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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Switching From Wegovy To Mounjaro — What You Need To Know
If you're currently taking Wegovy (semaglutide) and wondering whether Mounjaro (tirzepatide) might be a better option, you're not alone. Both are prescription-only weight loss injections licensed in the UK, but they work differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, whereas Wegovy is a GLP-1 agonist only. Any switch between medications must be managed by your prescriber or GP — never stop one medication and start another without clinical guidance. This guide explains the differences, eligibility criteria through NHS tier 3 services, and how to discuss a potential switch with your healthcare team.
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Calculate Your BMI Instantly
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 Differ — And Why You Might Switch
Mounjaro and Wegovy work through different mechanisms. Wegovy activates only the GLP-1 hormone pathway, helping reduce appetite and promote fullness. Mounjaro activates both GIP and GLP-1 pathways simultaneously — this dual action may provide stronger appetite suppression for some people. Clinical trial data showed Mounjaro users lost an average of 20.9% body weight at the maximum dose (15 mg), compared to approximately 14.9% with Wegovy (2.4 mg). However, individual results vary significantly.
You might consider switching if:
- Wegovy hasn't delivered the weight loss results you expected after adequate time and dose escalation
- You're experiencing side effects on Wegovy that are difficult to manage
- Your prescriber recommends trying a different GLP-1 or GIP/GLP-1 medication
- You want to explore whether the dual-mechanism approach of Mounjaro may work better for you
Any switch must be initiated and monitored by your GP, NHS specialist weight management service, or private prescriber following a clinical assessment.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS Access And Prescriber-Managed Switching
If you're accessing Wegovy through an NHS tier 3 specialist weight management service, any switch to Mounjaro would be managed entirely by that service. NICE has approved both semaglutide (Wegovy) and tirzepatide (Mounjaro) for weight management — your specialist team can discuss which medication may be more appropriate based on your weight loss response and tolerability.
If you're using Wegovy privately, you can discuss switching to Mounjaro with your private prescriber at your next consultation. They will review your weight loss progress, current side effects, and clinical suitability before issuing a Mounjaro prescription. Use our GLP-1 weight loss calculator to compare potential outcomes at different Mounjaro doses — this can help you discuss realistic expectations with your prescriber. Allow time between stopping Wegovy and starting Mounjaro as your prescriber advises (typically no washout is required, but timing depends on your individual case).
- 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.
Can I switch from Wegovy to Mounjaro on the NHS?
Yes, if you're receiving Wegovy through an NHS tier 3 specialist weight management service, you can discuss switching to Mounjaro with your service. NICE has approved both medications. Your specialist will assess whether Mounjaro is clinically suitable based on your progress and health profile. The switch would be managed entirely by the NHS service.
What's the approximate dose equivalent when switching from Wegovy to Mounjaro?
Wegovy 2.4 mg is roughly equivalent to Mounjaro 10 mg, though these are not exact conversions. Your prescriber may start you on a lower Mounjaro dose and titrate upwards based on your response, or switch you to an equivalent dose. Individual factors affect dosing decisions — always follow your prescriber's guidance.
Will I experience different side effects on Mounjaro compared to Wegovy?
Possibly. Both cause nausea, diarrhoea, constipation, and stomach pain as common side effects. Mounjaro's dual-action mechanism may produce slightly different side effect profiles in some people. If side effects were your reason for switching, discuss this with your prescriber — they may adjust your starting dose or offer additional support.
How long should I wait after stopping Wegovy before starting Mounjaro?
There is typically no mandatory washout period between stopping Wegovy and starting Mounjaro. However, your prescriber will advise you on the safest timing based on your individual health. Some prescribers may recommend a short interval; others may transition you directly. Always follow your prescriber's specific guidance.
Can I use the BMI calculator to check my progress on either Mounjaro or Wegovy?
Yes. Use our free BMI calculator to track your BMI before starting either medication and throughout your weight loss journey. Calculate your BMI regularly to monitor progress objectively, alongside the weight loss calculator, which can help you estimate potential outcomes at different Mounjaro doses.
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.