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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Semaglutide Benefits vs Mounjaro: What's the Difference?
Both semaglutide (Wegovy) and Mounjaro are prescription-only weight loss injections licensed in the UK, but they work differently. Semaglutide is a GLP-1 receptor agonist, while Mounjaro is a dual GIP and GLP-1 agonist — meaning it activates two hormone pathways instead of one. Clinical trials show both medications can support weight loss when combined with diet and physical activity, though individual results vary significantly. This guide compares the two medicines factually to help you understand the differences, though your GP or prescriber will assess which (if either) is suitable for your personal health needs.
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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 Semaglutide and Mounjaro Work Differently
Semaglutide (Wegovy) activates the GLP-1 hormone pathway — this signals fullness to the brain, reduces appetite, and slows how quickly food leaves the stomach. It's a single-mechanism medication with decades of GLP-1 research behind it.
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways simultaneously — the dual mechanism may provide additional support for appetite regulation and metabolic function. This dual approach is newer in the weight loss space.
- Semaglutide: once-weekly injection, starts at 0.25mg, maximum dose 2.4mg
- Mounjaro: once-weekly injection, starts at 2.5mg, maximum dose 15mg
- Both require dose escalation over weeks, not immediately at full strength
- Both slow gastric emptying (typically reducing over time with continued use)
Our GLP-1 weight loss calculator can help you understand potential weight loss projections with either medication at different dose levels.
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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 Results and Individual Suitability
The STEP-1 clinical trial (semaglutide) showed average weight loss of approximately 14.9% of body weight at 68 weeks on the 2.4mg dose. The SURMOUNT-1 trial (Mounjaro) showed average weight loss ranging from approximately 15% on 5mg to 20.9% on 15mg — though these results were achieved alongside reduced-calorie diet and physical activity.
Both medications may cause similar side effects: nausea, diarrhoea, vomiting, and constipation are very common in the first weeks. Neither medication is "better" — suitability depends on your individual health profile, other medications, and how your body responds.
Your GP or a licensed prescriber will assess whether semaglutide, Mounjaro, or another approach is appropriate for you based on your BMI, weight-related health conditions, and medical history. Use our free BMI calculator above to check your current BMI and understand your eligibility context.
- 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 is the main difference between semaglutide and Mounjaro?
Semaglutide (Wegovy) is a single GLP-1 receptor agonist. Mounjaro is a dual GIP and GLP-1 agonist, activating two hormone pathways. This dual mechanism may provide additional metabolic support, though both medications support weight loss when used alongside diet and exercise.
Which medication causes less nausea — semaglutide or Mounjaro?
Both medications have similar side effect profiles, with nausea being very common in the first weeks. Individual tolerance varies significantly — some people experience mild nausea, others severe. Your prescriber can advise on managing side effects during dose escalation.
Can I switch from semaglutide to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion formula — your prescriber will assess the appropriate starting dose if switching. Never change medications without medical guidance.
How much weight will I lose on semaglutide or Mounjaro?
Clinical trials show average weight loss of 14–21% of body weight, but individual results vary widely. Some people lose significantly more, others less. Results depend on genetics, lifestyle changes, dose, and how your body responds. Your prescriber will discuss realistic expectations.
How do I access semaglutide (Wegovy) or Mounjaro in the UK?
Both medications are prescription-only and require a clinical assessment. NHS access is through specialist tier 3 weight management services (availability varies by region). Private access is available through registered online prescribers and pharmacies following an online assessment.
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.