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 Mounjaro To Wegovy In The UK
If you're taking Mounjaro and considering switching to Wegovy, or vice versa, it's important to understand how these two prescription-only weight management injections work, how they differ, and what the NHS and UK prescribing guidelines say about switching between them. Both tirzepatide (Mounjaro) and semaglutide (Wegovy) are licensed in the UK for weight management, but they work slightly differently and have different dose ranges. Any switch between medications must be managed by your GP or a licensed prescriber following a clinical assessment. This guide explains what you need to know about both medicines and the switching process in a UK health context.
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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.
NHS Eligibility And Prescribing Context For Wegovy
Wegovy (semaglutide 2.4 mg) is a once-weekly GLP-1 receptor agonist licensed in the UK for weight management in adults. It's available through NHS specialist tier 3 weight management services, though access varies significantly by region and ICB (Integrated Care Board). The STEP-1 clinical trial showed that adults on Wegovy lost an average of approximately 15% of body weight over 68 weeks when combined with a reduced-calorie diet and increased physical activity—though individual results vary considerably.
To be eligible for Wegovy through the NHS, you typically need a BMI of 30 or above, or 27 or above with at least one weight-related health condition such as type 2 diabetes, high blood pressure, or cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS applies lower BMI thresholds. Unlike Mounjaro, which uses a dose titration schedule starting at 2.5 mg and increasing weekly, Wegovy's dose starts at 0.25 mg and escalates more gradually over 16 weeks to reach the maximum of 2.4 mg.
- Wegovy is a single GLP-1 agonist (versus Mounjaro's dual GIP and GLP-1 mechanism)
- NHS access is through specialist services—not standard GP prescription
- Regional availability and waiting lists vary across England, Scotland, Wales, and Northern Ireland
- Private prescription routes exist as an alternative if NHS waiting times are long
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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.
Switching Between Mounjaro And Wegovy: NHS And Private Routes
If you're already on Mounjaro through the NHS or private prescription and wish to switch to Wegovy, this must be done under the supervision of your prescriber. There is no official NHS-endorsed conversion chart between Mounjaro doses and Wegovy doses, although clinical practice suggests approximate equivalences—for example, Mounjaro 5 mg is loosely comparable to Wegovy 0.5 mg, and Mounjaro 15 mg to Wegovy 2.4 mg. However, any transition requires your prescriber's guidance because the two medicines have different pharmacology and titration schedules.
For NHS patients, a switch would typically involve discussion with your tier 3 specialist weight management service. If you're accessing either medicine privately, your prescriber will assess whether a switch is appropriate based on your individual response, tolerance, and health goals. Switching may be considered if you experience significant side effects on one medication, or if you and your prescriber agree that a different mechanism might suit you better. Use our free GLP-1 weight loss calculator to see how potential outcomes might differ between the two medicines, and always discuss any switch with your prescriber before making changes.
- 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 Mounjaro to Wegovy on the NHS?
Potentially, yes—but any switch must be managed by your NHS specialist weight management service or private prescriber. There is no automatic switching process. You'll need to discuss your reasons for switching (side effects, efficacy, preference) with your prescriber, who will assess whether Wegovy is appropriate for you and manage the transition safely.
Is Mounjaro or Wegovy more effective for weight loss?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss (around 15–21% of body weight) compared to Wegovy (around 15%), but individual results vary significantly. Effectiveness depends on your personal response, adherence to diet and exercise, and other health factors. Your prescriber can advise which is most suitable for you.
What's the key difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 receptor agonist only. This means Mounjaro activates two hormone pathways, whereas Wegovy activates one. Both are once-weekly injections, but Mounjaro starts at 2.5 mg and Wegovy starts at 0.25 mg, with different titration schedules.
How long does it take to switch from one medicine to the other?
The transition depends on your prescriber's guidance. Generally, you'll stop one medication and begin the other on a specified date. Because Wegovy has a longer initial titration phase (starting at 0.25 mg), the transition may take several weeks to reach an effective maintenance dose. Your prescriber will provide a detailed switching timeline.
Can I access Wegovy faster through a private prescription if NHS waiting lists are long?
Yes. Many UK adults access weight management medications through private online prescribing services following a clinical assessment, which can be faster than NHS routes. Private prescriptions are dispensed by GPhC-registered pharmacies. Costs vary by dose and provider. However, both NHS and private prescriptions require a valid prescription from a licensed UK prescriber—it's not available over the counter.
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