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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Changing From Wegovy To Mounjaro in the UK
Some people taking Wegovy (semaglutide) consider switching to Mounjaro (tirzepatide) due to side effects, weight loss plateaus, or personal preference. Both are prescription-only GLP-1 and dual GIP/GLP-1 receptor agonist medicines licensed for weight management in UK adults. Switching between them requires prescriber guidance and careful dose management. This guide covers UK prescribing context, approximate dose equivalence, and what to expect when changing medications.
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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 Prescribing and Dose Considerations
In the UK, both Wegovy and Mounjaro are prescription-only medicines available through NHS specialist tier 3 weight management services or private online prescribing. There is no official MHRA-approved conversion chart between semaglutide and tirzepatide — any switch must be managed by your prescriber based on your individual health, current dose, and response to treatment.
- Approximate equivalence: Wegovy 0.5 mg may align roughly with Mounjaro 5 mg; Wegovy 1.7 mg with Mounjaro 10 mg; Wegovy 2.4 mg with Mounjaro 15 mg — but these are clinical practice guides only, not prescribing standards
- Your prescriber may recommend stopping Wegovy and starting Mounjaro at a lower dose (e.g. 2.5 mg), then titrating upward over weeks to find your optimal dose
- Clinical trial data (SURMOUNT-1) shows tirzepatide achieved 15–21% average body weight loss; Wegovy (STEP trials) showed approximately 15% — individual results vary significantly
- The dual GIP/GLP-1 mechanism of Mounjaro differs from Wegovy's single GLP-1 pathway, so weight loss outcomes and side effects may differ between individuals
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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
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NHS vs Private Routes and Practical Steps
NHS route: If you're accessing Wegovy through an NHS tier 3 service, discuss any desire to switch directly with your specialist weight management team. They will assess whether Mounjaro is suitable and manage the transition. Waiting times for NHS specialist services vary significantly by region and can be lengthy.
Private route: If you're on Wegovy privately, you'll need to arrange a new private online consultation with a licensed prescriber to discuss switching to Mounjaro (tirzepatide). Always inform your prescriber of your current Wegovy dose and any side effects or weight loss response. Use our free GLP-1 weight loss calculator to track your progress and share this data with your prescriber. Never stop Wegovy abruptly — your prescriber will advise on timing of the switch.
- 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 directly from Wegovy to Mounjaro without stopping?
Not without prescriber guidance. Most switches involve stopping Wegovy for a set period, then starting Mounjaro at a lower dose (e.g. 2.5 mg) before titrating upward. Some prescribers may overlap medications briefly, but this is individualised. Always follow your prescriber's instructions — never switch without clinical guidance.
Will Mounjaro work better than Wegovy?
Not necessarily. Clinical trials show Mounjaro achieved slightly higher average weight loss (15–21% vs ~15% for Wegovy), but individual results vary significantly. Some people respond better to one medication than the other. Your prescriber will monitor your response and adjust accordingly.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP/GLP-1 receptor agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro has a once-weekly KwikPen dose range of 2.5–15 mg; Wegovy has a 0.25–2.4 mg once-weekly range. The dual mechanism may produce different appetite suppression and side effect profiles between individuals.
How long does the switch take?
Switching typically takes 2–6 weeks. You'll likely stop Wegovy, allow a brief washout period (3–7 days), then start Mounjaro at 2.5 mg. Doses then increase weekly or every 4 weeks depending on your prescriber's protocol. The entire titration to your maintenance dose may take 16–20 weeks.
Can I get Mounjaro through the NHS?
Yes — NICE has approved tirzepatide (Mounjaro) for use within NHS specialist tier 3 weight management services in the UK. Access is through regional NHS services, not standard GP prescriptions. Availability and waiting times vary significantly by region and local integrated care board (ICB) funding.
Ready to take control of your weight loss journey?
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