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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How To Switch From Wegovy To Mounjaro
Switching from Wegovy (semaglutide) to Mounjaro (tirzepatide) is a decision that should only be made with your prescriber's guidance. Both are prescription-only weight management injections licensed in the UK, but they work differently — Wegovy is a single GLP-1 receptor agonist, while Mounjaro is a dual GIP and GLP-1 receptor agonist. If you're considering a switch, this guide explains the clinical process, timing considerations, and what happens during the transition.
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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.
Understanding The Difference Between Wegovy And Mounjaro
Wegovy (semaglutide) activates GLP-1 receptors only, helping reduce appetite and slow gastric emptying. Mounjaro (tirzepatide) activates both GIP and GLP-1 receptors, potentially offering a dual mechanism of action. In clinical trials, Mounjaro demonstrated slightly greater average weight loss (around 20.9% at the maximum 15 mg dose) compared to Wegovy (approximately 14.9% at 2.4 mg), though individual results vary significantly.
The Mounjaro KwikPen delivers doses starting at 2.5 mg weekly, with escalation to a maximum of 15 mg. Wegovy starts at 0.25 mg and increases to a maximum of 2.4 mg. Because the doses and mechanisms differ, switching is not simply a matter of moving to an equivalent strength — your prescriber will assess your individual response and 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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
The Clinical Switching Process
Switching between weight loss medications requires prescriber approval and clinical reassessment. Your GP or specialist will review your weight loss results on Wegovy, any side effects you've experienced, your current health status, and your weight loss goals. They will decide whether Mounjaro is appropriate for you and determine your starting dose.
Most commonly, if switching, patients restart on the lowest Mounjaro dose (2.5 mg) regardless of their previous Wegovy dose. However, this depends entirely on your individual circumstances. The transition may involve a gap between your last Wegovy injection and your first Mounjaro injection — your prescriber will advise on timing. Throughout the switch, use our free GLP-1 weight loss calculator to track projections and monitor your progress with our free progress tracker.
- 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.
Do I need a new prescription to switch from Wegovy to Mounjaro?
Yes. Wegovy and Mounjaro are different medications requiring separate prescriptions. Your prescriber must assess whether Mounjaro is clinically appropriate for you, even if Wegovy worked well. This reassessment ensures the switch is safe given your health profile and current weight management progress.
What dose of Mounjaro should I start if I'm switching from Wegovy?
There is no official conversion chart between Wegovy and Mounjaro doses. Most prescribers restart patients on the lowest Mounjaro dose (2.5 mg weekly) when switching, regardless of their previous Wegovy dose. Your individual circumstances may vary — your prescriber will decide.
Why might someone switch from Wegovy to Mounjaro?
Common reasons include: plateaued weight loss on Wegovy, seeking potentially greater weight loss (as suggested by clinical trial data), intolerable side effects on Wegovy, or personal preference. Only your prescriber can assess whether switching is clinically appropriate for you.
Is there a waiting period between my last Wegovy injection and first Mounjaro injection?
Possibly, but this varies. Some prescribers may recommend starting Mounjaro immediately after your last Wegovy dose, whilst others may suggest a gap. Your prescriber will advise based on your individual health status and the timing of your previous injection.
Can I get Mounjaro on the NHS if I'm already on Wegovy?
Mounjaro is available through NHS tier 3 specialist weight management services in some regions, though availability and waiting times vary significantly. Access often depends on your ICB region. Your GP can advise on NHS eligibility and whether you can access Mounjaro through specialist services or private prescribing.
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.