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 Mounjaro To Wegovy
If you're considering switching from Mounjaro to Wegovy, you're looking at two different prescription-only weight loss injections. Both are GLP-1 receptor agonists approved in the UK, but Mounjaro (tirzepatide) has a dual mechanism — it activates both GIP and GLP-1 pathways — whereas Wegovy (semaglutide) activates the GLP-1 pathway alone. Switching between medications requires a new clinical assessment and prescription from your GP or licensed prescriber. This guide explains how the medications differ, approximate dose equivalences, and what you need to know before making a switch.
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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 Mounjaro And Wegovy Compare
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT clinical trials, average weight loss ranged from approximately 15% at the 5mg dose to 21% at the maximum 15mg dose over 72 weeks. Wegovy (semaglutide) is a GLP-1-only agonist; STEP trial data showed average weight loss of approximately 15% body weight at the 2.4mg maintenance dose.
- Mechanism: Mounjaro uses dual GIP+GLP-1; Wegovy uses GLP-1 only
- Injection frequency: Both are once weekly
- Starting dose: Mounjaro begins at 2.5mg; Wegovy at 0.25mg
- Maximum dose: Mounjaro reaches 15mg; Wegovy reaches 2.4mg
- Titration: Mounjaro escalates every 4 weeks; Wegovy escalates every 4 weeks
Individual results vary significantly, and what works best for one person may not be ideal for another. Your GP or prescriber can assess which medication suits your health profile.
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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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Approximate Dose Equivalence And Switching
If you're switching from Mounjaro to Wegovy, your prescriber will consider approximate dose equivalences, though there is no official established conversion chart. Generally, Mounjaro 5mg is roughly comparable to Wegovy 0.5mg, Mounjaro 10mg to Wegovy 1.7mg, and Mounjaro 15mg to Wegovy 2.4mg. However, any switch must be managed by your prescriber — never adjust doses yourself.
Your prescriber may recommend starting at a lower Wegovy dose and titrating upward to assess tolerance, or they may suggest a dose roughly equivalent to your current Mounjaro level. The switch should always involve a new clinical assessment to ensure Wegovy is appropriate for your current health status.
- 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 I switch from Mounjaro to Wegovy?
Reasons vary by individual. Some people experience side effects on Mounjaro that they hope to avoid on Wegovy, others want to try a different mechanism, or their prescriber may recommend a switch based on blood sugar control, cardiovascular health, or personal tolerance. Always discuss your reasons and expectations with your prescriber first.
Can I switch immediately, or do I need to stop Mounjaro first?
Your prescriber will advise on the best switching protocol. Some may recommend finishing your current Mounjaro pen before starting Wegovy; others may suggest an overlap period or a gap. Do not stop or switch medications without guidance from your GP or licensed prescriber.
Will my weight loss results be the same on Wegovy as on Mounjaro?
Clinical trial data suggests both medications deliver significant weight loss, but individual outcomes vary. Some people may lose more on Mounjaro due to its dual mechanism; others may achieve similar results on Wegovy. Your personal response depends on many factors including metabolism, adherence, diet, and activity level.
What if I don't tolerate Wegovy?
If you experience side effects on Wegovy that outweigh the benefits, discuss this with your prescriber. Options may include returning to Mounjaro, trying a different dose, adjusting diet and activity, or exploring other approaches. Do not stop medication abruptly — always discuss changes with your healthcare provider.
Do I need a new prescription to switch medications?
Yes. Switching from Mounjaro to Wegovy requires a new prescription and a clinical assessment. Contact your GP or licensed prescriber to request a clinical review. They will confirm Wegovy is appropriate for you, discuss the switch, and issue a new prescription if suitable.
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