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
Free tracker included with every download this week
Can You Switch From Mounjaro to Wegovy?
If you're currently taking Mounjaro (tirzepatide) and wondering about switching to Wegovy (semaglutide), the answer is yes—but only under prescriber guidance. Both are prescription-only GLP-1 and GLP-1 receptor agonist weight management injections licensed in the UK, but they work differently and require careful medical supervision to switch safely. Your prescriber will assess whether a change is appropriate for your individual health and circumstances.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
How Mounjaro and Wegovy Differ
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—meaning it activates two hormone pathways. This dual mechanism is why some people find it more effective for appetite suppression. Wegovy (semaglutide) is a single GLP-1 receptor agonist, activating only one pathway. In clinical trials, Mounjaro showed average weight loss of approximately 15–21% of body weight, whilst Wegovy demonstrated around 14.9% average weight loss in the STEP-1 trial—though individual results vary significantly. Dosing also differs: Mounjaro ranges from 2.5 mg to 15 mg weekly, whilst Wegovy ranges from 0.25 mg to 2.4 mg weekly. Because the mechanisms and doses differ, you cannot simply switch at the same dose—any transition must be managed by your prescriber, who will consider your current response, side effects, and health profile.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Why You Might Switch and How to Do It
Common reasons for switching include inadequate weight loss response, bothersome side effects, or cost considerations. If you've been on Mounjaro and wish to try Wegovy, contact your prescriber or GP to discuss your reasons and health goals. If approved, your prescriber will typically stop your current Mounjaro KwikPen and start Wegovy at a lower dose—usually 0.25 mg or 0.5 mg—rather than jumping to a higher dose immediately. This allows your body to adjust and lets your prescriber monitor your response. You'll need a new prescription, and there should be minimal gap between stopping one and starting the other to avoid loss of appetite-suppressing effect. Always follow your prescriber's specific instructions on timing and doses. Use our GLP-1 weight loss calculator to help track your progress during any transition period, and monitor how you feel week by week.
- 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.
What's the approximate dose conversion from Mounjaro to Wegovy?
Approximate equivalences exist (e.g., 5 mg Mounjaro ≈ 0.5 mg Wegovy; 15 mg Mounjaro ≈ 2.4 mg Wegovy), but there is no official established conversion chart. Your prescriber will determine your starting Wegovy dose based on your current Mounjaro dose, response, and individual health. Never attempt to self-convert doses.
Will I lose weight faster or slower on Wegovy than Mounjaro?
Individual results vary significantly. Clinical data shows slightly lower average weight loss with Wegovy versus Mounjaro, but this doesn't predict your personal outcome. Some people lose weight equally well on Wegovy; others may lose less. Your prescriber can help set realistic expectations based on your health profile.
Can I switch back to Mounjaro if Wegovy doesn't work for me?
Yes, you can switch back if Wegovy is not effective for you. This requires another discussion with your prescriber, who will assess your response and decide whether returning to Mounjaro or trying a different approach is best. Switching between medications is normal in weight management.
Will there be a gap in coverage when switching between the two?
Your prescriber will work to minimise any gap. Ideally, you'll collect your Wegovy prescription shortly before finishing your Mounjaro supply, or start on the same week your Mounjaro runs out. A very short gap is usually acceptable, but avoid long periods without medication if possible.
Do I need a new prescription for Wegovy after being on Mounjaro?
Yes. Each medication requires its own separate prescription. Your prescriber will issue a new Wegovy prescription once they've agreed the switch with you. If accessing through the NHS, this may involve recontacting your tier 3 weight management service. Private prescribers can often issue new prescriptions more quickly during a consultation.
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.