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 Before and After vs Wegovy — What's the Difference?
When considering weight loss injections in the UK, many people ask whether Mounjaro (tirzepatide) or Wegovy (semaglutide) produces better results. Both are prescription-only, once-weekly injectable medicines licensed for weight management, but they work through different mechanisms and showed different outcomes in clinical trials. This guide compares the evidence head-to-head, explains why individual results vary significantly, and helps you understand what each medication may offer. Neither assures specific weight loss — your prescriber will assess which is more suitable for your circumstances.
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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.
Clinical Trial Results: SURMOUNT vs STEP Data
Clinical trials provide the most reliable data for comparing weight loss medications. Mounjaro (tirzepatide) was tested in the SURMOUNT-1 trial over 72 weeks alongside a reduced-calorie diet and increased physical activity. Participants taking the maximum 15 mg dose lost an average of approximately 20.9% of their body weight. Those on 10 mg lost around 19.5%, and those on 5 mg lost about 15%.
Wegovy (semaglutide) was tested in the STEP-1 trial over 68 weeks under similar lifestyle conditions. Participants on the maximum 2.4 mg dose lost an average of 14.9% of their body weight, which was lower than the highest Mounjaro dose but still significant.
Important context: these are clinical trial averages with carefully selected participants. Individual results vary substantially based on genetics, starting weight, adherence, diet quality, physical activity, and other health factors. A prescriber will discuss realistic expectations during your consultation.
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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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How They Work Differently: The Mechanism Behind Results
Wegovy activates a single hormone pathway (GLP-1 only), which signals fullness, slows stomach emptying, and reduces appetite. It's been used for type 2 diabetes management for years before being approved for weight loss.
Mounjaro activates two hormone pathways simultaneously (GIP and GLP-1), which is why it carries the label "dual receptor agonist". The additional GIP pathway may contribute to greater appetite suppression and improved metabolic effects in some people. This dual mechanism is what distinguishes tirzepatide from earlier GLP-1-only medications like semaglutide.
However, a more powerful mechanism doesn't assures better results for every individual. Some people respond excellently to Wegovy, whilst others prefer Mounjaro. A prescriber will consider your medical history, other medications, and personal factors when recommending which to try.
- 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.
Does Mounjaro always cause more weight loss than Wegovy?
No. Whilst clinical trials show Mounjaro achieved slightly higher average weight loss (20.9% vs 14.9%), individual results vary significantly. Some people lose more on Wegovy, others on Mounjaro. Genetics, lifestyle adherence, and personal metabolism play major roles. Your prescriber will assess which suits you based on your health profile.
How long do Mounjaro or Wegovy "before and after" results take?
Clinical trials measured results over 12–16 weeks to see meaningful changes, with maximum effects at 52–72 weeks. Most people notice appetite reduction within 1–2 weeks, but visible weight loss typically appears after 4–8 weeks of consistent use alongside diet and exercise.
What's the difference between tirzepatide and semaglutide?
Tirzepatide (Mounjaro) is a dual GIP+GLP-1 receptor agonist. Semaglutide (Wegovy) is a GLP-1-only agonist. The dual mechanism may explain why Mounjaro showed higher average weight loss in trials, but both require a reduced-calorie diet and physical activity to be effective.
Can I switch from Wegovy to Mounjaro if results plateau?
Yes, many people do switch medications if weight loss plateaus. There's no official conversion chart — your prescriber will recommend starting doses and monitor your response. This is a decision made during follow-up consultations with your healthcare provider.
Will I lose weight the same as the clinical trial participants?
Unlikely. Clinical trial participants were carefully selected and closely monitored. Real-world results vary based on individual adherence, diet quality, physical activity, sleep, stress, and genetics. Use our free GLP-1 weight loss calculator to estimate potential results, but discuss realistic expectations with your prescriber. Track progress using our free progress tracker to monitor your personal journey.
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.