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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Empty Mounjaro Pen vs Wegovy — What's the Difference?
When your Mounjaro KwikPen runs empty and you're considering Wegovy as an alternative, it's useful to understand how these two weight loss injections differ. Both are prescription-only medications available in the UK for weight management, but they work through different mechanisms and have different dose schedules. This guide compares the two based on clinical trial data, eligibility, and individual suitability so you can have an informed discussion with your GP or prescriber about which might be right for you.
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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 separate hormone pathways in the body. This dual mechanism may help regulate appetite more effectively by working through two different signalling routes simultaneously.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. Both medications help reduce appetite and support feeling fuller after smaller meals, but Mounjaro's dual approach may offer a different effect profile.
- Mounjaro: Dual GIP + GLP-1 activation, once-weekly KwikPen injection
- Wegovy: GLP-1 activation only, once-weekly auto-injection pen
- Tirzepatide (Mounjaro's active ingredient): Generally used at higher doses (up to 15 mg weekly)
- Semaglutide (Wegovy's active ingredient): Used at up to 2.4 mg weekly for weight management
Clinical trial data from SURMOUNT-1 (Mounjaro) and STEP-1 (Wegovy) showed both medications support weight loss when used alongside a 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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Clinical Trial Data and Individual Suitability
The SURMOUNT-1 trial showed that Mounjaro at the maximum dose (15 mg) helped participants lose an average of approximately 20.9% of their body weight over 72 weeks. The STEP-1 trial showed Wegovy at 2.4 mg helped participants lose approximately 14.9% of body weight over 68 weeks. However, these are clinical trial averages — individual results vary considerably based on genetics, lifestyle changes, dose tolerance, and adherence.
Your prescriber will assess which medication suits your individual health circumstances, not just trial averages. Factors include your medical history, other medications, side effect tolerance, and whether you've used either medication before. Switching between them requires prescriber guidance. Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses, and our BMI calculator to track your starting point and progress.
- 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.
When my Mounjaro KwikPen is empty, can I switch straight to Wegovy?
Not without prescriber guidance. Although both are once-weekly injections for weight loss, switching between medications requires clinical assessment. Your prescriber needs to review your response to Mounjaro, any side effects, and your current health status before recommending Wegovy. There's no official automatic conversion between the two — each requires a fresh prescription and clinical decision.
Which medication causes fewer side effects — Mounjaro or Wegovy?
Both have similar common side effects: nausea, diarrhoea, vomiting, and constipation, especially at the start. Individual side effect experiences vary widely. Some people tolerate one better than the other due to differences in mechanism and dose schedules. Only your prescriber can assess which might suit your body better based on your health history and any previous medication responses.
Is Mounjaro more effective for weight loss than Wegovy?
Clinical trials showed Mounjaro at maximum dose produced slightly higher average weight loss (20.9% vs 14.9%), but these are averages from specific trial populations. Individual results vary considerably. "More effective" is not the same as "right for you" — effectiveness depends on your genetics, lifestyle adherence, and tolerance. Your prescriber will consider your individual circumstances, not just trial rankings.
How do I know which medication I'm eligible for on the NHS?
NHS eligibility for both Mounjaro and Wegovy is the same: BMI 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease, sleep apnoea). Lower thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. Availability and prescribing decisions vary by NHS region — discuss options with your GP or specialist weight management service.
What happens if I've had side effects on Mounjaro — will Wegovy be different?
Possibly, but not assured. Both medications activate GLP-1 receptors, so if you experienced nausea or digestive side effects, Wegovy might also cause similar issues. However, Mounjaro's dual GIP+GLP-1 mechanism may produce a different side effect profile in some people. Only your prescriber can advise based on which specific side effects you experienced and why you might tolerate the alternative better.
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.