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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How to Administer Mounjaro vs Wegovy in the UK
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they differ significantly in how they're administered and how they work. Understanding these differences helps you discuss options with your GP or prescriber. Both require a clinical assessment before prescription, and both are typically used alongside a reduced-calorie diet and increased physical activity. Our GLP-1 weight loss calculator can help you explore potential outcomes with each medication at different doses.
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Calculate Your BMI Instantly
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.
Injection Administration and Mechanism Differences
Mounjaro (tirzepatide) is administered as a once-weekly injection using a pre-filled KwikPen device. It's a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways to regulate appetite and blood sugar. The starting dose is 2.5 mg, with escalation every 4 weeks up to a maximum of 15 mg per week.
Wegovy (semaglutide) is also a once-weekly injection but uses a different pre-filled auto-injection pen. It's a GLP-1 receptor agonist only, activating a single hormone pathway. Wegovy starts at 0.25 mg and escalates over 16+ weeks to a maximum of 2.4 mg per week.
- Mounjaro: dual mechanism (GIP + GLP-1), faster escalation schedule, higher maximum dose
- Wegovy: single mechanism (GLP-1 only), longer escalation period, lower maximum dose
- Both: once-weekly injections, both require prescription, both available through NHS specialist services or private prescribing
Clinically BackedWeight Loss
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Clinical Trial Data and Individual Suitability
The SURMOUNT-1 trial showed Mounjaro at 15 mg achieved approximately 20.9% average body weight loss over 72 weeks. The STEP-1 trial showed Wegovy at 2.4 mg achieved approximately 14.9% average weight loss over 68 weeks. However, individual results vary significantly, and these are clinical trial averages alongside reduced-calorie diet and exercise.
Choosing between Mounjaro and Wegovy depends on your personal circumstances, medical history, and prescriber assessment — not trial data alone. Both are suitable for adults with 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 prescriber will assess which is appropriate for you based on your full health picture.
- 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.
What's the main difference in how Mounjaro and Wegovy work?
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways. Wegovy is a GLP-1 receptor agonist only — it activates one pathway. This dual mechanism in Mounjaro may contribute to different appetite regulation, but both aim to support weight loss alongside lifestyle changes.
How often do you inject Mounjaro versus Wegovy?
Both are once-weekly injections. You use a pre-filled pen device for both. The difference is the escalation schedule — Mounjaro escalates faster (every 4 weeks) with a higher maximum dose (15 mg), while Wegovy has a longer escalation (16+ weeks) to a lower maximum (2.4 mg).
Which medication caused more weight loss in trials?
In clinical trials, Mounjaro at 15 mg showed approximately 20.9% average weight loss, while Wegovy at 2.4 mg showed approximately 14.9% average weight loss. However, individual results vary significantly, and trial results don't predict personal outcomes.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between medications must be managed by your prescriber. There's no official conversion chart between doses. Your prescriber will advise on whether a switch is appropriate and how to do it safely, including any washout period needed.
How do I get Mounjaro or Wegovy prescribed in the UK?
Both require a clinical assessment by a UK-licensed prescriber. You can access them through NHS tier 3 specialist weight management services (via GP referral) or private online prescribing (no NHS referral needed). Eligibility is assessed individually based on BMI and health conditions.
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.