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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Mounjaro vs Wegovy — Understanding Your Weight Support Options
Mounjaro and Wegovy are both prescription-only weight loss injections available in the UK, but they work differently and show different results in clinical trials. Both are weekly weight loss injections given via a pre-filled pen, but they use different mechanisms to support weight management. This guide compares the two to help you understand the key differences when discussing weight support options with your prescriber. Your GP or a licensed prescriber will assess which option, if any, is suitable for your individual health needs following a clinical assessment.
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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 and Wegovy Work — Key Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) both signal fullness and reduce appetite. This dual action may help regulate appetite more effectively than a single pathway alone.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating just the GLP-1 pathway. It has a well-established track record and shows consistent results in clinical practice.
Clinical trial comparison: In the SURMOUNT-1 trial, Mounjaro at 15 mg showed average weight loss of approximately 20.9% of body weight. In the STEP-1 trial, Wegovy at 2.4 mg showed average weight loss of approximately 14.9% of body weight over similar timeframes. However, individual results vary significantly, and trial averages do not predict personal outcomes. Both medications work alongside reduced-calorie diet and increased physical activity.
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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.
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Dosing, Access, and Which Is Right for You
Mounjaro (the KwikPen pre-filled device) starts at 2.5 mg weekly and increases to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg weekly and increases to a maximum of 2.4 mg weekly. Both require a UK prescription and clinical assessment to confirm eligibility.
Access in the UK is through NHS specialist weight management services (with significant regional variation and waiting lists) or private online prescribing following an online clinical assessment. There is no universally "better" option — suitability depends on your individual health profile, preferences, and prescriber recommendation. Try our free GLP-1 weight loss calculator to explore projected outcomes at different doses, and use the BMI calculator to understand your starting point. Your prescriber will discuss contraindications, side effects, and which medication aligns best with your health 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.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials show Mounjaro at maximum dose may produce slightly higher average weight loss (approximately 20.9% vs 14.9%), but individual results vary significantly. Neither is universally "better" — prescriber suitability assessment depends on your personal health, contraindications, and response likelihood.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible, but only under prescriber guidance. There is no official conversion chart — approximate equivalence exists (e.g., Wegovy 2.4 mg roughly corresponds to Mounjaro 15 mg), but any switch requires careful medical oversight and dose adjustment.
What's the difference between Mounjaro and Ozempic?
Mounjaro and Ozempic both contain semaglutide, but Ozempic is licensed for type 2 diabetes (up to 2 mg) and Wegovy is the licensed weight management version (up to 2.4 mg). Ozempic is not licensed for weight loss in the UK. Mounjaro contains tirzepatide (different medication entirely).
How long does it take to see weight loss results with either injection?
Most people notice changes in appetite within the first 1–2 weeks. Measurable weight loss typically appears within 4–8 weeks. Full dose titration takes 16+ weeks, and sustained results develop over several months alongside lifestyle changes.
Do I need to follow a special diet while taking either medication?
Both Mounjaro and Wegovy work most effectively alongside a reduced-calorie diet and increased physical activity. Your prescriber may recommend specific dietary guidance. These medications support weight loss but are not replacements for lifestyle change.
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.