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: Ingredient and Mechanism Comparison
Mounjaro and Wegovy are both prescription-only weight management injections available in the UK, but they contain different active ingredients and work through different mechanisms. Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist, whilst Wegovy contains semaglutide, a GLP-1 receptor agonist only. Understanding these differences can help you and your GP or prescriber assess which medication might be more suitable 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 natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action means it works on two separate appetite and satiety mechanisms simultaneously. In the SURMOUNT clinical trial, participants achieved an average weight loss of approximately 15–21% of body weight at the maximum dose of 15 mg.
Wegovy (semaglutide) activates only the GLP-1 receptor pathway. Whilst a single mechanism, GLP-1 is a well-established hormone for appetite regulation. In the STEP clinical trials, semaglutide showed an average weight loss of approximately 14.9% of body weight at the maximum dose of 2.4 mg.
Both medications also slow gastric emptying (how quickly food leaves the stomach), though this effect typically reduces over time. Individual results vary significantly, and weight loss occurs 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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Which Medication Might Be Right for You?
Both Mounjaro and Wegovy are prescription-only and require a UK-licensed prescriber to assess your suitability based on your BMI, weight-related health conditions, medical history, and current medications. Neither medication is "better" — suitability depends on individual factors including your medical profile, response to treatment, and any contraindications.
Your GP or online prescriber will consider: your BMI (typically 30 or above, or 27+ with weight-related conditions), any existing health conditions like type 2 diabetes or high blood pressure, medication interactions, and your personal preference. Some people may respond better to one mechanism than the other. Use our free BMI calculator above to check your personal BMI, and our GLP-1 weight loss calculator to estimate potential outcomes with different medications.
- 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 ingredients?
Mounjaro contains tirzepatide, which activates both GIP and GLP-1 hormone pathways (dual mechanism). Wegovy contains semaglutide, which activates only the GLP-1 pathway. The dual mechanism of Mounjaro may provide additional appetite suppression, though individual response varies.
Does having two mechanisms mean Mounjaro is more effective?
Not necessarily. Clinical trial data shows both medications produce meaningful weight loss — Mounjaro averaged 15–21% and Wegovy averaged 14.9%. Effectiveness depends on individual factors, not just mechanism. Your prescriber will assess which suits your health profile best.
Can I switch from Wegovy to Mounjaro or vice versa?
Potentially yes, but only under prescriber guidance. There is no official established conversion chart between the medications. Any switch must be managed by your GP or prescriber to ensure safe dosing and monitoring. Do not attempt to switch yourself.
Are Mounjaro and Wegovy natural ingredients?
Both are synthetic medications — they are manufactured pharmaceutical compounds, not natural extracts. They mimic the action of natural hormones (GIP and GLP-1) produced in your own body. All prescription weight loss medications are synthetic and require clinical assessment before use.
Which medication is available on the NHS in the UK?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) have been approved by NICE for NHS use in specialist tier 3 weight management services. However, NHS availability varies by region and waiting times can be long. Both are also available privately through GPhC-registered pharmacies following online clinical assessment.
Ready to take control of your weight loss journey?
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