The UK's most complete GLP-1 guide
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- 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 — What's the Difference?
If you've heard about weight loss injections in the UK, you've likely encountered both Mounjaro and Wegovy. Though both are prescription-only medications for weight management, they work differently and have distinct dosing schedules. This guide compares these two GLP-1 receptor agonist treatments, explaining how each works, their clinical evidence, side effects, and how to access them through the NHS or privately. Understanding the differences helps you have informed conversations with your GP or prescriber about which option might suit your health needs.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) work together to reduce appetite, increase feelings of fullness, and support blood sugar regulation. This dual mechanism is the key distinction.
Wegovy (semaglutide) activates only the GLP-1 pathway. Whilst highly effective, it uses a single hormone mechanism. Clinical trials showed both medications support meaningful weight loss when combined with reduced-calorie diet and increased physical activity.
Clinical data from the SURMOUNT-1 trial (Mounjaro) showed average weight loss of approximately 20.9% body weight at the maximum 15mg dose. STEP-1 trial data (Wegovy) showed average weight loss of around 14.9% body weight at the maximum 2.4mg dose. However, individual results vary significantly, and these figures represent clinical trial averages alongside lifestyle changes.
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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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Doses, Access, and Practical Differences
Mounjaro starts at 2.5mg weekly, escalating to a maximum of 15mg. Wegovy starts at 0.25mg weekly, escalating to a maximum of 2.4mg. Both are once-weekly injections. Use our free BMI calculator above to check whether you may be eligible for either medication — generally, adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions, may qualify.
In the UK, both are available through NHS specialist tier 3 weight management services (with potential waiting lists) or privately via online prescribing following clinical assessment. Neither is available as a standard GP prescription. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different doses, though actual results depend on individual circumstances and adherence to lifestyle changes.
Side effects are similar between both medications: nausea, diarrhoea, vomiting, constipation, and stomach pain are very common, especially during dose escalation. Both require a prescription from a UK-licensed healthcare professional and a clinical assessment to confirm suitability.
Switching between medications requires prescriber guidance — there is no official conversion chart. If you're considering either option, discuss with your GP or a licensed prescriber whether Mounjaro's dual mechanism or Wegovy's established track record might better suit your individual 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.
Is Mounjaro the same as Wegovy?
No. Both are injectable weight loss medications, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. This means Mounjaro activates two hormone pathways simultaneously, whilst Wegovy activates one. Clinical trials showed different average weight loss outcomes, though individual results vary.
Which medication causes fewer side effects?
Both have similar side effect profiles. Nausea, diarrhoea, vomiting, and constipation are very common, especially during dose escalation. Side effects typically reduce as the body adjusts. Your prescriber can help determine which is more suitable based on your medical history and tolerance to GLP-1 medications.
How do I access Mounjaro or Wegovy on the NHS in the UK?
Both are available through NHS specialist tier 3 weight management services. You'll need a GP referral. Availability and waiting times vary significantly by region. Privately, both are available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber.
Can I switch from Wegovy to Mounjaro?
Yes, but only under prescriber guidance. There is no official conversion chart — your prescriber will manage any switch based on your individual response and health. Never switch medications without professional advice. Use our free BMI calculator to track your progress during any medication change.
Which medication is more effective for weight loss?
Clinical trials showed Mounjaro achieved slightly higher average weight loss percentages than Wegovy, but both support meaningful results when combined with lifestyle changes. Individual outcomes vary greatly. Your GP or prescriber can discuss which mechanism might suit your health needs and medical history better.
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.