The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
Does Mounjaro Actually Work? Clinical Evidence Explained
Yes, clinical evidence shows Mounjaro (tirzepatide) can help with weight loss in UK adults — but results vary significantly between individuals. In the SURMOUNT-1 clinical trial, participants using Mounjaro lost an average of 15–21% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. However, "work" means something different for everyone. This guide explains what the research actually shows, how the injection works, and what to realistically expect.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
What Clinical Trials Show About Mounjaro
The SURMOUNT-1 trial is the main clinical evidence for Mounjaro (tirzepatide) in weight management. Here's what researchers found:
- 5 mg dose: Average weight loss of approximately 15% body weight
- 10 mg dose: Average weight loss of approximately 19.5% body weight
- 15 mg dose (maximum): Average weight loss of approximately 20.9% body weight
These results were achieved over 72 weeks (approximately 1.5 years) in adults with obesity or overweight with weight-related health conditions. Participants also followed a reduced-calorie diet and increased physical activity — the medication worked alongside these lifestyle changes, not instead of them. The tirzepatide KwikPen is administered once weekly through dose titration, starting at 2.5 mg and gradually increasing over several months. Individual responses varied significantly: some participants lost more, some lost less.
Clinically BackedWeight Loss
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
How Mounjaro Works — The Mechanism Behind Results
Mounjaro works differently to other weight loss injections because it's a dual GIP and GLP-1 receptor agonist. Rather than activating just one hormone pathway like Wegovy (semaglutide), Mounjaro activates two: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help explain why some people see better results with Mounjaro versus Wegovy, though results vary. The medication helps regulate appetite, reduce food cravings, and promote feeling fuller after smaller meals. It also slows how quickly food leaves your stomach — though this effect typically lessens over time. For Mounjaro to work effectively, it must be combined with a reduced-calorie diet and increased physical activity. Medication alone is not enough.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is Mounjaro assured to work for weight loss?
No. While clinical trials show Mounjaro helps most people lose weight, individual results vary significantly. Some people lose more than the average, some less. Success depends on the dose, your lifestyle changes, adherence to the medication, and individual metabolism. Your GP or prescriber will assess whether it's suitable for you.
How does Mounjaro compare to Wegovy for weight loss?
Both are prescription-only weight loss injections, but Mounjaro is a dual GIP/GLP-1 agonist whilst Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (up to 20.9%) versus Wegovy (around 14.9%), but individual results vary. Only a prescriber can advise which is more suitable for you.
How long does it take to see results from Mounjaro?
Weight loss typically begins within the first few weeks for many people, but the first month may show modest results. Most significant weight loss occurs between weeks 8–20 as your dose increases through titration. The full SURMOUNT trial results were measured at 72 weeks, so sustained commitment is needed to achieve maximum benefit.
Can I get Mounjaro on the NHS in the UK?
Mounjaro is approved by NICE for use within NHS specialist weight management services. However, NHS access varies significantly by region and involves referral to a tier 3 specialist service, often with substantial waiting lists. Private prescription is also available through UK-licensed online prescribers following a clinical assessment.
What does "Mounjaro actually works" really mean?
It means the evidence shows tirzepatide helps most people lose a clinically meaningful amount of weight when combined with diet and exercise changes. "Works" doesn't mean studied results, dramatic transformations, or a substitute for lifestyle changes. It means evidence-based benefit, with individual variation. Use our free BMI calculator and GLP-1 weight loss calculator to understand your personal context and potential outcomes.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.