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
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Mounjaro Weight Loss: Before and After in the UK
When considering Mounjaro (tirzepatide) for weight loss in the UK, many people want to understand what realistic results look like. Clinical trial data shows that Mounjaro can help adults lose an average of 15–21% of their body weight over a year, though individual results vary significantly. The NHS and NICE now recommend Mounjaro as an option for eligible adults with obesity or overweight with weight-related health conditions. This guide explains what the evidence shows, how NHS prescribing works, and what to realistically expect from treatment.
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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 Evidence Shows About Mounjaro Weight Loss
Clinical trial data comes from the SURMOUNT-1 study, which followed adults taking Mounjaro alongside a reduced-calorie diet and increased physical activity for 72 weeks. Results showed:
- At 5 mg dose: approximately 15% average body weight loss
- At 10 mg dose: approximately 19.5% average body weight loss
- At 15 mg dose (maximum): approximately 20.9% average body weight loss
These are averages — individual results vary considerably. Some people lose more, some less. Weight loss also depends on diet quality, physical activity level, medication adherence, and individual metabolism. Mounjaro works as a weekly weight loss injection that helps reduce appetite and support feeling fuller after smaller meals. NICE and the MHRA (UK medicines regulator) have approved Mounjaro based on this evidence, and it's now available through NHS tier 3 specialist weight management services.
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.
NHS Eligibility and What "Before and After" Means in UK Context
Before starting Mounjaro: Typically requires a formal NHS assessment through a tier 3 specialist weight management service. You'll have your BMI measured, health conditions reviewed, and weight loss history discussed. The "before" is your baseline weight, health markers (blood pressure, blood sugar), and fitness level.
After starting Mounjaro: Weight loss typically becomes noticeable between weeks 4–12, with continued loss over 12 months. Beyond weight, many people report improved blood sugar control, lower blood pressure, reduced joint pain, and better energy. These changes happen gradually — gradual weight management can lead to muscle loss, which is why sustainable progress matters. Use our free progress tracker to log your weight loss journey week by week, or use our BMI calculator to see how your changing weight affects your health category over time.
- 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.
Can Mounjaro clinically studied weight loss results like a "before and after"?
No. Mounjaro helps with weight loss, but results depend on diet, activity, and individual factors. Clinical trials showed average loss of 15–21%, but individual results vary. "Before and after" transformations shown online may represent best outcomes, not typical results. Your GP can discuss realistic expectations for your situation.
What is the difference between Mounjaro and other weight loss injections like Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is GLP-1 only. Mounjaro showed slightly higher average weight loss in clinical trials (~20.9% vs ~15%), but both require prescription and clinical assessment. The best option depends on your individual health profile and prescriber recommendation.
How quickly will I see weight loss with Mounjaro in the NHS?
Weight loss typically becomes noticeable after 4–12 weeks as you progress through dose increases. The NHS recommends losing 0.5–1 kg per week for sustainable results. Don't expect dramatic changes immediately — Mounjaro works gradually alongside diet and activity changes.
What happens if I don't lose weight on Mounjaro?
If you're not seeing expected progress after 3–4 months at the target dose, speak with your prescriber. They may adjust your dose, review your diet and activity, check for medication interactions, or discuss alternative options. Weight loss medication works best with lifestyle changes.
Is Mounjaro available through the NHS for weight loss in the UK?
Yes. NICE recommends Mounjaro (tirzepatide) as an option for eligible NHS patients. Access is through tier 3 specialist weight management services. Eligibility requires BMI ≥30, or BMI 27–29.9 with weight-related health conditions. Waiting lists vary by region — ask your GP for a referral.
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.