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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Ozempic vs Mounjaro for Weight Loss: Key Differences
If you're researching weight loss injections in the UK, you may have encountered both Ozempic and Mounjaro. While both contain similar active ingredients, they have different purposes and licensing in the UK. Understanding the distinction between these prescription-only medications is important before discussing options with your GP or a licensed prescriber. This guide clarifies what each medication is, how they work, and which is actually licensed for weight management in the UK.
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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.
Understanding Ozempic and Mounjaro: The Key Distinction
This is a crucial point: Ozempic is not licensed for weight loss in the UK. Ozempic (semaglutide up to 2 mg) is approved by MHRA for managing type 2 diabetes. Wegovy (semaglutide 2.4 mg) is the licensed weight management version of semaglutide, and Mounjaro (tirzepatide) is a separate dual GIP and GLP-1 receptor agonist also licensed for weight management. Using Ozempic for weight loss would be off-label prescribing. If you're seeking a weekly weight loss injection, you're looking at either Wegovy (GLP-1 agonist) or Mounjaro (dual GIP/GLP-1 agonist). Both are prescription-only and require clinical assessment. Your GP or a licensed prescriber will assess which medication, if any, is appropriate for your individual health circumstances, BMI, and weight-related health conditions.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Mounjaro vs Wegovy: Mechanism and Clinical Evidence
Mounjaro works via dual mechanism—activating both GIP and GLP-1 pathways—which may provide greater appetite suppression than GLP-1 alone. In the SURMOUNT clinical trial, participants on Mounjaro 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks. Wegovy (semaglutide) activates GLP-1 only, with STEP trial data showing average weight loss of around 14.9% at 68 weeks on the maximum 2.4 mg dose. Both work alongside reduced-calorie diet and increased physical activity. Individual results vary significantly. Use our GLP-1 weight loss calculator to estimate potential outcomes at different doses, and our BMI calculator to check your current BMI category according to NHS guidelines.
- 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.
Why is Ozempic sometimes mentioned for weight loss if it's not licensed for it?
Ozempic (semaglutide 2 mg for diabetes) became widely discussed for weight loss during the pandemic when people noticed weight loss as a side effect. However, it is not licensed for weight management in the UK. Wegovy (semaglutide 2.4 mg) is the licensed weight management version. Using Ozempic off-label for weight loss is not standard practice and would require a specific clinical decision by a prescriber.
Is Mounjaro better than Wegovy for weight loss?
Clinical trials show Mounjaro may support slightly greater average weight loss than Wegovy, but individual results vary significantly. Mounjaro uses a dual GIP/GLP-1 mechanism versus Wegovy's single GLP-1 mechanism. Neither is universally "better"—suitability depends on your health profile, medical history, and individual response. A prescriber will assess which is appropriate for you.
Can I get Mounjaro or Wegovy on the NHS in the UK?
Access through NHS specialist tier 3 weight management services is possible, but availability varies significantly by region and ICB. Waiting lists can be lengthy. Private online prescribing through GPhC-registered pharmacies is an alternative route. Both require clinical assessment and a valid prescription from a UK-licensed prescriber.
What does the Mounjaro KwikPen contain?
The Mounjaro KwikPen is a pre-filled auto-injection device containing tirzepatide. Each pen holds four weekly doses and is colour-coded by strength (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg). You inject once weekly into your stomach, thigh, or upper arm. Store unopened pens in the fridge; after first use, they can be kept at room temperature for up to 30 days.
How does mounjaro dose titration work?
Mounjaro dose titration means gradually increasing your dose every 4 weeks, starting at 2.5 mg. You move through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg (the maximum). Each dose is a once-weekly injection. Your prescriber decides the titration schedule based on your tolerance and response. This gradual approach helps minimise side effects.
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