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 Pen vs Wegovy — UK Dosing & Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly weight loss injections available in the UK, but they work slightly differently and use different pen designs. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. Understanding the differences in their dosing schedules, pen mechanisms, and clinical trial results can help you and your GP decide which might be more suitable for your circumstances. Both are prescription-only and require a clinical assessment before being prescribed.
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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.
Mounjaro KwikPen vs Wegovy Auto-Injector — Key Differences
Mounjaro (tirzepatide) uses an easy-to-use KwikPen device with a mechanical click system. Each pen contains 4 weekly doses in a pre-filled cartridge. Doses range from 2.5 mg (starting dose) up to 15 mg (maximum). You rotate through doses over 20+ weeks as your prescriber increases the amount based on tolerance and response.
Wegovy (semaglutide) uses a pre-filled auto-injection pen. Doses start at 0.25 mg and increase to 2.4 mg maximum over 17 weeks. Each pen contains one dose only, so you receive a new pen weekly.
In clinical trials, Mounjaro showed average weight loss of 15–21% of body weight (SURMOUNT-1 trial), whilst Wegovy showed approximately 15% average loss (STEP-1 trial). Both work alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly, and results depend on your starting weight, adherence, and overall lifestyle changes.
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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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How to Choose — Individual Suitability & NHS Context
Both medications require a prescription from a UK-licensed prescriber following a clinical assessment. Your eligibility depends on your BMI, weight-related health conditions (such as type 2 diabetes or high blood pressure), and overall health profile. NHS access is through specialist tier 3 weight management services, where waiting lists vary by region. Private prescribing is also available through GPhC-registered online consultations without NHS referral.
Your prescriber will assess which medication is more suitable based on your medical history, current medications, and preference. There is no universally "better" option — both are effective, and individual response varies. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to confirm your current classification against NHS thresholds.
- 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 is the main difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only. Both reduce appetite and support feeling fuller, but Mounjaro's dual mechanism may contribute to slightly higher average weight loss in clinical trials (15–21% vs 15%).
How many doses are in each pen, and how often do I inject?
Mounjaro's KwikPen contains 4 weekly doses in one cartridge. Wegovy's auto-injector pen contains 1 dose per pen, so you receive a new pen each week. Both are injected once per week, on the same day each week.
What are the maximum doses for each medication?
Mounjaro's maximum dose is 15 mg once weekly. Wegovy's maximum dose is 2.4 mg once weekly. Both start at lower doses and escalate every 4 weeks (Mounjaro) or weekly (Wegovy) based on tolerance and response.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official established conversion chart — approximate equivalences suggest Mounjaro 10 mg is roughly comparable to Wegovy 1.7 mg, but any switch requires clinical guidance and dose adjustment.
How do I get Mounjaro or Wegovy prescribed in the UK?
You can access either medication through NHS specialist weight management services (tier 3) by GP referral, or privately through GPhC-registered online prescribing services. Both routes require a clinical assessment. Your prescriber will determine eligibility based on BMI and weight-related health conditions.
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.