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 Monjaro Work vs Wegovy?
Monjaro and Wegovy are both prescription weight loss injections available in the UK, but they work through different mechanisms. Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and blood sugar. Wegovy (semaglutide) is a GLP-1 receptor agonist only, working through a single pathway. Understanding how each works can help you and your GP discuss which might suit your individual health needs. Both are prescription-only and require clinical assessment before prescribing.
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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 Each Medication Works — Mechanism and Trial Data
Monjaro (tirzepatide) activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors. This dual action may help regulate appetite more effectively by triggering two natural satiety signals in the brain. In the SURMOUNT-1 clinical trial, participants on 15 mg Mounjaro lost an average of 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and physical activity.
Wegovy (semaglutide) activates GLP-1 receptors only. It reduces appetite and helps you feel fuller after smaller meals. In the STEP-1 clinical trial, participants on 2.4 mg Wegovy lost approximately 14.9% of their body weight over 68 weeks with the same lifestyle support.
- Mounjaro: dual mechanism, once-weekly injection, doses up to 15 mg
- Wegovy: single GLP-1 mechanism, once-weekly injection, maximum 2.4 mg
- Individual weight loss results vary significantly between people
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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.
Which Medication Is Right for You?
Choosing between Monjaro and Wegovy depends on your individual health profile, medical history, and what your prescriber recommends. Both are licensed for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, hypertension, or sleep apnoea. Neither is suitable during pregnancy or whilst breastfeeding. Your GP or a licensed prescriber will assess whether either medication is appropriate for you based on your personal health circumstances, current medications, and weight loss goals. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current position on the NHS health scale.
- 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 how Monjaro and Wegovy work?
Monjaro is a dual GIP+GLP-1 receptor agonist, meaning it activates two hormone pathways to regulate appetite. Wegovy is a GLP-1 agonist only, activating one pathway. The dual mechanism in Monjaro may provide additional appetite control, which is why it showed slightly higher average weight loss in clinical trials. However, individual results vary significantly.
Which weight loss injection works better — Monjaro or Wegovy?
Clinical trial data shows Monjaro achieved average weight loss of 20.9% at maximum dose, compared to Wegovy at 14.9%. However, these are averages — individual results vary considerably. "Better" depends on your personal health factors, side effect tolerance, and prescriber recommendation. Your GP or prescriber will advise which suits you best.
How quickly does Monjaro start working?
Most people notice changes within the first few weeks of starting Monjaro, though the effect builds gradually as your dose increases. Clinical trials lasted 72 weeks to measure full results. Weight loss typically continues over several months as you remain on the medication and maintain a reduced-calorie diet and physical activity.
Can I switch from Wegovy to Monjaro?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart — your prescriber will advise on the appropriate starting dose if you switch. Approximate equivalences exist (Wegovy 2.4 mg is roughly equivalent to Monjaro 10 mg), but any change requires clinical guidance and monitoring.
Do Monjaro and Wegovy have the same side effects?
Both cause similar common side effects including nausea, diarrhoea, constipation, and stomach pain, especially when starting or increasing doses. Side effect severity and duration vary between individuals. Some people tolerate one medication better than the other. Your prescriber will discuss side effect management and can adjust your dose if needed.
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.