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 — How They Compare
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work through different mechanisms and have different dosing schedules. This guide compares how to use each pen, their clinical trial results, and key differences to help you understand what your prescriber might discuss with you. Both require a clinical assessment before prescription and are used alongside a reduced-calorie diet and increased physical activity for best results.
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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 Pens Work Differently
Mounjaro (tirzepatide) uses a dual GIP and GLP-1 receptor agonist mechanism — it activates two hormone pathways simultaneously. This dual action may help reduce appetite, support feeling fuller after meals, and regulate fat utilisation. The KwikPen delivers doses ranging from 2.5 mg to 15 mg, with a minimum of 4 weeks between dose increases.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. It may help reduce appetite and slow how quickly food leaves the stomach. Wegovy pens deliver doses from 0.25 mg to 2.4 mg, typically titrated over 16–20 weeks.
Clinical trial data (SURMOUNT-1 for Mounjaro and STEP-1 for Wegovy) showed both can support meaningful weight loss when combined with lifestyle changes. Individual results vary significantly — your prescriber will assess which approach suits your circumstances.
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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 Results and Individual Suitability
In clinical trials, Mounjaro at 15 mg showed average weight loss of approximately 20.9% of body weight over 72 weeks, whilst Wegovy at 2.4 mg showed approximately 14.9% over 68 weeks — but these are averages and individual results vary considerably. Both medications require a clinical assessment to determine eligibility: you'll typically need a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
Your prescriber will consider factors including any other medications you take, your medical history, and personal preference when deciding which injection might be appropriate for you. Both are prescription-only and available through NHS specialist weight management services or private prescribing pathways in the UK.
- 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 how Mounjaro and Wegovy work?
Mounjaro is a dual GIP and GLP-1 receptor agonist (activates two hormone pathways), whilst Wegovy is a GLP-1 receptor agonist only (single pathway). The dual mechanism may be why some clinical trials showed slightly higher average weight loss with Mounjaro, though individual results vary significantly.
How often do I inject Mounjaro compared to Wegovy?
Both are once-weekly injections. Mounjaro uses a KwikPen with doses from 2.5 mg to 15 mg, whilst Wegovy uses a pen with doses from 0.25 mg to 2.4 mg. Both start at lower doses and increase gradually if tolerated.
Can I switch from Wegovy to Mounjaro or vice versa?
Any switch between medications must be managed by your prescriber — there is no official conversion chart. Approximate equivalences exist (e.g., Wegovy 2.4 mg ≈ Mounjaro 15 mg), but switching requires medical guidance and a period of transition.
Which injection will I be prescribed — Mounjaro or Wegovy?
Your prescriber will assess your individual circumstances, medical history, and preferences. Both are UK-licensed for weight management and require a clinical assessment. Availability and regional variation in NHS services also affect which option is offered.
Do Mounjaro and Wegovy have similar side effects?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain, especially at first. Side effect profiles are broadly similar. Your prescriber will discuss potential side effects and how to manage them as your dose increases. ```
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