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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How To Change From Mounjaro To Wegovy In The UK
If you're taking Mounjaro (tirzepatide) and considering a switch to Wegovy (semaglutide), you'll need to work with your prescriber to make the change safely. Both medications are GLP-1 and GIP receptor agonists approved in the UK for weight management, but they work slightly differently and have different dosing schedules. This guide explains the NHS and private routes for switching, how doses compare, and what you need to know about making the transition.
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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.
Understanding The Difference Between Mounjaro And Wegovy
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways — while Wegovy (semaglutide) is a GLP-1-only agonist. This is a key difference: Mounjaro's dual mechanism may explain why clinical trials showed slightly higher average weight loss (approximately 20.9% on the maximum dose) compared to Wegovy (approximately 14.9% in STEP trial data). Mounjaro is injected once weekly using a KwikPen, and Wegovy is also once weekly. Individual results vary significantly. Mounjaro uses a titration schedule starting at 2.5 mg, escalating to a maximum of 15 mg over approximately 20 weeks. Wegovy starts at 0.25 mg and escalates to 2.4 mg over approximately 16 weeks. Because the mechanisms and doses are different, you cannot simply switch doses directly — your prescriber must assess which dose of Wegovy is appropriate for you based on your individual response and health status.
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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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NHS Route Versus Private Route For Switching
If you're currently accessing Mounjaro through an NHS tier 3 specialist weight management service, switching to Wegovy would typically stay within the same NHS pathway. However, NHS availability for both medications varies significantly by region (ICB). Some areas may have Wegovy in their formulary while others do not, and waiting times for specialist assessment can be lengthy. If you're on a private prescription for Mounjaro, you can switch to Wegovy through a private online prescriber following a clinical assessment. Your prescriber will review your current dose, weight loss progress, and any side effects you've experienced to determine the appropriate starting dose of Wegovy. Many private prescribers can manage the transition smoothly, though you may need to stop Mounjaro and begin Wegovy on a new schedule rather than switching directly mid-week.
- 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 approximate dose equivalence when switching from Mounjaro to Wegovy?
Approximate equivalences are: Mounjaro 5 mg ≈ Wegovy 0.5 mg; 10 mg ≈ 1.7 mg; 15 mg ≈ 2.4 mg. However, these are rough guides only — your prescriber may start you at a lower Wegovy dose and titrate upward based on your tolerance and results. Individual response varies.
Do I need to stop Mounjaro before starting Wegovy?
Yes, you should stop Mounjaro before beginning Wegovy. Your prescriber will advise on the exact timing. Because Mounjaro is once-weekly and Wegovy is also once-weekly, there may be a short gap between your last Mounjaro injection and your first Wegovy dose to allow your body to adjust.
Can I switch through my GP or do I need a specialist?
If you're on an NHS prescription through a tier 3 weight management service, your specialist will manage the switch. If you're on a private prescription, you can switch through a private prescriber via an online consultation. Your GP may not prescribe these medications directly — most GPs refer to specialist services.
Why would I switch from Mounjaro to Wegovy if Mounjaro shows higher weight loss?
Reasons include side effects (Wegovy may suit you better), cost differences between private providers, NHS formulary availability in your region, or personal preference. Some people experience fewer GI side effects with Wegovy. Your prescriber can discuss whether switching aligns with your health goals.
Will I lose my current weight loss progress if I switch?
No. Switching medications does not erase your progress. However, there may be a brief transition period (typically a few days to weeks) before Wegovy reaches therapeutic effectiveness. Maintaining your diet and exercise habits during the switch is important to minimise any temporary weight regain.
Ready to take control of your weight loss journey?
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