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 Versus Wegovy: Key Differences Explained
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight management injections licensed in the UK, but they work in different ways. Both are once-weekly injections that may help reduce appetite and support weight loss alongside lifestyle changes. The main difference is that Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Clinical trial data, eligibility criteria, and individual health needs all play a role in which medication a prescriber might recommend. This guide compares both medications factually to help you understand the differences.
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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.
Mechanism, Doses, and Clinical Trial Data
Mounjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may enhance appetite regulation and fat utilisation more broadly than a single pathway. In the SURMOUNT-1 clinical trial, participants on the maximum 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. The KwikPen device allows doses from 2.5 mg up to 15 mg, with minimum 4-week intervals between dose increases.
Wegovy (semaglutide) activates the GLP-1 pathway only. In the STEP-1 clinical trial, participants on the maximum 2.4 mg dose lost an average of approximately 14.9% of body weight over 68 weeks with the same lifestyle support. Wegovy dosing escalates from 0.25 mg to a maximum of 2.4 mg, also with minimum intervals between increases.
Important: These are clinical trial averages. Individual results vary significantly based on adherence, lifestyle changes, metabolism, and other health factors. Always use our free GLP-1 weight loss calculator to explore potential outcomes at different doses based on your starting weight.
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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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Which Medication Might Suit You?
Neither Mounjaro nor Wegovy is automatically "better" — prescriber choice depends on your individual health profile, medical history, and clinical assessment. Both medications require a prescription from a UK-licensed healthcare professional, whether through NHS specialist weight management services or private online prescribing. Eligibility is the same for both: BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Your GP or prescriber will assess which medication is appropriate for you, considering factors like your metabolic profile, existing medications, and any contraindications. Some prescribers may switch between medications or try one if another is unsuitable. Always discuss any concerns or side effects with your healthcare provider, and track your progress with our free progress tracker.
- 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?
Mounjaro showed higher average weight loss in clinical trials (approximately 20.9% vs 14.9%), but individual results vary significantly. Higher trial results don't assures better personal outcomes — suitability depends on your unique health profile and how your body responds. Your prescriber will assess which is most appropriate for you.
Can you switch between Mounjaro and Wegovy?
Yes, some people do switch between medications if one isn't tolerated well or results plateau. Approximate dose equivalences exist (e.g., Mounjaro 15 mg ≈ Wegovy 2.4 mg), but any switch must be managed by your prescriber. Never self-switch without medical guidance.
Which costs more in the UK?
Private costs vary by provider and dose. Mounjaro and Wegovy have similar price ranges, typically £129–£320 monthly depending on dose. NHS access is free but varies by region and involves waiting for specialist services. Ask your prescriber for current pricing.
Do Mounjaro and Wegovy have the same side effects?
Both commonly cause nausea, diarrhoea, constipation, and vomiting, particularly early on. Side effect severity and duration vary between individuals. Most side effects improve as your body adjusts. Inform your GP of any persistent or concerning side effects.
How long do you need to take Mounjaro or Wegovy?
Both are typically used for ongoing weight management, not as short-term courses. Many people take them long-term to maintain weight loss. If you stop either medication, weight often returns. Your prescriber will discuss duration and long-term management plans with you.
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.