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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Mounjaro Starter Kit vs Wegovy — What's the Difference?
If you're considering prescription weight loss treatment in the UK, you may have heard about both Mounjaro and Wegovy. Both are licensed injectable medications that may help with weight management, but they work differently and start at different doses. Understanding how they compare — including their starter kit doses, mechanisms, and clinical evidence — can help you have an informed conversation with your prescriber. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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.
Mounjaro vs Wegovy: Mechanism, Starter Doses, and Trial Data
Mechanism of action: The key difference is how these medications work. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two separate hormone pathways to regulate appetite and blood sugar. Wegovy (semaglutide) is a GLP-1 receptor agonist only, targeting one pathway. This dual mechanism in Mounjaro may explain why some clinical trial participants achieved slightly higher average weight loss.
- Mounjaro starter dose: 2.5 mg once weekly for the first 4 weeks
- Wegovy starter dose: 0.25 mg once weekly for the first 4 weeks
Clinical trial data shows Mounjaro participants lost an average of approximately 15–21% body weight (SURMOUNT-1 trial), while Wegovy participants lost approximately 14.9% body weight (STEP-1 trial) — though individual results vary significantly and both are used alongside a reduced-calorie diet and increased physical activity.
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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.
How to Choose: Individual Suitability and Your Prescriber's Role
Both Mounjaro and Wegovy are prescription-only medicines that require a clinical assessment before being prescribed. Your GP or a licensed prescriber will evaluate your medical history, current medications, BMI, and weight-related health conditions to determine which — if either — is suitable for you. Neither medication is automatically "better" than the other; suitability depends on your individual circumstances.
Both require dose titration (gradual increases over weeks), and both are most effective when combined with dietary changes and physical activity. Check your eligibility using our free BMI calculator, then discuss your options with a healthcare professional who can assess your personal suitability.
- 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 main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist (two hormone pathways), while Wegovy is a GLP-1 agonist (one pathway). Both are once-weekly injections, but Mounjaro's dual action may contribute to slightly higher average weight loss in clinical trials. Individual results vary.
Which starter dose is lower?
Wegovy's starter dose is lower — 0.25 mg compared to Mounjaro's 2.5 mg. However, the doses are not directly comparable because the two medications have different potencies and mechanisms. Your prescriber will recommend the appropriate starting dose for you.
Can I switch from Mounjaro to Wegovy or vice versa?
Any switch between medications must be managed by your prescriber. There is no official conversion chart between Mounjaro and Wegovy. If a change is needed, your healthcare provider will guide the transition and adjust your dose accordingly.
Which medication will I lose more weight with?
Clinical trials show Mounjaro had a slightly higher average weight loss (~20.9% at maximum dose) compared to Wegovy (~14.9%), but individual results vary significantly. Some people respond better to one medication than the other. Your prescriber will assess which is most suitable for your situation.
Do I need a NHS referral to get either medication?
On the NHS, you typically need a referral to a specialist tier 3 weight management service, though availability and waiting times vary by region. Privately, both medications are available through UK-licensed prescribers following an online clinical assessment — no NHS referral required. Costs range from approximately £129–£320 per month depending on dose and provider.
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.