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 15mg Pen vs Wegovy — What's the Difference?
Both Mounjaro and Wegovy are prescription-only weight management injections available in the UK, but they work differently. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. The Mounjaro 15mg KwikPen is the maximum licensed dose in the UK; Wegovy's highest dose is 2.4mg. This guide compares the two medications factually so you can understand what prescribers might consider when assessing your suitability.
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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 15mg and Wegovy Work Differently
Mounjaro (tirzepatide) activates two hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. This dual action may help regulate appetite, reduce cravings, and support feeling fuller after meals more effectively than a single pathway.
Wegovy (semaglutide) activates the GLP-1 pathway only, which similarly reduces appetite and slows gastric emptying, but through one mechanism rather than two.
In clinical trials, the SURMOUNT-1 study showed Mounjaro at 15mg helped participants lose an average of 20.9% of their body weight over 72 weeks. The STEP-1 trial showed Wegovy at 2.4mg resulted in approximately 14.9% average weight loss over 68 weeks. However, individual results vary significantly, and results were achieved alongside reduced-calorie diet and increased physical activity. Our free GLP-1 weight loss calculator can help you understand potential weight loss ranges at different doses.
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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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Eligibility, Doses, and Which Might Suit You
Both medications are prescribed to UK adults with a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions (such as type 2 diabetes, hypertension, or cardiovascular disease). South Asian, Chinese, Black African, and Black Caribbean adults may have lower BMI eligibility thresholds.
Mounjaro dosing ranges from 2.5mg to 15mg weekly; Wegovy from 0.25mg to 2.4mg weekly. Neither medication assures weight loss, and suitability depends on your individual health profile. A prescriber conducting a clinical assessment will consider which medication aligns with your medical history, current medications, and health goals. Check your personal BMI using our free BMI calculator to see if you may be eligible for either medication.
- 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 15mg and Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist (activates two hormone pathways), while Wegovy is a GLP-1 agonist only (one pathway). The dual mechanism in Mounjaro may provide additional appetite suppression, though individual responses vary. Both are prescription-only and require clinical assessment.
Can I switch between Mounjaro and Wegovy on the NHS?
Switching between medications would be a prescriber decision based on your response, tolerability, and health profile. On the NHS, access to either medication is through specialist tier 3 weight management services, and switching protocols vary by region. Discuss options with your prescriber.
Which medication causes fewer side effects?
Both medications have similar common side effects: nausea, diarrhoea, constipation, and stomach pain. Side effects are typically dose-dependent and often reduce over time. Individual tolerance varies significantly. A prescriber can discuss which medication might suit your health profile best.
How do I get Mounjaro 15mg or Wegovy in the UK?
Both are prescription-only and available through NHS specialist weight management services (following GP referral) or private prescribing following an online clinical assessment with a UK-licensed prescriber. Neither is available over-the-counter. Your prescriber will assess eligibility and suitability based on your BMI and health profile.
Do I need to use our weight loss calculator to check which is right for me?
Our free GLP-1 weight loss calculator helps you explore potential weight loss projections at different doses, but it cannot determine which medication suits you. Only a qualified prescriber can assess your individual health, medical history, and medications to recommend the most suitable option. Always consult your GP or a licensed prescriber.
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.