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 — What's the Difference?
Both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK, but they work in different ways and have different dose ranges. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. Understanding the key differences can help you and your GP or prescriber decide which might be suitable for you. Both are used alongside a reduced-calorie diet and increased physical activity for best results.
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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.
Key Differences Between Mounjaro and Wegovy
Active ingredient and mechanism: Mounjaro contains tirzepatide and activates two hormone pathways (GIP and GLP-1), while Wegovy contains semaglutide and activates the GLP-1 pathway only. This dual action in Mounjaro may contribute to slightly higher average weight loss in clinical trials.
- Mounjaro (tirzepatide): Doses from 2.5 mg to 15 mg once weekly. Average weight loss ~15–21% body weight in clinical trials (SURMOUNT-1).
- Wegovy (semaglutide): Doses from 0.25 mg to 2.4 mg once weekly. Average weight loss ~14.9% body weight in clinical trials (STEP-1).
Both are injected once weekly using pre-filled pens. Individual results vary significantly, and weight loss is achieved alongside dietary changes and physical activity, not as a standalone treatment. Use our GLP-1 weight loss calculator to estimate 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.
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How to Access Mounjaro and Wegovy in the UK
Both medications are prescription-only and require a clinical assessment before prescribing. On the NHS, access is through specialist tier 3 weight management services, usually following GP referral. Waiting times vary significantly by region and can be lengthy in many areas.
Privately, both are available through UK-licensed prescribers via online consultations followed by dispensing through GPhC-registered pharmacies. This route doesn't require NHS referral but involves private costs. Your prescriber will assess your eligibility based on BMI (usually 30 or above, or 27–29.9 with weight-related health conditions) and overall health. Use our BMI calculator above to check your current BMI and understand where you fall on the NHS eligibility spectrum.
- 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 or Wegovy more effective for weight loss?
Clinical trials show Mounjaro achieves slightly higher average weight loss (~20.9% at maximum dose) than Wegovy (~14.9%), but individual results vary significantly. The "best" medication depends on your personal health profile, medical history, and how your body responds. Your prescriber will help determine which is more suitable for you.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart—approximate equivalences exist (e.g., Wegovy 2.4 mg ≈ Mounjaro 15 mg) but switching requires careful monitoring. Never switch medications without medical guidance.
What's the difference between Ozempic and Wegovy?
Both contain semaglutide, but Ozempic (up to 2 mg) is licensed for type 2 diabetes management, while Wegovy (up to 2.4 mg) is licensed specifically for weight loss. Using Ozempic for weight loss is off-label in the UK. Wegovy is the correct licensed version for weight management.
How long do I need to take Mounjaro or Wegovy?
Both medications are typically ongoing treatments. If you stop, weight regain is common within months because the underlying appetite regulation returns to baseline. Treatment duration should be discussed with your prescriber based on your goals and health status.
Are there side effects common to both medications?
Yes. Both commonly cause nausea, diarrhoea, constipation, vomiting, and stomach pain, especially during dose escalation. These often improve over time. Serious side effects are rare but include pancreatitis (severe stomach pain) and gallbladder issues. Report any concerning symptoms to your prescriber immediately. ```
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