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 5mg Pen vs Wegovy — Which Is Right for You?
If you're considering prescription weight loss medication in the UK, you've likely heard of both Mounjaro and Wegovy. Both are licensed for weight management, but they work differently and may suit different individuals. This guide compares the two medications, their mechanisms, clinical trial data, and how your prescriber will help you choose. Understanding the differences helps you have an informed conversation with your GP or a licensed prescriber about which option might be more appropriate for your circumstances.
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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 — The Key Difference
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways in your body. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) together help regulate appetite, reduce food cravings, and support feelings of fullness after meals. This dual mechanism is unique to Mounjaro and may contribute to its weight loss outcomes.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates the single GLP-1 pathway. While this is a well-established and effective mechanism, it uses only one of the two hormone systems that Mounjaro targets. The Mounjaro KwikPen delivers the tirzepatide dose once weekly, similar to Wegovy's injection frequency. Both medications slow gastric emptying and help regulate blood sugar control, though through slightly different pathways.
- Mounjaro: dual mechanism (GIP + GLP-1)
- Wegovy: single mechanism (GLP-1 only)
- Both: once-weekly injection, prescription-only, require clinical assessment
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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.
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Clinical Trial Data and Weight Loss Outcomes
The SURMOUNT-1 trial for Mounjaro showed average weight loss of approximately 20.9% of body weight at the maximum 15mg dose over 72 weeks, alongside a reduced-calorie diet and increased physical activity. The STEP-1 trial for Wegovy showed approximately 14.9% average weight loss at the 2.4mg dose over 68 weeks under similar conditions.
Important context: These are clinical trial averages — individual results vary significantly. A 5mg Mounjaro dose (often an early titration step) typically produces more modest results than the maximum dose. Your own weight loss will depend on your metabolism, adherence to lifestyle changes, dose reached, and individual response. Neither medication assures specific outcomes. Your prescriber will assess which is more suitable based on your health history, any contraindications, and personal circumstances. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
Eligible UK adults typically have a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Both medications are prescription-only and require assessment by a UK-licensed prescriber — either through NHS specialist tier 3 weight management services or via private online prescribing with clinical assessment. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds due to higher health risks at lower weights.
Your GP or prescriber will consider your full medical history, current medications, and individual circumstances to recommend the most appropriate option. Mounjaro and Wegovy are not interchangeable — the choice depends on clinical suitability, not personal preference alone. Both require lifestyle changes alongside medication for best results. Check your eligibility using our free BMI calculator, and discuss prescription options with your healthcare provider.
- 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's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist (two hormone pathways), while Wegovy is a GLP-1 agonist only (one pathway). This means Mounjaro may work through two different mechanisms to help with appetite and weight regulation, whereas Wegovy uses a single well-established pathway. Both are once-weekly injections and prescription-only in the UK.
Which causes more weight loss — Mounjaro 5mg or Wegovy?
Clinical trials showed Mounjaro's maximum dose (15mg) produced greater average weight loss (~21%) than Wegovy 2.4mg (~15%). However, a 5mg Mounjaro dose is typically an early titration step, not the maximum dose. Individual results vary greatly depending on adherence, metabolism, and other factors. Your prescriber will monitor your progress and adjust as needed.
Can I switch from Wegovy to Mounjaro?
Switching between medications is possible but must be managed by your prescriber. While approximate equivalences exist (roughly 5mg Mounjaro ≈ 0.5mg Wegovy), there is no official conversion chart. Any switch requires clinical assessment and must be done safely under medical supervision. Your prescriber will assess whether switching is appropriate and manage the transition.
How do I get Mounjaro or Wegovy on the NHS in the UK?
Both medications are available through NHS specialist tier 3 weight management services, typically accessed via GP referral. Availability varies significantly by region (ICB area), and waiting lists can be long in many areas. Private prescribing is also available through UK-licensed prescribers and registered pharmacies without NHS referral.
Do I need a prescription for either medication?
Yes, both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only medicines in the UK. Neither can be obtained without a prescription from a qualified healthcare professional. You'll need a clinical assessment before either can be prescribed, whether through NHS services or private prescribing providers. Always ensure any prescriber is UK-licensed and any pharmacy is GPhC-registered.
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