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 Does Mounjaro Work vs Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight management injections licensed in the UK, but they work through different biological mechanisms. Understanding how each one works helps you and your prescriber decide which might be most suitable for your individual situation. This guide explains the science behind both medications, compares the clinical trial evidence, and clarifies how they're accessed through the UK healthcare system. Use our free BMI calculator to check your eligibility range, and our GLP-1 weight loss calculator to explore potential outcomes.
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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.
Mechanism Differences: Dual Action vs Single Action
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways in your body. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are both natural gut hormones released after eating. By activating both receptors simultaneously, Mounjaro amplifies the signals that regulate appetite, promote feeling fuller faster, and support healthy blood sugar control.
Wegovy is a single GLP-1 receptor agonist — it activates only the GLP-1 pathway. This means it works through one mechanism rather than two. Both medications slow gastric emptying (how quickly food moves from your stomach), but the dual action of Mounjaro in clinical trials resulted in slightly higher average weight loss compared to Wegovy alone. In the SURMOUNT-1 trial, Mounjaro at 15 mg achieved approximately 20.9% average body weight loss over 72 weeks, whilst in the STEP-1 trial, Wegovy 2.4 mg achieved approximately 14.9% average body weight loss over 68 weeks. However, individual results vary significantly — some people respond better to one medication than the other, and prescriber assessment is essential.
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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.
Clinical Evidence and Dose Titration
Both medications are started at low doses and gradually increased over several weeks — a process called dose titration. Mounjaro starts at 2.5 mg weekly and can be escalated to a maximum of 15 mg. Wegovy starts at 0.25 mg and escalates to a maximum of 2.4 mg. The SURMOUNT and STEP clinical trials both demonstrated weight loss results alongside a reduced-calorie diet and increased physical activity — neither medication works as a standalone treatment. Your prescriber will assess which dose range and medication suits your health profile, BMI category, and any existing health conditions. Access in the UK is through NHS specialist weight management services (with regional variation and waiting lists) or private clinical assessment followed by pharmacy dispensing.
- 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 Mounjaro more effective than Wegovy?
Clinical trials show Mounjaro achieved slightly higher average weight loss (20.9% vs 14.9%), but individual results vary greatly. Both are effective for many people. Your prescriber will assess which is most suitable for your specific health situation based on clinical assessment, not trial averages alone.
How long does it take to see weight loss on Mounjaro?
Most people begin noticing weight changes within the first 4–6 weeks, though some take longer. Dose titration (gradual increases every 4 weeks) means full effects typically emerge over 12–16 weeks. Consistent results require combined use with a reduced-calorie diet and increased physical activity.
Can I switch from Wegovy to Mounjaro?
Yes, switching is possible and sometimes recommended based on response or side effects. Any switch must be managed by your prescriber — there is no simple conversion chart between doses. Never change medications without clinical guidance. Your prescriber will assess the timing and new dose.
Do I need an NHS referral to get Mounjaro in the UK?
For NHS access, you typically need referral to a tier 3 specialist weight management service from your GP. Private access is available through UK-licensed prescribers following an online clinical assessment, without NHS referral. Availability and waiting lists vary significantly by region.
What's the difference between Mounjaro and Ozempic?
Both contain semaglutide but serve different purposes. Ozempic is approved for type 2 diabetes (up to 2 mg dose). Wegovy is the licensed weight management version of semaglutide (up to 2.4 mg). Mounjaro contains tirzepatide (a different drug entirely) and uses the dual GIP+GLP-1 mechanism.
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.