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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Going From Mounjaro to Wegovy in the UK
If you're currently taking Mounjaro and considering Wegovy, or your GP has suggested a switch, it's important to understand how these two GLP-1 medications compare and what the process involves in the UK. Both tirzepatide (Mounjaro) and semaglutide (Wegovy) are prescription-only weight management injections licensed by the MHRA, but they work differently and have distinct dosing schedules. This guide explains the NHS and UK prescribing context for making this switch safely.
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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.
Understanding the Difference: Mounjaro vs Wegovy in NHS Care
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. This difference in mechanism means they work on slightly different pathways in the body. Mounjaro typically reaches a maximum dose of 15 mg once weekly, whereas Wegovy's maximum is 2.4 mg once weekly. In the SURMOUNT clinical trials, Mounjaro demonstrated average weight loss of approximately 15–21% of body weight, while the STEP trials showed Wegovy achieved around 14.9% average weight loss—though individual results vary considerably.
- Mounjaro uses the KwikPen delivery device
- Wegovy uses an auto-injection pen
- Mounjaro requires longer dose titration (up to 20 weeks to maximum)
- Wegovy reaches maximum dose more quickly (around 16 weeks)
- Both require a prescription and clinical assessment
If your GP or specialist recommends switching, they will assess whether Wegovy is appropriate based on your individual health, current weight loss progress, and any side effects you've experienced on Mounjaro.
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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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Switching Via NHS Services vs Private Route in the UK
In the UK, if you've been prescribed Mounjaro through an NHS tier 3 specialist weight management service, any switch to Wegovy would typically be managed by that same service. Your specialist will review your progress, discuss your reasons for wanting to switch, and determine whether Wegovy is a suitable next step—or whether continuing Mounjaro at a different dose might be better. There is no official established dose conversion chart between Mounjaro and Wegovy, so switching requires individual clinical judgment. If you're accessing Mounjaro privately, you can request a switch through your prescriber, who will assess your suitability for Wegovy following a fresh clinical consultation. Use our free GLP-1 weight loss calculator to understand projected outcomes with either 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.
Can I switch directly from Mounjaro to Wegovy without stopping in between?
No. Your prescriber will advise a washout period (typically 1–2 weeks) between stopping Mounjaro and starting Wegovy to allow your body to clear the previous medication. Starting Wegovy involves dose titration from 0.25 mg, so you'll restart at the lowest dose regardless of your Mounjaro dose.
What dose of Wegovy would I start on if I was taking 15 mg Mounjaro?
There is no official conversion chart. Approximate guidance suggests 15 mg Mounjaro is roughly equivalent to 2.4 mg Wegovy, but you'll still start Wegovy at 0.25 mg and titrate upwards. Your prescriber will make the final decision based on your individual response and health.
Will I lose more weight on Wegovy than Mounjaro?
Clinical trial data shows similar average weight loss between the two (around 15–21% for Mounjaro, 14.9% for Wegovy), but individual results vary greatly. Some people respond better to one medication than the other. Weight loss depends on your body, adherence, diet, and activity level—not the medication alone.
Can the NHS switch me from Mounjaro to Wegovy if I want to?
If you're being a UK pharmacy through NHS tier 3 specialist services, your team will consider a switch if clinically justified (e.g., side effects, inadequate response, or clinical guidance changes). However, switching purely by choice may not be available on the NHS in all regions due to cost and service restrictions.
What happens to my weight if I stop Mounjaro and wait before starting Wegovy?
Weight typically begins to increase again after stopping any GLP-1 medication, though the rate varies. This is why prescribers try to minimise the gap between stopping one and starting the next. Your specialist will discuss this risk and aim to start Wegovy quickly to maintain momentum.
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.