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 2.5mg Pen vs Wegovy: Key Differences
If you're considering weight loss injections in the UK, you may have come across both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are prescription-only GLP-1 receptor agonist medications licensed for weight management in UK adults, but they work in different ways and have distinct dosing schedules. Understanding how the Mounjaro 2.5mg starting dose compares to Wegovy's approach can help you have a more informed conversation with your GP or prescriber about which option might suit your individual circumstances.
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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) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways rather than one. This dual action may contribute to its effects on appetite regulation and food cravings. The 2.5mg KwikPen is the starting dose, typically used for the first four weeks before increasing to 5mg.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. Clinical trial data shows different weight loss trajectories: Mounjaro participants in the SURMOUNT-1 trial lost an average of approximately 15% body weight at 5mg and 20.9% at the maximum 15mg dose. Wegovy participants in the STEP-1 trial achieved approximately clinically studied weight loss at the maximum 2.4mg dose. Both medications work alongside reduced-calorie diet and increased physical activity—individual results vary significantly.
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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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Which Is Right for You? A Prescriber Decision
Neither Mounjaro nor Wegovy is universally "better"—suitability depends on your individual health profile, medical history, and how your body responds. Your GP or specialist prescriber will assess whether you meet UK eligibility criteria (BMI 30 or above, or 27–29.9 with weight-related health conditions) and consider any contraindications or existing medications. Some people respond better to dual-action tirzepatide; others find semaglutide equally effective. Both are prescription-only medicines requiring clinical assessment. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses, then discuss the most suitable option with your prescriber based on your NHS NICE guidance and 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.
What is the difference between Mounjaro's 2.5mg starting dose and Wegovy's starting dose?
Mounjaro starts at 2.5mg once weekly, whereas Wegovy begins at 0.25mg once weekly. These different starting points reflect their different mechanisms—Mounjaro is a dual GIP+GLP-1 agonist, Wegovy is GLP-1 only. Both require minimum four weeks at each dose before escalation, so the total time to reach higher doses differs between medications.
Which causes more weight loss, Mounjaro or Wegovy?
Clinical trial averages show Mounjaro achieved up to clinically studied weight loss at maximum dose, and Wegovy approximately 14.9%. However, these are population averages—individual results vary considerably. Some people lose more weight on one medication than the other. Your prescriber will help identify which suits your circumstances.
Can I switch from Mounjaro 2.5mg to Wegovy if I don't tolerate it?
Switching between medications is possible and is managed by your prescriber. There is no official conversion chart—doses are not directly equivalent. Any medication change should only happen following clinical guidance from your GP or weight management specialist. Do not stop or switch without professional advice.
Is Mounjaro available on the NHS in the UK?
Yes, tirzepatide (Mounjaro) is NICE-approved for NHS weight management services. However, availability is through specialist tier 3/4 services, not routine GP prescription. Access varies significantly by region and waiting lists can be substantial. Private prescription remains an alternative route.
What makes the Mounjaro KwikPen different from Wegovy pens?
Both are pre-filled auto-injection pens for once-weekly use. The main difference is the active ingredient—Mounjaro contains tirzepatide, Wegovy contains semaglutide. Both are stored in a refrigerator until opened, then can be kept at room temperature for up to 30 days. Injection technique and site rotation are similar for both medications.
Ready to take control of your weight loss journey?
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