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 Semaglutide to Mounjaro in the UK
Switching from semaglutide (Wegovy) to Mounjaro (tirzepatide) is a decision some people in the UK consider after assessing their weight loss progress and individual response. Both are once-weekly weight loss injections, but they work differently — semaglutide is a single GLP-1 receptor agonist, while Mounjaro is a dual GIP and GLP-1 receptor agonist. Before switching, you'll need to understand the cost differences between NHS and private options, how the transition works, and what eligibility requirements apply. This guide covers everything you need to know about making the switch in the UK.
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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.
Cost and Access: NHS vs Private Route for Mounjaro
Access to Mounjaro in the UK differs significantly depending on whether you pursue an NHS or private route. NHS access is through specialist tier 3/4 weight management services — typically via referral from your GP — but availability varies substantially by region and waiting lists can extend from months to over a year in many areas. Private access is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber, and is often faster. Private Mounjaro costs typically range from £129 to £320 per month depending on the dose you require. The 2.5 mg starting dose costs less than higher doses, and costs increase as you progress through dose titration towards the 15 mg maximum. Unlike some medications, Mounjaro pricing is relatively transparent across regulated UK providers, though slight variations exist. If you're currently paying for semaglutide privately, switching to Mounjaro may cost more or less depending on your current dose and which private provider you use.
- NHS Mounjaro: free once approved through specialist services
- Private Mounjaro: £129–£320/month (varies by dose and provider)
- NHS waiting lists: highly variable by region and ICB
- Private prescriptions: typically available within 7–14 days
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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.
The Switching Process: NHS, Private, and What to Expect
If you're currently on semaglutide (Wegovy) via NHS, switching to Mounjaro requires discussion with your specialist weight management service — they may offer the switch if Mounjaro is available in your region. If you're on private semaglutide, you can switch to private Mounjaro by completing an online consultation with a different prescriber who assesses your eligibility for Mounjaro. This assessment considers your BMI, weight-related health conditions, and response to previous GLP-1 treatment. There's typically no need to overlap doses — your prescriber will advise when to stop semaglutide and start Mounjaro. The KwikPen delivery system is similar to Wegovy, and the once-weekly injection schedule remains the same. Your prescriber may start you on the 2.5 mg starting dose even if you were on a higher semaglutide dose, then titrate upwards over weeks as your body adapts to the new medication.
- 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.
Is Mounjaro more expensive than Wegovy on the NHS?
Both are free once prescribed through NHS specialist weight management services. However, NHS access to either medication is limited and varies by region. Waiting lists for NHS specialist services can be lengthy, whereas private Mounjaro and Wegovy have similar costs (£129–£320/month depending on dose).
Can my GP switch me from semaglutide to Mounjaro?
No — your GP cannot initiate the switch on the NHS. Only NHS specialist tier 3/4 weight management services can prescribe Mounjaro. If you're on private semaglutide, you can approach a private prescriber directly for a Mounjaro assessment. Use our free GLP-1 weight loss calculator to compare potential outcomes with different medications.
How long does the switching process take on the NHS?
This varies significantly by region and ICB. Some areas have specialist services with capacity; others have waiting lists of 6–12+ months. Ask your GP about waiting times for your local specialist weight management service.
What happens to side effects when switching from semaglutide to Mounjaro?
Side effect profiles are similar — both cause nausea, diarrhoea, and stomach discomfort commonly — but individual responses differ. Some people tolerate one better than the other. Your prescriber will monitor your response closely after switching.
Can I switch between Mounjaro and Wegovy multiple times?
Technically yes, but frequent switching is not typical clinical practice. Switching is usually done once, after discussion with your prescriber about whether a change is justified by your progress and individual response to the first medication.
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.