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's the Difference Between Mounjaro and Wegovy?
Mounjaro and Wegovy are both prescription-only weight management injections available in the UK, but they work differently. Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist, while Wegovy contains semaglutide, a GLP-1 receptor agonist only. Understanding how these active ingredients work can help you and your GP discuss which option might be suitable for your individual circumstances. Both require a clinical assessment before prescription and are used alongside diet and lifestyle changes.
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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 the Active Ingredients Differ
The key difference lies in how these medications work at a hormonal level. Mounjaro's tirzepatide activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may help regulate appetite, reduce food cravings, and support feelings of fullness after eating. Wegovy's semaglutide activates only the GLP-1 pathway, which also reduces appetite and slows stomach emptying but through a single mechanism.
Clinical trial data provides context: in the SURMOUNT-1 trial, Mounjaro helped participants lose an average of 20.9% body weight at the maximum 15mg dose. In the STEP-1 trial, Wegovy achieved approximately 14.9% average weight loss at the maximum 2.4mg dose. These are clinical trial averages alongside a reduced-calorie diet and increased physical activity — individual results vary significantly. Your prescriber will consider which mechanism may suit your health profile and medical history.
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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 Option Is Right for You?
Both Mounjaro and Wegovy are prescription-only medicines requiring a clinical assessment before being prescribed. Eligibility criteria are similar: UK 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, may be considered. Your GP or a licensed prescriber will evaluate your individual circumstances, including your medical history, current medications, and health goals, to assess which option is appropriate.
Use our free BMI calculator above to check your current BMI category, and our GLP-1 weight loss calculator to explore projected results at different doses. Both tools can provide helpful context for conversations with your prescriber about which medication pathway may suit you best.
- 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 tirzepatide (Mounjaro) better than semaglutide (Wegovy)?
Clinical trials show Mounjaro achieved higher average weight loss percentages than Wegovy, but this does not mean it's "better" for everyone. Individual response varies significantly. Suitability depends on your medical history, other medications, and how your body responds. Your prescriber assesses which is appropriate for you personally — not which has higher average outcomes.
Why does Mounjaro have a dual mechanism?
Mounjaro activates both GIP and GLP-1 because dual activation may amplify appetite suppression and metabolic effects compared to GLP-1 activation alone. The GIP pathway influences how your body handles glucose and may enhance weight loss. This dual action is why Mounjaro is chemically distinct from semaglutide, which targets GLP-1 only.
Can I switch from Wegovy to Mounjaro?
Switching between medications is possible but must be managed by your prescriber. There is no official established conversion chart — approximate equivalences exist based on clinical practice, but any switch requires individual assessment. Never stop or change medication without prescriber guidance.
Are the side effects different between Mounjaro and Wegovy?
Both medications have similar common side effects: nausea, diarrhoea, vomiting, constipation, and stomach pain. Individual side effect profiles vary. Some people tolerate one better than the other. Your prescriber discusses potential side effects for whichever medication they recommend based on your health profile.
How long do I need to take Mounjaro or Wegovy?
These are ongoing medications, not short-term treatments. Weight loss is typically sustained while taking the injection; weight may gradually return if you stop. Duration is determined by your prescriber and depends on your response, health goals, and whether benefits outweigh any side effects for you personally.
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.