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 Semaglutide — Key Differences for UK Patients
If you're considering prescription weight loss medication in the UK, you may have heard of both Mounjaro and semaglutide. While both are licensed for weight management in the UK and work as GLP-1-related medications, they differ in their mechanisms, dosing schedules, and clinical trial results. Understanding these differences is important when discussing options with your GP or a licensed prescriber. This guide explains how Mounjaro (tirzepatide) and semaglutide (marketed as Wegovy for weight loss) compare in the UK context, including NHS eligibility and access routes.
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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 Semaglutide Work Differently
The key difference between these two medications 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. In contrast, semaglutide (Wegovy) is a GLP-1 receptor agonist only, activating one pathway. Both medications help regulate appetite and support feeling fuller after meals, but Mounjaro's dual mechanism may explain why clinical trials showed slightly higher average weight loss with tirzepatide. Mounjaro is administered once weekly using a KwikPen auto-injector. Semaglutide (Wegovy) is also a once-weekly injection but uses a different pen device. Starting doses differ too — Mounjaro begins at 2.5 mg and escalates to a maximum of 15 mg, while Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg. Both are prescription-only medications requiring a clinical assessment before prescribing.
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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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NHS Eligibility and UK Access Routes
In the UK, both Mounjaro and Wegovy are approved for weight management and have similar NHS eligibility criteria. Adults with a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease), may be eligible. However, NHS access is limited and varies by region — both medications are typically prescribed through specialist tier 3 weight management services, not standard GP practices. Waiting times can be substantial. Private prescribing is available in the UK through GPhC-registered online pharmacies following a clinical assessment. Use our free GLP-1 weight loss calculator to estimate potential outcomes with either medication at different doses. Calculate your BMI using our free BMI calculator above to check your eligibility threshold based on your current height and weight.
- 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 between Mounjaro and Wegovy?
Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) activates one (GLP-1 only). This dual mechanism may explain why Mounjaro showed slightly higher average weight loss in clinical trials — approximately 20.9% body weight at the maximum dose compared to Wegovy's 14.9%. Both are once-weekly injections and prescription-only.
Can I get Mounjaro or Wegovy on the NHS in the UK?
Both are NHS-approved, but access is limited to specialist tier 3 weight management services through referral from your GP. Availability and waiting times vary significantly by region and ICB (integrated care board). Private prescribing is available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber.
Which medication causes fewer side effects?
Both Mounjaro and Wegovy cause similar side effects, with nausea, diarrhoea, and constipation being most common, especially at the start of treatment. Individual responses vary widely. Side effects often improve as your body adjusts. Always discuss potential side effects with your prescriber before starting either medication.
How do the doses compare between Mounjaro and Wegovy?
Mounjaro starts at 2.5 mg weekly and escalates to a maximum of 15 mg. Wegovy starts at 0.25 mg weekly and reaches a maximum of 2.4 mg. Approximate equivalences exist — for example, Mounjaro 10 mg is roughly equivalent to Wegovy 1.7 mg — but any switch between medications must be managed by your prescriber.
How long does weight loss take with either medication?
Clinical trials typically show meaningful weight loss over 12–16 weeks, with continued loss up to 68 weeks. Results vary between individuals and depend on adherence, diet, and physical activity. Both medications work best alongside a reduced-calorie diet and increased exercise. Track your progress with our free progress tracker to monitor changes over time.
Ready to take control of your weight loss journey?
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