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 for Diabetes — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription weight loss injections licensed in the UK, but they work differently and have distinct clinical profiles. Understanding how they compare helps you and your GP or prescriber make an informed decision about which might be right for your situation. This guide breaks down the key differences between these two GLP-1 medications, including their mechanisms, trial results, and NHS availability.
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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.
How Mounjaro and Wegovy Work Differently
The fundamental difference between Mounjaro and Wegovy lies in their mechanism of action. Wegovy (semaglutide) is a GLP-1 receptor agonist, meaning it activates only the GLP-1 hormone pathway — this signals fullness to the brain, slows stomach emptying, and helps regulate blood sugar. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating both the GIP and GLP-1 pathways simultaneously. This dual mechanism may enhance appetite suppression and fat utilisation more effectively than GLP-1 alone.
Clinical trial data shows this difference translates to weight loss outcomes. In the SURMOUNT-1 trial, participants on Mounjaro 15 mg lost an average of 20.9% of their body weight over 72 weeks. In the STEP-1 trial, participants on Wegovy 2.4 mg lost an average of 14.9% of their body weight over 68 weeks. However, individual results vary considerably, and the "best" medication depends on your personal health profile, not average trial results.
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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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Doses, Prescribing, and Practical Considerations
Mounjaro uses a Kwikpen device with doses from 2.5 mg to 15 mg, given once weekly. Wegovy is also once weekly but uses doses from 0.25 mg up to 2.4 mg. Both require a gradual dose increase over several weeks. In the UK, both are prescription-only and accessed either through NHS specialist weight management services (long waiting lists in many regions) or private online prescribing following a clinical assessment. If you have type 2 diabetes alongside excess weight, your GP can discuss which medication aligns with your diabetes control goals. Your prescriber will assess your kidney function, history of pancreatitis, and other medical conditions before recommending either option.
- 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 Mounjaro or Wegovy if I have type 2 diabetes?
Yes, both are licensed for weight management in adults with type 2 diabetes and excess weight. However, if you're already on diabetes medications like insulin or sulphonylureas, your prescriber may need to adjust those doses because these weight loss injections improve blood sugar control. Always inform your prescriber of all current medications.
Which causes fewer side effects, Mounjaro or Wegovy?
Both have similar common side effects — nausea, vomiting, diarrhoea, and constipation — particularly during dose escalation. Individual tolerance varies. Some people tolerate one better than the other, but there's no clear evidence that one is universally "easier" than the other. Your prescriber can help assess your risk based on your medical history.
Is Mounjaro available on the NHS in the UK?
Mounjaro has been approved by NICE for NHS use in specialist tier 3 weight management services, but availability varies significantly by region. Many NHS services have long waiting lists. Private access is available through GPhC-registered pharmacies following an online clinical assessment.
How long do I need to stay on these injections?
Both Mounjaro and Wegovy are long-term medications. Weight typically returns if you stop treatment, so most people continue indefinitely or until they reach their health goals. Your prescriber will monitor your progress and discuss duration with you individually.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching is possible, but any change must be managed by your prescriber. There is no official conversion chart between the two — your prescriber will assess whether a switch is appropriate and manage the transition carefully to minimise side effects.
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.