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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What Are Mounjaro Injections and How Do They Compare to Wegovy?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that works differently from Wegovy (semaglutide). Both are GLP-1-type medications used for weight loss, but Mounjaro uses a dual mechanism — targeting both GIP and GLP-1 hormone pathways — whereas Wegovy targets GLP-1 only. Understanding the differences between these injections can help you have an informed conversation with your GP or prescriber about which option might be more suitable for your situation.
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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 — Key Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. This means it activates two natural hormone pathways simultaneously — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may support appetite regulation, increase feelings of fullness, and help with fat utilisation.
Wegovy (semaglutide) is a single GLP-1 receptor agonist, activating only the GLP-1 pathway. It works by reducing appetite and helping you feel fuller after smaller meals.
- Mounjaro: Once-weekly injection, doses from 2.5 mg to 15 mg
- Wegovy: Once-weekly injection, doses from 0.25 mg to 2.4 mg
- Clinical trials: Mounjaro showed average weight loss of 15–21% of body weight; Wegovy showed approximately 14.9% average weight loss
Both injections are used alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly based on personal factors, lifestyle adherence, and how your body responds to the medication. Calculate your estimated weight loss with our free GLP-1 weight loss calculator to see potential outcomes at different doses.
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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.
Eligibility, Access, and Choosing Between Them in the UK
Both Mounjaro and Wegovy are prescription-only and require a clinical assessment before being prescribed. Eligibility is similar: adults with a BMI of 30 or above, or 27 or above with weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease).
In the UK, you can access both through:
- NHS specialist weight management services (tier 3 or 4) — availability varies by region
- Private online consultations with a UK-licensed prescriber — typically faster access but costs apply
Your prescriber will assess your medical history, current medications, and health goals to determine which medication is most appropriate for you. Neither injection is "better" — suitability depends on individual circumstances. Check your personal BMI using our free BMI calculator to see where you fall in the eligibility range, and discuss both options with your healthcare provider.
- 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?
Clinical trials show Mounjaro achieved average weight loss of 15–21% compared to Wegovy's approximately 14.9%. However, individual results vary significantly, and effectiveness depends on personal factors, adherence to lifestyle changes, and how your body responds. Both are effective options — your prescriber will recommend the most suitable for your situation.
Can I switch from Wegovy to Mounjaro?
Yes, switching between these medications is possible and should be managed by your prescriber. Mounjaro 5 mg is approximately equivalent to Wegovy 0.5 mg, though these are not official conversion equivalencies. Any switch requires a new clinical assessment and prescription.
What is tirzepatide versus semaglutide?
Tirzepatide (Mounjaro's active ingredient) is a dual GIP and GLP-1 agonist. Semaglutide (Wegovy's active ingredient) is a GLP-1 agonist only. The dual mechanism in tirzepatide may work differently for some people, though both medications are effective for weight management.
How long do Mounjaro and Wegovy injections take to work?
Most people notice appetite reduction within the first 1–2 weeks, though this varies. Significant weight loss typically becomes apparent after 4–8 weeks of consistent use at a stable dose. Both require patience and lifestyle changes to be effective.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are approved by NICE for NHS use, but access is through specialist tier 3 weight management services. Availability and waiting times vary significantly by region. Private access via online prescribers is also available but involves costs — typically ranging from £130–£320 per month depending on dose and provider.
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.