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 and Weight Loss: How It Compares to Wegovy
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection that works differently from Wegovy, using a dual GIP and GLP-1 mechanism. Both medications are licensed for weight management in UK adults and may support weight loss alongside lifestyle changes, but they have different dosing, trial outcomes, and mechanisms. This guide compares the two to help you understand how they differ—though individual results vary and only a licensed prescriber can assess which is right for you.
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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: The Key Difference
Mounjaro uses a dual GIP and GLP-1 receptor agonist mechanism, activating two natural hormone pathways to help reduce appetite and regulate blood sugar. Wegovy uses GLP-1 only, a single hormone pathway. This dual action in Mounjaro may explain why clinical trial results showed slightly higher average weight loss, though both are effective weight loss injections.
- Mounjaro: Dual hormone mechanism, 15 mg maximum dose, once weekly
- Wegovy: Single GLP-1 pathway, 2.4 mg maximum dose, once weekly
In the SURMOUNT-1 trial, Mounjaro at 15 mg resulted in approximately 20.9% average body weight loss over 72 weeks. In the STEP-1 trial, Wegovy at 2.4 mg showed approximately 14.9% average weight loss over 68 weeks—results vary significantly between individuals. Both work by slowing gastric emptying and supporting feeling fuller after smaller meals.
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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, Eligibility, and Your Next Steps
Mounjaro starts at 2.5 mg weekly and escalates every 4 weeks to a maximum of 15 mg, while Wegovy begins at 0.25 mg and increases to 2.4 mg over approximately 16 weeks. Both are prescription-only and available through NHS specialist weight management services or private prescribing following clinical assessment. Eligibility for both is the same: BMI 30 or above, or BMI 27–29.9 with a weight-related health condition such as type 2 diabetes, high blood pressure, or cardiovascular disease.
Your GP or a licensed prescriber will assess which medication suits your individual circumstances. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your current status—this context helps your prescriber make an informed decision tailored to you.
- 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 better than Wegovy for weight loss?
Clinical trials suggest Mounjaro may result in slightly higher average weight loss (20.9% vs 14.9%), but individual results vary significantly. Both are effective. The "better" option depends on your health history, side effect profile, and how your body responds—only a prescriber can determine which is suitable for you after assessment.
Can I switch from Wegovy to Mounjaro?
Yes, switching between these medications is possible and managed by your prescriber. Mounjaro 5 mg is approximately equivalent to Wegovy 0.5 mg, and Mounjaro 15 mg to Wegovy 2.4 mg, though these are approximate conversions. Any switch requires clinical supervision and dose adjustment.
How long does it take to see weight loss with Mounjaro?
Most people begin noticing changes within 4–8 weeks, though weight loss becomes more significant after 12 weeks as doses increase. Full results typically appear after 6–12 months of treatment. Weight loss continues throughout the titration period and beyond with consistent use.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are NICE-approved for NHS specialist weight management services, but access is limited and varies by region. Waiting lists can be lengthy. Private prescribing is available through GPhC-registered pharmacies following an online clinical assessment, typically costing £129–£320 monthly depending on dose.
What's the main difference between these weight loss injections?
Mounjaro activates two hormone pathways (GIP and GLP-1), while Wegovy activates one (GLP-1 only). This dual mechanism in Mounjaro may contribute to higher average weight loss in trials, but both require weekly injection, prescription, and clinical assessment before use.
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.