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
What Does Mounjaro Do To Your Body?
Mounjaro (tirzepatide) works differently from other weight loss injections because it activates two natural hormone pathways in your body — GIP and GLP-1 — rather than just one. This dual action affects appetite, blood sugar, and how your stomach empties food, making it distinct from Wegovy, which is a GLP-1 receptor agonist only. Understanding how Mounjaro works as a weekly weight loss injection can help you and your GP decide if it's the right option 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.
How Mounjaro vs Wegovy Affect Your Body Differently
Mounjaro (tirzepatide) activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors. This dual mechanism may produce stronger appetite suppression and help regulate how your body uses fat. Clinical trials (SURMOUNT) showed average weight loss of 15–21% body weight at maximum dose over 72 weeks.
Wegovy (semaglutide) activates GLP-1 receptors only. STEP trial data showed average weight loss of approximately 14.9% body weight at 68 weeks on the maximum 2.4 mg dose.
- Both slow gastric emptying (food leaving your stomach)
- Both signal fullness to your brain
- Mounjaro's dual action may produce greater weight loss on average
- Individual results vary significantly regardless of mechanism
Try our free GLP-1 weight loss calculator to project potential weight loss at different doses with either medication.
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.
Which One Affects Your Body Better for You?
Neither medication is inherently "better" — suitability depends on your individual health, medical history, and how your body responds. Dual action doesn't automatically mean better results for everyone. Some people respond excellently to Wegovy; others benefit more from Mounjaro. A prescriber will assess factors including existing health conditions, other medications, and contraindications before recommending either option.
Both medications require a UK prescription and clinical assessment. Both are typically prescribed as part of a weight management plan including reduced-calorie diet and increased physical activity. Side effects vary between individuals. Your GP or online prescriber can discuss which mechanism aligns better with your specific health profile and goals.
- 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.
How is Mounjaro's GIP+GLP-1 mechanism different from Wegovy's GLP-1 only?
Mounjaro activates two hormone pathways (GIP and GLP-1) while Wegovy activates one (GLP-1 only). The dual mechanism may produce stronger appetite suppression and fat regulation in Mounjaro, but individual responses vary. Both are effective weekly weight loss injections — the best choice depends on your personal health profile.
Will Mounjaro work better than Wegovy for weight loss?
Clinical trial averages favour Mounjaro slightly (15–21% vs clinically studied weight loss), but individual results vary considerably. Some people lose more with Wegovy; others with Mounjaro. A prescriber will recommend based on your health history, not trial averages. Your body's response is what matters most.
Do Mounjaro and Wegovy cause the same side effects?
Both cause similar common side effects (nausea, stomach pain, diarrhoea, constipation) because they share the GLP-1 mechanism. Side effect severity and duration vary between individuals. Discuss tolerability with your prescriber — some people manage one better than the other.
How quickly does Mounjaro start working in your body?
Effects typically begin within the first few weeks, though appetite suppression and fullness signals strengthen gradually as doses increase. Most people notice changes between weeks 4–12. Use our free progress tracker to log changes in appetite, energy, and weight over time.
Can my GP switch me between Mounjaro and Wegovy if one doesn't suit me?
Yes. If one medication isn't working well or causing side effects, your prescriber can switch you to the other. There's no official conversion chart — switching requires a new prescription and clinical reassessment. Approximate dose equivalence exists (5mg Mounjaro ≈ 0.5mg Wegovy) but any switch must be managed by your prescriber.
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.