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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Losing Weight With Mounjaro: How It Compares to Wegovy
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections licensed in the UK, but they work differently and produce different results. Mounjaro uses a dual GIP and GLP-1 mechanism, whilst Wegovy is a GLP-1-only medication. Understanding the differences between these two popular weekly weight loss injections can help you and your prescriber decide which might suit you best. This guide compares both medications based on clinical trial data, dosing, side effects, and individual suitability — helping you make an informed decision about your weight loss options in the UK.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may enhance appetite suppression and support feeling fuller after smaller meals. In the SURMOUNT-1 clinical trial, participants taking the maximum 15mg dose lost an average of approximately 20.9% of their body weight over 72 weeks.
Wegovy (semaglutide) activates the GLP-1 pathway only. Based on the STEP-1 trial, participants taking the maximum 2.4mg dose lost an average of around 14.9% of body weight over 68 weeks. Both medications are given as once-weekly injections via a KwikPen device. Individual results vary significantly, and weight loss occurs alongside reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses for your weight.
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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
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Dosing, Side Effects, and Individual Suitability
Mounjaro escalates from 2.5mg over 20 weeks to a maximum of 15mg weekly, whilst Wegovy escalates more gradually from 0.25mg over 17 weeks to 2.4mg weekly. Both require clinical assessment and prescription. Common side effects include nausea, diarrhoea, vomiting, and stomach pain — typically most noticeable during dose escalation. Neither is suitable during pregnancy, and both may reduce absorption of oral contraceptives.
Your prescriber will assess which suits you based on your health history, current medications, and personal preferences. Neither medication assures specific weight loss outcomes. Access in the UK is through NHS specialist weight management services (variable waiting times by region) or private online prescribing following clinical assessment. Your GP or prescriber can provide personalised guidance on suitability for your individual circumstances.
- 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 more effective than Wegovy for weight loss?
Clinical trials show Mounjaro (average 20.9% loss) produced greater average weight loss than Wegovy (average 14.9% loss). However, individual results vary considerably. Some people respond better to one than the other. Effectiveness depends on your metabolism, lifestyle adherence, and personal health factors. Only a prescriber can assess which is more suitable for you.
What is the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1-only agonist, activating one pathway. This difference in mechanism may explain why Mounjaro produced higher average weight loss in clinical trials, but both require lifestyle changes alongside medication.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber — they cannot be directly substituted. Approximate dosing equivalence exists (Wegovy 2.4mg ≈ Mounjaro 15mg), but any change requires clinical assessment, a new prescription, and proper dose titration. Never switch without prescriber guidance.
How quickly will I lose weight on Mounjaro or Wegovy?
Safe, sustainable weight loss is typically 0.5–1kg per week. Both medications work by reducing appetite and supporting fullness. Weight loss usually becomes noticeable within 4–8 weeks, but varies individually. Use our BMI calculator to track your starting point, and our progress tracker to log weekly changes alongside your prescriber's monitoring.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are NHS-approved but access is through specialist tier 3 weight management services, not standard GP prescription. Waiting times vary significantly by region and ICB. Private prescription through online assessment is also available. Ask your GP about NHS options in your area, or explore private routes if NHS waiting lists are lengthy.
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.