
- Type 2 diabetes. To improve blood sugar control alongside diet and exercise.
- Weight management. For adults living with obesity, or overweight with a weight-related condition, alongside a reduced-calorie diet and increased physical activity.
What the trials found
The headline evidence comes from the SURMOUNT programme. Large, randomised, placebo-controlled trials published in the New England Journal of Medicine and The Lancet:
| Trial | Who | Result |
|---|---|---|
| SURMOUNT-1 (2022) | Adults with obesity or overweight, without diabetes | Average weight loss of up to 20.9% at the 15 mg dose over 72 weeks, versus 3.1% on placebo |
| SURMOUNT-4 (2023) | People who stopped vs continued tirzepatide | Those who stopped regained around two-thirds of their lost weight within a year. Those who continued kept losing |
| SURMOUNT-5 (2025) | Head-to-head versus semaglutide 2.4 mg (Wegovy) | Average weight loss of 20.2% with tirzepatide versus 13.7% with semaglutide over 72 weeks |
| SURMOUNT-OSA (2024) | Adults with obesity and obstructive sleep apnoea | Apnoea events per hour fell by roughly half to two-thirds versus placebo |
These are averages across thousands of people. Roughly a third of people in SURMOUNT-1 lost 25% or more of their body weight at the top dose. While others lost considerably less. Your own result depends on dose, adherence, food choices, activity, sleep and starting point.
Doses and the KwikPen
In the UK, Mounjaro comes in a multi-dose KwikPen. Each pen contains four weekly doses (28 days) of one strength. Everyone starts at 2.5 mg and moves up in steps, with at least four weeks at each dose:
| Stage | Dose | Notes |
|---|---|---|
| Starting dose | 2.5 mg once weekly | For the first 4 weeks. This is a "get used to it" dose, not a weight-loss dose |
| Step-up | 5 mg → 7.5 mg → 10 mg → 12.5 mg | Increase only if needed and tolerated, after at least 4 weeks at the current dose |
| Maximum | 15 mg once weekly | The highest licensed dose |
You do not have to reach 15 mg. Many people get excellent results (and fewer side effects) staying at 5 mg or 10 mg. The goal is the lowest dose that keeps working for you.
Each full injection from a UK KwikPen is exactly 60 clicks and 0.6 ml of fluid, whatever the strength. The concentration changes, not the volume. Older UK pens (V1, 3.2 ml) contained enough overfill for a "fifth dose"; the current V2 pens (2.6 ml) do not. Our Mounjaro click calculator and fifth dose guide explain both in detail.
Who can get Mounjaro in the UK
On the NHS
NICE recommended tirzepatide for weight management in December 2024 (TA1026). It covers adults with an initial BMI of at least 35 kg/m² plus at least one weight-related condition, such as type 2 diabetes, high blood pressure or sleep apnoea. Lower BMI thresholds (usually reduced by 2.5 kg/m²) apply for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds.
Because of the sheer numbers eligible, NHS England is rolling it out in phases through its interim commissioning guidance, prioritising those with the highest clinical need first. Availability varies by area. Your GP practice can tell you what is commissioned locally.
Privately
Most UK users currently buy Mounjaro privately from GPhC-registered online pharmacies and clinics, typically following an online consultation reviewed by a UK prescriber. Legitimate providers will verify your identity, weight and medical history. If a site skips those checks, treat it as a red flag (see our fake medicines guide). For Mounjaro specifically, Monj.co.uk manually verifies the pharmacies it recommends, which removes most of the legwork.
Side effects in brief
The most common side effects are gastrointestinal (nausea, diarrhoea, vomiting, constipation, indigestion and reduced appetite) and they are usually worst in the first weeks or after a dose increase. The MHRA has also strengthened warnings about acute pancreatitis across all GLP-1 medicines, and the patient leaflet covers gallbladder problems and dehydration from vomiting or diarrhoea.
Severe, persistent stomach pain (especially radiating to your back, with vomiting), signs of a serious allergic reaction, or symptoms of severe dehydration. Report suspected side effects to the MHRA Yellow Card scheme. Our full side effects guide covers management in detail.
Frequently asked questions
Is Mounjaro available on the NHS? +
Yes, but in a phased rollout. NICE recommends tirzepatide for adults with a BMI of at least 35 kg/m² plus a weight-related condition (TA1026), and NHS England is prioritising those with the greatest clinical need first. Ask your GP what is available in your area. Many people currently access it privately while NHS services scale up.
How much weight will I lose on Mounjaro? +
In the SURMOUNT-1 trial, people lost an average of up to 20.9% of their body weight at the 15 mg dose over 72 weeks, alongside diet and activity changes. Real-world results vary widely. Dose, adherence, food quality, protein intake, activity and sleep all matter.
Is Mounjaro the same as Ozempic or Wegovy? +
No. Mounjaro contains tirzepatide, which mimics two gut hormones (GIP and GLP-1). Wegovy and Ozempic both contain semaglutide, which mimics GLP-1 only. In the SURMOUNT-5 head-to-head trial, tirzepatide produced greater average weight loss than semaglutide 2.4 mg.
How long does a Mounjaro pen last? +
Each UK KwikPen contains four weekly doses. 28 days of treatment. If you miss a dose, you can take it within 4 days (96 hours); after that, skip it and take the next dose on your usual day.
Do I have to keep increasing the dose? +
No. You step up only if you and your prescriber agree it is needed and you are tolerating treatment well. Many people maintain on 5 mg or 10 mg. The 2.5 mg starting dose is for tolerability, not weight loss.
References & official sources
Show all 7 cited sources, with study authors +
- NICE TA1026. Tirzepatide for managing overweight and obesity (December 2024): www.nice.org.uk/guidance/ta1026
- NHS England. Interim commissioning guidance: implementation of NICE TA1026: www.england.nhs.uk/long-read/interim-commissioning-guidance-nice-ta1026-tirzepatide/
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). NEJM 2022: www.nejm.org/doi/full/10.1056/NEJMoa2206038
- Aronne LJ et al. Continued Treatment With Tirzepatide for Maintenance of Weight Reduction (SURMOUNT-4). JAMA 2024: jamanetwork.com/journals/jama/fullarticle/2812936
- Aronne LJ et al. Tirzepatide as Compared with Semaglutide (SURMOUNT-5). NEJM 2025: www.nejm.org/doi/full/10.1056/NEJMoa2416394
- electronic medicines compendium. Mounjaro KwikPen SmPC & patient leaflet: www.medicines.org.uk/emc/search?q=mounjaro
- MHRA. GLP-1 receptor agonists: strengthened warnings on acute pancreatitis: www.gov.uk/drug-safety-update