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 Wegovy To Mounjaro In The UK
If you're currently taking Wegovy (semaglutide) and considering switching to Mounjaro (tirzepatide), it's important to understand how this transition works in the UK prescribing context. Both are prescription-only weight loss injections, but they work differently and require careful medical management. Your GP or prescriber will need to assess whether switching is appropriate for your individual health situation, and any change should be made under professional guidance rather than independently. This guide explains the practical steps, dose considerations, and NHS access routes for switching between these two 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.
How Switching From Wegovy To Mounjaro Works In The UK
Switching from Wegovy to Mounjaro requires stopping one medication and starting another under medical supervision—it is not recommended to take both simultaneously. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whereas Wegovy (semaglutide) is a GLP-1 agonist only, meaning Mounjaro has two hormone pathways rather than one.
There is no official NHS guidance on exact dose equivalence between the two medications. However, clinical practice suggests approximate conversions: Wegovy 0.5 mg is roughly equivalent to Mounjaro 5 mg, Wegovy 1.7 mg to Mounjaro 10 mg, and Wegovy 2.4 mg to Mounjaro 15 mg. Your prescriber will assess your current response to Wegovy and may start you on a lower Mounjaro dose, then titrate upwards over weeks according to tolerance and effect.
The transition typically involves a gap of a few days between your last Wegovy dose and starting Mounjaro. Your prescriber will advise the exact timing based on your individual situation. After starting Mounjaro, you will follow the standard dose escalation schedule: 2.5 mg weekly for 4 weeks, then stepping up every 4 weeks as tolerated up to your target dose.
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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.
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NHS Access And Private Routes For Switching
NHS route: If you are currently on Wegovy through an NHS tier 3 specialist weight management service, you may request a switch to Mounjaro through the same service. However, NHS availability of Mounjaro varies by region and ICB (Integrated Care Board). Some areas have approved tirzepatide for use; others may have limited or no access. You would need to discuss this with your specialist team—they will assess whether the switch is clinically justified for your progress and health goals.
Private route: Many people switch via a private online prescriber following a clinical assessment. This route is often faster than waiting for NHS tier 3 review. You will need a new prescription and clinical consultation specific to Mounjaro. Costs typically range from £150–£300+ per month depending on dose. Use our free GLP-1 weight loss calculator to compare projected outcomes at different doses of both medications.
Keep a record of your weight, any side effects, and your current Wegovy dose before your assessment—this helps your prescriber make an informed decision. Use our free progress tracker to log your Wegovy results before switching, so you and your prescriber can evaluate whether a change is likely to benefit you. Both medications carry similar side effects (nausea, diarrhoea, constipation), so switching alone may not resolve these if you experienced them on Wegovy. However, some people find the dual mechanism of Mounjaro more effective for appetite control. Individual response varies significantly.
- 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 Wegovy to Mounjaro?
Some people switch because they want to try the dual GIP+GLP-1 mechanism of Mounjaro for potentially stronger appetite suppression, or because they experienced limited weight loss or difficult side effects on Wegovy. Others switch due to availability—Mounjaro may be more accessible in their NHS region or via private prescribers. A prescriber will assess whether switching is clinically appropriate based on your individual progress.
Can I switch immediately without a gap between medications?
No. You should not take Wegovy and Mounjaro together. There is typically a gap of a few days between your last Wegovy dose and starting Mounjaro. Your prescriber will advise the exact timing. Taking both simultaneously risks cumulative hormonal effects and increased side effects. Always follow your prescriber's instructions.
Will I start Mounjaro at the same dose I was on with Wegovy?
No. Even though approximate dose equivalences exist (e.g., Wegovy 2.4 mg ≈ Mounjaro 15 mg), most prescribers start Mounjaro at the standard starting dose of 2.5 mg and titrate upwards over weeks based on tolerance. This is more conservative but safer. Your prescriber may adjust this based on your Wegovy response and current health status.
Is Mounjaro available on the NHS for switching from Wegovy?
NICE has approved tirzepatide (Mounjaro) for use within NHS specialist weight management services. However, regional availability varies significantly. Some ICBs have approved access; others have limited stock or no access. You would need to discuss switching with your NHS tier 3 team—they will advise what is available in your area and whether switching is supported.
What if I experience side effects when switching to Mounjaro?
Initial nausea, diarrhoea, or constipation are common when starting any GLP-1 or GIP/GLP-1 medication and typically improve within weeks as your body adjusts. If side effects are severe or persistent, inform your prescriber immediately. They may slow your dose escalation, adjust your diet, or in rare cases, recommend switching back to Wegovy or another medication.
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