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 or Mounjaro for Weight Loss — What's the Difference?
If you're exploring prescription weight loss options in the UK, you may have heard of both Ozempic and Mounjaro. While both are semaglutide and tirzepatide-based injectables, they have important differences in how they work, dosing, and clinical evidence. This guide explains the facts to help you understand what a prescriber might consider when deciding which is right for you. Remember, both require a prescription and a clinical assessment — the choice depends entirely on individual health circumstances.
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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 Ozempic and Mounjaro Differ — Mechanism and Trials
The key difference between these two weekly weight loss injections lies in their mechanism of action. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body — both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). This dual mechanism may help regulate appetite and blood sugar more comprehensively.
Ozempic (semaglutide) is a GLP-1 receptor agonist only, activating just the GLP-1 pathway. Note that Ozempic is licensed for type 2 diabetes management, not specifically for weight loss in the UK — using it for weight loss would be off-label use.
For weight loss, Wegovy (also semaglutide) is the licensed UK alternative to Mounjaro. Clinical trials show SURMOUNT data for Mounjaro indicated average weight loss of approximately 15–21% body weight, while STEP trials for Wegovy showed approximately 14.9% average loss. Individual results vary significantly.
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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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Prescriber Decision and Individual Suitability
Whether a GP or prescriber recommends Mounjaro, Wegovy, or suggests exploring other options depends entirely on your individual health profile, medical history, and how you respond to treatment. Both are prescription-only medications requiring a clinical assessment before being dispensed.
Use our free GLP-1 weight loss calculator above to understand projected weight loss ranges with these medications, and our BMI calculator to check if you meet eligibility criteria. Your prescriber will evaluate contraindications, interactions with other medications, and your weight-related health conditions before deciding which medication — if any — is appropriate for you.
Access in the UK is through NHS specialist tier 3 weight management services (with potentially long waiting lists) or via private online prescribing following a clinical assessment.
- 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 Ozempic licensed for weight loss in the UK?
No. Ozempic (semaglutide 0.5–2 mg) is licensed for type 2 diabetes management. Using it for weight loss would be off-label. Wegovy (semaglutide 2.4 mg) is the licensed UK alternative for weight management. Both are manufactured by Novo Nordisk but are distinct products with different approved uses.
How much weight can you lose on Mounjaro vs Wegovy?
Clinical trials showed Mounjaro (SURMOUNT) helped participants lose approximately 15–21% of body weight on average, while Wegovy (STEP trials) showed approximately 14.9% average loss. However, individual results vary significantly based on diet, exercise, dose reached, and personal metabolism.
Why would a prescriber choose Mounjaro over Wegovy?
The dual GIP+GLP-1 mechanism of Mounjaro may be more effective for some individuals, or a prescriber may choose based on your medical history, previous medication response, tolerance of side effects, or specific weight-related health conditions. Only your prescriber can assess suitability.
Can you switch between Mounjaro and Wegovy?
Yes, switching is possible, but it must be managed by your prescriber. There is no official established conversion chart — approximate equivalences exist but any transition requires professional medical guidance to ensure safe dosing and monitoring.
How do I access these medications on the NHS?
Both require referral to a specialist tier 3 weight management service via your GP. NHS availability varies by region and waiting times can be lengthy. Private prescribing through a UK-licensed prescriber is an alternative, available without NHS referral.
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.