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?
Yes, switching from Mounjaro (tirzepatide) to Wegovy (semaglutide) is possible in the UK, but it requires clinical assessment and prescriber approval. Both are prescription-only GLP-1-based weight loss injections licensed for adults with obesity or overweight with health complications. The decision to switch depends on your individual response, side effects, and NHS or private prescribing pathway. This guide covers eligibility, how the switch works, and what to expect in the UK healthcare context.
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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 Eligibility and the Switching Process
Switching between weight loss injections in the UK is possible but requires prescriber guidance. Both Mounjaro and Wegovy are approved by MHRA and recommended by NICE for weight management in eligible adults. NHS eligibility criteria are the same for both: BMI of 30 kg/m² or above, or BMI of 27–29.9 kg/m² with a weight-related health condition such as type 2 diabetes, high blood pressure, or cardiovascular disease.
If you're on Mounjaro through an NHS tier 3 specialist weight management service, switching to Wegovy would require discussion with your prescriber. They will assess whether a switch is clinically appropriate based on your response to Mounjaro, any side effects you've experienced, and your personal health circumstances. There is no automatic conversion between doses — while approximate equivalences exist (5mg Mounjaro ≈ 0.5mg Wegovy, 10mg ≈ 1.7mg, 15mg ≈ 2.4mg), any dose adjustment must be managed by your prescriber.
- Discuss side effects or tolerance issues with your prescriber before requesting a switch
- NHS services vary by region — availability of Wegovy may differ from Mounjaro depending on your local ICB (Integrated Care Board)
- Tier 3 services prioritise continuing existing treatment unless there is a clinical reason to change
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Routes
If you're currently on Mounjaro through a private prescriber, switching to Wegovy is more straightforward — you can request a switch during your next online consultation, and a new prescription will be issued if clinically appropriate. Private prescribing offers more flexibility than NHS services, though you remain responsible for ensuring any switch is medically safe.
Regardless of your current pathway, your prescriber will consider: your weight loss progress so far, side effects you've experienced (nausea, constipation, appetite suppression), your BMI improvement, and any other health changes. Both medications work through GLP-1 receptor agonism but with different mechanisms — Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is GLP-1 only. Use our free GLP-1 weight loss calculator to compare projected weight loss at different doses, and our BMI calculator to monitor your progress during treatment.
- 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.
Why would someone switch from Mounjaro to Wegovy?
Reasons include inadequate weight loss response, troublesome side effects (persistent nausea or constipation), cost considerations, or personal preference. Some people tolerate one medication better than the other despite similar mechanisms. Your prescriber can advise whether a switch is appropriate.
What happens to my dose when I switch from Mounjaro to Wegovy?
There is no direct automatic conversion. Your prescriber will typically restart you on a lower Wegovy dose (0.25mg) and titrate upward over weeks, similar to initial treatment. Approximate equivalences exist but individual response varies significantly.
Can I switch between Mounjaro and Wegovy on the NHS?
Switching is possible but depends on your local NHS tier 3 service and clinical assessment. NHS services generally prioritise continuing treatment rather than switching. Discuss your concerns about efficacy or side effects with your prescriber before requesting a change.
Is there a waiting time to switch medications through the NHS?
Yes, NHS weight management services often have waiting lists. If switching is approved, you may face additional waiting time before Wegovy becomes available. Private prescribing offers faster access to alternative medications.
How do Mounjaro and Wegovy compare in terms of weight loss?
Clinical trials show similar average weight loss (around 15–21% with Mounjaro, 15% with Wegovy at maximum doses), though Mounjaro's dual mechanism may offer slightly different efficacy. Individual results vary significantly, and side effect tolerance often determines which is better for you personally.
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.