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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Can You Switch From Mounjaro To Wegovy In The UK?
Some people prescribed tirzepatide (Mounjaro) may consider switching to semaglutide (Wegovy), another weight loss injection available in the UK. Whether you can switch depends on your prescriber, your response to treatment, and NHS or private access routes. Both are prescription-only medications for adults meeting UK eligibility criteria. This guide explains how switching works in UK healthcare, the key differences between these GLP-1 and dual GIP/GLP-1 medications, and what to discuss with your GP or prescriber.
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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.
NHS Prescribing And Switching Between Weight Loss Injections
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are MHRA-approved prescription-only medications for weight management in UK adults. However, switching between them is not straightforward on the NHS. If you're receiving either medication through NHS tier 3 specialist weight management services, any switch must be managed by your specialist team — they will assess whether changing medication is clinically appropriate based on your individual response, side effects, and health needs.
On the NHS, tirzepatide (Mounjaro) has been approved by NICE for use within specialist weight management services, whilst semaglutide access varies by region and individual circumstances. Switching typically requires discussion with your prescriber about why a change might benefit you. If you're on a Mounjaro KwikPen and wish to explore Wegovy instead, your NHS specialist will consider factors like: how well Mounjaro has worked for you, any side effects you've experienced, your dose titration progress, and local formulary guidance.
- Tier 3 NHS services: Prescribers manage switching based on clinical judgment and regional protocols
- Weight loss context: Clinical trials showed Mounjaro achieved approximately 15–21% average weight loss, whilst Wegovy (semaglutide) achieved approximately 15% on average — individual results vary significantly
- Dose differences: Mounjaro doses (2.5–15 mg weekly) are not directly interchangeable with Wegovy doses (0.25–2.4 mg weekly) — any switch requires prescriber guidance on appropriate dosing
Clinically BackedWeight Loss
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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.
Private Prescribing And Switching In The UK
If you're currently receiving Mounjaro through a private online prescriber and wish to switch to Wegovy, you can discuss this directly with your prescriber during your next consultation. Private prescribers can assess whether Wegovy is a suitable alternative for you and manage the transition. This route typically offers more flexibility — you can switch medications more readily than through NHS services, though each medication still requires a separate clinical assessment and prescription.
When switching privately, your prescriber may recommend: stopping Mounjaro and starting Wegovy at its standard starting dose (0.25 mg), or in some cases starting at a slightly higher dose depending on your previous response. Use our free GLP-1 weight loss calculator to explore projected outcomes at different Wegovy doses and compare with your previous Mounjaro results. Always inform your new prescriber about your dose history on Mounjaro and any side effects you experienced, as this helps them tailor your Wegovy regimen.
- 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?
Switching is possible but must be managed by your NHS specialist weight management service. Your prescriber will assess whether Wegovy is clinically appropriate for you based on your response to Mounjaro, any side effects, and local NHS guidance. Direct requests for a switch don't assures approval — clinical need must be demonstrated.
Are Mounjaro and Wegovy doses the same?
No. Mounjaro (tirzepatide) ranges from 2.5–15 mg weekly, whilst Wegovy (semaglutide) ranges from 0.25–2.4 mg weekly. These are not equivalent doses despite similar weight loss outcomes in clinical trials. Any switch requires your prescriber to determine an appropriate starting dose for Wegovy based on your individual circumstances.
Why might someone switch from Mounjaro to Wegovy?
Reasons include: persistent side effects on Mounjaro (nausea, diarrhoea, constipation), inadequate weight loss response, cost considerations, or individual prescriber preference. Some people may also find the single GLP-1 mechanism of Wegovy works better for them than Mounjaro's dual GIP/GLP-1 action, though individual responses vary.
What's the difference between tirzepatide and semaglutide for weight loss?
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist, whilst semaglutide (Wegovy) is a GLP-1-only agonist. Both regulate appetite and blood sugar, but the dual mechanism of tirzepatide may offer additional benefits — clinical trials showed slightly higher average weight loss with Mounjaro, though results vary between individuals.
Can I switch back to Mounjaro if Wegovy doesn't work?
Yes, switching medications in either direction is possible if your prescriber believes it's clinically appropriate. Discuss any concerns about weight loss progress or side effects with your GP or specialist. Using our BMI calculator and progress tracker can help you and your prescriber assess how well each medication is working for you over time.
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.