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
Semaglutide Injections vs Mounjaro: What's the Difference?
Both semaglutide injections (Wegovy) and Mounjaro (tirzepatide) are prescription-only weight loss medications licensed in the UK. The key difference is their mechanism: semaglutide is a GLP-1 receptor agonist only, while Mounjaro is a dual GIP and GLP-1 receptor agonist. This means Mounjaro activates two hormone pathways, whereas semaglutide targets one. Both are given as once-weekly injections and require clinical assessment before prescription. Which is right for you depends on individual factors your GP or prescriber will assess. Our GLP-1 weight loss calculator can help you 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.
How Semaglutide and Mounjaro Work Differently
Semaglutide (Wegovy) is a GLP-1 receptor agonist, meaning it activates the GLP-1 hormone pathway. This helps reduce appetite, promotes feeling fuller after smaller meals, and slows gastric emptying. It's manufactured by Novo Nordisk.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways simultaneously. This dual mechanism may offer additional appetite suppression and metabolic support compared to GLP-1 alone. It's manufactured by Eli Lilly.
- Semaglutide maximum dose: 2.4 mg once weekly
- Mounjaro maximum dose: 15 mg once weekly
- Semaglutide typical starting dose: 0.25 mg
- Mounjaro typical starting dose: 2.5 mg
Clinical trials showed both achieve meaningful weight loss when combined with reduced-calorie diet and increased physical activity. Individual results vary significantly.
Clinically BackedWeight Loss
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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 and Weight Loss Outcomes
In the STEP-1 clinical trial, semaglutide (2.4 mg) resulted in an average weight loss of approximately 14.9% of body weight over 68 weeks. In the SURMOUNT-1 trial, Mounjaro at 15 mg achieved approximately 20.9% average weight loss at 72 weeks.
These results are averages from clinical trials conducted alongside lifestyle changes. Your personal outcome may differ based on adherence, diet, exercise, metabolism, and individual response. Use our GLP-1 weight loss calculator above to explore estimated outcomes at different doses. A BMI check with our free BMI calculator can also help contextualise your starting point.
- 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.
Is semaglutide the same as Wegovy?
Semaglutide is the active ingredient; Wegovy is the brand name for semaglutide approved for weight loss. Ozempic is also semaglutide but approved for type 2 diabetes at lower doses. In the UK, Wegovy is the licensed weight management version at doses up to 2.4 mg.
Which causes fewer side effects — semaglutide or Mounjaro?
Both have similar common side effects: nausea, diarrhoea, constipation, and stomach pain, especially early on. Individual tolerance varies. Some people tolerate one better than the other. Your prescriber can discuss which suits you based on your medical history.
Can I switch from Mounjaro to semaglutide or vice versa?
Switching between medications is possible but must be managed by your prescriber. There's no official established conversion chart — switching requires a new clinical assessment and dose adjustment period. Never change medications without guidance.
How long does semaglutide take to work for weight loss?
Weight loss typically begins within the first 4–8 weeks, though some people see changes sooner. The first 4 weeks at the starting dose (0.25 mg) is an adjustment phase. Most meaningful weight loss occurs when you reach maintenance doses (1.7–2.4 mg).
Can my GP prescribe semaglutide for weight loss, or do I need a specialist?
NHS access typically requires referral to a specialist tier 3 weight management service, though regional availability varies widely. Private prescription is available through online clinical assessment by UK-licensed prescribers. Your GP can advise on NHS options in your area. ```
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.