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 Used For — How It Compares to Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work through different mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. This difference affects how they're used and their clinical trial results. Understanding what each medication does can help you and your GP or prescriber decide which might be appropriate for your individual health needs. Both require a clinical assessment before prescription.
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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) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite more comprehensively and support fat utilisation alongside appetite reduction.
Wegovy (semaglutide) activates only the GLP-1 pathway, which reduces appetite and makes you feel fuller after smaller meals. It also slows how quickly food leaves the stomach.
In clinical trials, Mounjaro showed an average weight loss of approximately 20.9% at the maximum 15 mg dose, whilst Wegovy showed approximately 14.9% at the 2.4 mg dose — though individual results vary significantly. Both are used alongside a reduced-calorie diet and increased physical activity. Your prescriber will assess which approach may suit your health profile best, considering factors like your BMI, existing health conditions, and any medications you're taking.
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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.
Clinical Trials and Eligibility Considerations
Mounjaro data comes from the SURMOUNT-1 trial, whilst Wegovy data comes from the STEP-1 trial. These trials tested different populations and used different study designs, so direct outcome comparison should be made carefully. Both medications are appropriate for UK adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Lower thresholds may apply for South Asian, Black African, Black Caribbean, and Chinese adults. Neither medication is suitable during pregnancy or whilst breastfeeding. A prescriber will take a full medical history, check your current medications, and assess your individual suitability before issuing a UK prescription. Use our free GLP-1 weight loss calculator to explore potential outcomes.
- 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.
What's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist (two hormone pathways), whilst Wegovy is a GLP-1 agonist only (one pathway). Mounjaro may offer a slightly different effect on appetite and metabolism, but individual responses vary. Clinical trial averages differed, but real-world results depend on many factors.
Which medication causes more weight loss?
In clinical trials, Mounjaro showed slightly higher average weight loss (20.9% at 15 mg) compared to Wegovy (14.9% at 2.4 mg). However, individual results vary significantly, and some people respond better to one medication than the other. Your prescriber will help determine which is most suitable for you.
Can I switch from Wegovy to Mounjaro?
Yes, switching is possible, but must be managed by your prescriber. There is no official established conversion chart between the two medications, though approximate dose equivalences exist based on clinical practice. Any switch requires a new assessment and prescription.
Are there differences in side effects between Mounjaro and Wegovy?
Both medications share common side effects like nausea, diarrhoea, constipation, and stomach pain — these are typical of all GLP-1 and GIP/GLP-1 agonists. Individual tolerance varies. Always discuss potential side effects with your prescriber before starting either medication.
Which is available on the NHS in the UK?
Both Mounjaro and Wegovy may be prescribed through NHS specialist tier 3 weight management services, though availability varies significantly by region and ICB. Private access is available through registered online prescribers without NHS referral. Waiting times and access differ between areas.
Ready to take control of your weight loss journey?
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