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 Mounjaro Work for Everyone? Understanding Individual Response
Not everyone responds equally to Mounjaro (tirzepatide), and understanding why is important when considering GLP-1 weight loss medication. Clinical trials show average weight loss of 15–21% of body weight, but individual results vary significantly. Some people experience modest weight loss, while others see more substantial changes. This variation depends on factors like starting weight, lifestyle changes, genetics, and how your body responds to the dual GIP and GLP-1 mechanism. If you're wondering whether Mounjaro is right for you, comparing it with Wegovy (semaglutide) and understanding prescriber eligibility criteria can help you make an informed decision. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses.
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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.
Clinical Trial Data: Mounjaro vs Wegovy Effectiveness
Mounjaro (SURMOUNT trials): In the SURMOUNT-1 clinical trial, participants on Mounjaro 15 mg achieved an average weight loss of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. At 10 mg, the average was 19.5%. At the lower 5 mg dose, participants lost approximately 15% on average.
Wegovy (STEP trials): In the STEP-1 clinical trial, participants on Wegovy 2.4 mg achieved an average weight loss of approximately 14.9% of their body weight over 68 weeks under similar lifestyle conditions.
Why the difference? Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways, whilst Wegovy activates GLP-1 only. This dual mechanism may explain why some people experience greater weight loss with Mounjaro. However, individual response varies — some people respond equally well to Wegovy, and some people on Mounjaro experience minimal weight loss regardless of dose titration.
Important: Clinical trial averages do not predict individual outcomes. Your prescriber will assess which medication and dose is most suitable based on your medical history, current health conditions, and previous response to GLP-1 medications if applicable.
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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 Mounjaro Doesn't Work for Everyone: Individual Factors
Several factors affect how well Mounjaro works for you personally:
- Starting weight and metabolism: Percentage-based weight loss varies by individual metabolism and starting body composition. Someone starting at 130 kg may lose more total weight than someone starting at 85 kg, even at the same percentage.
- Lifestyle adherence: Mounjaro is prescribed alongside a reduced-calorie diet and increased physical activity. Without these lifestyle changes, medication efficacy is significantly reduced.
- Dose tolerance: Not everyone can tolerate higher doses. Gastrointestinal side effects (nausea, constipation, diarrhoea) may limit how high your dose can go, potentially affecting outcomes.
- Genetic factors: Individual genetics influence appetite signalling and GLP-1 receptor sensitivity — some people's bodies respond more robustly to GLP-1 activation than others.
- Medical conditions: Type 2 diabetes, PCOS, and other metabolic conditions affect weight loss response. Your GP or prescriber will assess your individual suitability based on your Mounjaro UK prescription eligibility.
- 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.
If Mounjaro doesn't work for me, can I switch to Wegovy?
Yes. If your prescriber determines that Mounjaro is not producing adequate results or side effects are problematic, switching to Wegovy (semaglutide) is a clinical option. Both are UK-licensed GLP-1-based medications. Any medication change must be managed by your prescriber — never stop or switch without guidance. The approximate dose equivalence is Mounjaro 15 mg ≈ Wegovy 2.4 mg, though this is not a direct conversion.
What does "not working" mean for Mounjaro?
"Not working" typically means: minimal weight loss after 12+ weeks despite good adherence to diet and exercise, or side effects are too severe to increase to an effective dose. Clinical guidelines suggest assessing effectiveness after 12–16 weeks at a stable dose. Very modest weight loss (under 2–3% body weight) may prompt your prescriber to consider alternatives or investigate other factors like thyroid function or medication interactions.
How long should I wait to see if Mounjaro works?
Clinical guidance suggests waiting at least 12–16 weeks on a stable therapeutic dose before assessing effectiveness. Your dose will typically be increased every 4 weeks (Mounjaro dose titration follows 2.5 mg → 5 mg → 7.5 mg → 10 mg progression). Weight loss typically becomes more noticeable weeks 8–12. If you've reached your target dose and see minimal change by week 16, discuss alternatives with your prescriber.
Is Mounjaro or Wegovy objectively more effective?
Based on clinical trial data, Mounjaro 15 mg showed slightly higher average weight loss (20.9%) than Wegovy 2.4 mg (14.9%). However, individual variation is substantial — some people respond better to Wegovy, some to Mounjaro. There is no "better" medication for everyone. Your prescriber will consider your individual health profile, previous medication response, side effect tolerance, and medical history when recommending which medication is most suitable for you.
What should I do if Mounjaro hasn't caused weight loss after 3 months?
Contact your prescriber. They will review your dose, check your adherence to lifestyle changes, and investigate other factors (thyroid function, medication interactions, underlying conditions). Your prescriber may: increase your dose if you're tolerating it well, continue at the current dose for longer, refer you for specialist assessment, or discuss switching to an alternative medication. Never stop or change doses without guidance from your prescriber.
Ready to take control of your weight loss journey?
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