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 15mg vs Wegovy: How They Compare
If you're considering weight loss injections in the UK, you may be wondering how the 15mg Mounjaro pen compares to Wegovy. Both are prescription-only once-weekly injections licensed for weight management, but they work differently and have different evidence bases. Understanding the key differences—including their mechanisms, clinical trial results, and suitability for your individual health—is essential before discussing options with your GP or prescriber. This guide breaks down the facts to help you have an informed conversation.
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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.
Mounjaro vs Wegovy: Mechanism and Clinical Trial Evidence
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways to regulate appetite and blood sugar. In the SURMOUNT-1 clinical trial, adults on the maximum 15mg dose lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. In the STEP-1 clinical trial, adults on the maximum 2.4mg dose lost an average of approximately 14.9% of their body weight over 68 weeks, also with lifestyle changes.
- Mounjaro's dual mechanism may explain its numerically higher average weight loss in trials
- Both medications slow gastric emptying and reduce hunger signals
- Individual results vary significantly—clinical trial averages do not predict personal outcomes
- Both require ongoing lifestyle changes to achieve and maintain weight loss
The SURMOUNT and STEP trial data provide the evidence base, but your prescriber will assess which is suitable for your individual health and medical history.
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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, Eligibility, and How to Access in the UK
The 15mg Mounjaro pen represents the maximum maintenance dose—you begin on 2.5mg and titrate (increase gradually) every 4 weeks up to 15mg. Wegovy starts at 0.25mg and escalates to a maximum of 2.4mg. Both are prescription-only and require a clinical assessment to check eligibility.
In the UK, both medications are available through NHS specialist tier 3 weight management services or private prescribing following an online assessment. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease). Your GP or licensed prescriber will assess which medication, if any, is appropriate based on your medical history, current medications, and individual circumstances. Try our GLP-1 weight loss calculator to estimate potential outcomes, and use the BMI calculator to check your current BMI category.
- 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 does Mounjaro have higher weight loss results than Wegovy in clinical trials?
Mounjaro is a dual GIP and GLP-1 agonist (activates two hormone pathways), while Wegovy is GLP-1 only. The SURMOUNT trial showed approximately 20.9% average weight loss at 15mg, compared to STEP's approximately 14.9% at 2.4mg Wegovy. However, individual results vary, and a prescriber will determine which is suitable for you.
Can I switch from Wegovy to Mounjaro, or vice versa?
Switching between these medications should only be managed by your prescriber. There is no official conversion chart between Mounjaro and Wegovy doses. Your prescriber may recommend starting over with a titration schedule if switching. Never attempt to convert doses yourself.
Which injection should I choose—Mounjaro or Wegovy?
This decision is made with your GP or licensed prescriber based on your medical history, current medications, eligibility, and individual health factors. Neither medication is "better"—the right choice depends on your personal circumstances. Both require a clinical assessment before prescription.
How long does a 15mg Mounjaro pen last?
Each KwikPen contains 4 doses (4 weeks' supply) at the dose strength on the pen. A 15mg pen provides four 15mg injections. You would receive a new pen every 4 weeks if continuing at this dose.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are licensed in the UK. NHS access is through specialist tier 3 weight management services, though availability and waiting times vary significantly by region. Private access is available through GPhC-registered online pharmacies following a clinical assessment, without NHS referral.
Ready to take control of your weight loss journey?
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