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 Get The Best Out Of Mounjaro vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and produce different results in clinical trials. Understanding how each medication functions, what the evidence shows, and which might suit your individual circumstances can help you and your prescriber make an informed choice. This guide compares both options factually so you can have an informed conversation with your GP or a licensed prescriber about which approach might work best for you.
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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.
Mechanism: GIP+GLP-1 vs GLP-1 Only
Mounjaro (tirzepatide) works via a dual mechanism — it activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptor pathways. This dual action may help regulate appetite more comprehensively and support fat utilisation. The KwikPen starts at 2.5 mg and escalates up to a maximum of 15 mg once weekly.
Wegovy (semaglutide) is a single GLP-1 agonist — it activates the GLP-1 pathway only. This has been well-established in treating type 2 diabetes and is now approved for weight management. Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg once weekly.
The key difference: Mounjaro's dual mechanism may produce stronger appetite suppression, while Wegovy's single mechanism has a longer track record of safety data. Both are delivered by once-weekly injection, making adherence straightforward. Use our GLP-1 weight loss calculator to compare projected results at different doses.
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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.
Trial Data: SURMOUNT vs STEP — What The Evidence Shows
Clinical trials provide the strongest evidence for real-world outcomes. SURMOUNT-1 (Mounjaro trial) showed average weight loss of approximately 20.9% of body weight at the 15 mg dose over 72 weeks. STEP-1 (Wegovy trial) showed average weight loss of approximately 14.9% at the 2.4 mg dose over 68 weeks. However, individual results vary significantly — some participants lost substantially more, others less.
Both trials included participants following a reduced-calorie diet and increased physical activity. Neither medication alone assures weight loss. Your prescriber will discuss realistic expectations based on your personal health profile, medical history, and current medications. Both require clinical assessment before prescription in the UK.
- 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.
Which is better, Mounjaro or Wegovy?
Neither is universally "better" — it depends on your individual health, medical history, and how you respond. Mounjaro showed slightly higher average weight loss in trials, but Wegovy has longer safety data. Your prescriber will assess which suits you based on contraindications, other medications, and clinical need. Both require prescription and ongoing clinical support.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible, but it must be managed by your prescriber. Approximate dose equivalences exist (e.g. Wegovy 0.5 mg ≈ Mounjaro 5 mg), but there's no official conversion chart. Your prescriber will assess the timing and whether a washout period is needed. Never switch without clinical guidance.
How long does it take to see results with Mounjaro vs Wegovy?
Both typically show results within 4–8 weeks at therapeutic doses, though some people notice appetite changes sooner. Significant weight loss usually becomes apparent by 12–16 weeks. Remember, both work best alongside a reduced-calorie diet and increased physical activity — medication alone doesn't studied results.
Are side effects different between Mounjaro and Wegovy?
Both share similar common side effects: nausea, diarrhoea, constipation, and stomach pain. Mounjaro side effects profile is extensive but similar in type to Wegovy. Severity and duration vary between individuals. Always discuss potential side effects with your prescriber before starting either medication.
Do I need an NHS referral to get Mounjaro or Wegovy in the UK?
For NHS access, you'll need referral to a tier 3 specialist weight management service. However, both are also available privately through GPhC-registered online prescribers following an online clinical assessment. Availability and waiting times vary significantly by region and route — discuss options with your GP.
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.