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 15 mg Pen vs Wegovy: What's the Difference?
If you're considering weight loss medication in the UK, you've likely come across both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are prescription-only injections licensed for weight management, but they work differently and may suit different people. This guide compares the 15 mg Mounjaro KwikPen dose with Wegovy to help you understand the key differences. Remember, a prescriber will assess which is right for your individual circumstances following a clinical assessment.
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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 Mounjaro and Wegovy Work Differently
The main difference between these medications lies in how they work. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways in the body. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one pathway. This dual mechanism in Mounjaro may contribute to different weight loss outcomes. In clinical trials, the SURMOUNT-1 study showed Mounjaro 15 mg (the maximum dose) resulted in approximately 20.9% average body weight loss over 72 weeks. The STEP-1 trial showed Wegovy 2.4 mg (its maximum dose) resulted in approximately 14.9% average body weight loss. However, individual results vary significantly, and these are clinical trial averages alongside reduced-calorie diet and increased physical activity.
- Mounjaro: dual mechanism (GIP + GLP-1)
- Wegovy: single mechanism (GLP-1 only)
- Both reduce appetite and support feeling fuller after meals
- Both are once-weekly injections
- Both require a prescription and clinical assessment
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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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Eligibility, Prescription, and Personal Suitability
Both medications have similar UK eligibility criteria: a BMI of 30 or above, or 25–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. However, which medication is prescribed depends on individual assessment by a UK-licensed prescriber. Some people may be offered one, the other, or either — based on medical history, other medications, side effect tolerance, and personal preference. Neither is "better" universally; suitability is individual. Use our free GLP-1 weight loss calculator to explore potential weight loss scenarios at different doses. Your GP or prescriber will discuss which option aligns best with your health profile following a proper 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 Mounjaro 15 mg more effective than Wegovy?
Clinical trials show Mounjaro 15 mg resulted in approximately 20.9% average weight loss, while Wegovy 2.4 mg showed approximately 14.9%. However, these are clinical averages — individual results vary significantly. Which is "more effective" depends on the individual. Your prescriber will advise based on your circumstances.
Why does Mounjaro have a dual mechanism when Wegovy doesn't?
Mounjaro's dual GIP and GLP-1 mechanism was designed to activate two appetite-regulating pathways simultaneously. Wegovy's single GLP-1 mechanism is also effective. Different mechanisms don't necessarily mean one is superior — both are licensed for weight management in the UK.
Can I switch from Wegovy to Mounjaro or vice versa?
Any switch between medications must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg), but switching requires clinical guidance. Never switch without consulting your prescriber.
How long does a Mounjaro 15 mg KwikPen last?
Each Mounjaro KwikPen contains four once-weekly doses, so one pen lasts approximately four weeks. You'll need approximately 13 pens per year if remaining on the 15 mg maintenance dose. Check storage instructions — unopened pens must be refrigerated at 2–8°C.
Which medication is available on the NHS in the UK?
Both Mounjaro and Wegovy are NICE-approved and available through NHS specialist weight management services (tier 3). However, NHS access varies significantly by region and availability may be limited. Private access is available through GPhC-registered online pharmacies following clinical assessment.
Ready to take control of your weight loss journey?
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