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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Does Everyone Lose Weight on Mounjaro?
Not everyone loses weight on Mounjaro, even when taking it as prescribed. Clinical trials show average weight loss across groups, but individual results vary significantly. Some people respond excellently, while others see modest changes. This guide explains what affects your personal outcome and how the NHS and UK prescribers assess whether Mounjaro is right for you. Weight loss depends on many factors beyond the medication itself—including diet, activity level, genetics, and how your body responds to the GLP-1 and GIP pathways that Mounjaro activates.
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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 the Clinical Evidence Actually Shows
The SURMOUNT-1 clinical trial, which formed the basis for Mounjaro's approval in the UK, showed average weight loss of approximately 15–21% of body weight at the maximum 15 mg dose over 72 weeks. However, "average" masks the wide range of individual responses. Some participants lost over 25% of their body weight, whilst others lost less than 10%. The trial also included a control group receiving a placebo, who lost an average of 3% through lifestyle changes alone—meaning some of Mounjaro's effect is about medication, and some is about the diet and exercise changes people make whilst taking it.
- SURMOUNT-1 median weight loss at 15 mg: approximately 22.5% body weight
- Approximately 85% of participants achieved at least clinically studied weight loss
- Only about 50% achieved 20% or greater weight loss
- Baseline weight, age, and metabolic factors all influenced individual outcomes
Your GP or prescriber will discuss realistic expectations based on your personal health context, not just population averages. Use our free GLP-1 weight loss calculator to see projected weight loss ranges based on different doses—this gives a more personalised picture than trial averages alone.
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.
Why Some People Don't Lose Weight on Mounjaro
Several factors explain why individual results differ so dramatically on a weekly weight loss injection like Mounjaro KwikPen:
- Genetic response: How your body responds to GLP-1 and GIP receptor activation varies—some people's appetite suppression is much stronger than others
- Adherence to lifestyle changes: Mounjaro works best alongside reduced-calorie diet and increased physical activity—without these, results are minimal
- Underlying medical conditions: Thyroid disorders, PCOS, certain medications, and metabolic conditions can reduce weight loss response
- Dose tolerance: Some people cannot tolerate higher doses due to side effects, limiting therapeutic benefit
- Duration: Weight loss is not immediate—significant loss typically takes 12–16 weeks to become apparent
If you're not seeing results after 12 weeks at a therapeutic dose, your prescriber may adjust your approach or explore other options. The NHS prescribing guidance emphasises that weight loss medication is a tool, not a assures.
- 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.
What percentage of people don't lose weight on Mounjaro?
Approximately 15% of trial participants did not achieve meaningful weight loss (5% or more). Some people experience minimal loss despite taking the medication correctly. Non-response can be due to genetics, underlying conditions, or insufficient adherence to diet and exercise changes. Your GP can help identify reasons if you're not seeing results after 12 weeks at a therapeutic dose.
How long does it take to see weight loss on Mounjaro?
Most people begin noticing appetite suppression within 1–2 weeks, but measurable weight loss typically takes 4–8 weeks to become apparent. Significant weight loss (10%+ of body weight) usually takes 12–16 weeks at a therapeutic dose. The medication works gradually alongside calorie reduction and physical activity—rapid results are not expected or safe.
Can you lose weight on a low Mounjaro dose?
Yes, but results are typically smaller. The 2.5 mg starting dose is for tolerability, not therapeutic effect. Weight loss generally increases as you titrate to higher doses (5 mg, 7.5 mg, 10 mg, 15 mg). Many people see modest loss on lower doses, then more significant loss as they reach their maintenance dose over several months.
What should I do if Mounjaro isn't working for me?
First, ensure you're following dietary and activity recommendations—medication alone is insufficient. Give it at least 12 weeks at a therapeutic dose before concluding it's not working. If you're experiencing significant side effects preventing you from taking it properly, discuss alternatives with your GP or prescriber. Some people respond better to other GLP-1 medications or benefit from additional support.
Is Mounjaro assured to work on the NHS?
No. The NHS approves Mounjaro for eligible patients, but individual response is not assured. Your GP or tier 3 specialist service will monitor your progress and may adjust your treatment plan if you're not responding. Weight loss medication works best as part of a comprehensive approach including diet, activity, and behavioural support.
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.