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 Ozempic for Weight Loss — What's the Difference?
If you're exploring prescription weight loss options in the UK, you may have heard both Mounjaro and Ozempic mentioned. Although both contain similar active ingredients from the same manufacturer, they are licensed for different purposes and have important distinctions. This guide explains the key differences between these weekly weight loss injections, how they work, and what you need to know to make an informed decision with your GP or prescriber.
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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.
Key Differences: Mechanism, Dose, and Licensed Use
The most important distinction: Mounjaro and Ozempic both come from Novo Nordisk, but Ozempic is NOT licensed for weight loss in the UK. Ozempic (semaglutide up to 2 mg) is approved for type 2 diabetes management only. Mounjaro (tirzepatide, 2.5–15 mg) is specifically licensed for weight management in UK adults.
How they work differently: Ozempic is a GLP-1 receptor agonist — it activates one hormone pathway to regulate blood sugar and appetite. Mounjaro uses a dual mechanism, activating both GIP and GLP-1 pathways, which may provide a stronger appetite-suppressing effect. In clinical trials, Mounjaro demonstrated slightly higher average weight loss (~15–21% body weight) compared to GLP-1-only medications like Wegovy (~15% body weight).
Dosing and frequency: Both are once-weekly injections using a pre-filled KwikPen device. However, their dose ranges differ significantly — Ozempic goes up to 2 mg for diabetes, whereas Mounjaro's therapeutic weight loss range is 2.5–15 mg weekly.
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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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Clinical Evidence and Individual Suitability
Mounjaro's weight loss benefits are supported by the SURMOUNT-1 clinical trial, which showed participants lost an average of 20.9% of their body weight at the maximum 15 mg dose over 72 weeks, alongside a reduced-calorie diet and physical activity. Ozempic was not tested for weight loss in UK trials and is therefore not licensed for this indication.
Both medications require a prescription and clinical assessment from a UK-licensed prescriber. Your GP or a specialist will evaluate your BMI, health conditions, and medical history to determine which medication is appropriate for you. Use our free BMI calculator to check your starting point, then discuss prescription options with your prescriber. Individual results vary significantly, and lifestyle changes remain essential alongside any medication.
- 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.
Can I use Ozempic for weight loss in the UK?
No. Ozempic is licensed only for type 2 diabetes management in the UK, not for weight loss. Using it for weight loss is off-label and not recommended. Mounjaro is the licensed tirzepatide product specifically approved for weight management in UK adults. Always use a medication only for its licensed indication unless explicitly advised by your prescriber.
Which causes more weight loss — Mounjaro or Ozempic?
Mounjaro's dual GIP+GLP-1 mechanism has shown higher average weight loss (~21% at maximum dose) in SURMOUNT trials compared to GLP-1-only medications. However, individual results vary significantly based on dose, lifestyle, and personal factors. Only a clinical assessment can determine which is suitable for you.
Is Mounjaro available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for use within NHS specialist weight management services (tier 3/4). NHS access depends on your local ICB and availability, and waiting lists vary by region. Private prescription access is also available through UK-licensed online prescribers following a clinical assessment.
What are the main side effects of Mounjaro?
Common side effects include nausea, diarrhoea, constipation, and stomach pain — typically most noticeable when starting or increasing doses. Most people tolerate the medication well, and side effects often improve over time. Inform your prescriber of any concerning symptoms, and never stop medication without medical guidance.
How do Mounjaro and Wegovy compare for weight loss?
Both are once-weekly injections licensed for weight management. Mounjaro (dual GIP+GLP-1) showed slightly higher average weight loss (~21%) than Wegovy (GLP-1 only, ~15%) in clinical trials. However, both require prescription and clinical assessment. Your prescriber will recommend the most suitable option based on your individual health profile and medical history.
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.