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 Wegovy: 3-Month Weight Loss Results
When comparing weight loss medications, understanding what to expect in the first three months is important for making informed decisions. Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only GLP-1 based medications licensed in the UK for weight management, but they work differently and deliver different results. This guide compares the clinical evidence for both, explaining what the NHS and clinical trials tell us about early results.
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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
The two medications come from different clinical trials that measured weight loss over time. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist tested in the SURMOUNT-1 trial, whilst Wegovy (semaglutide) is a GLP-1 only agonist tested in the STEP trials. At the 3-month mark (12 weeks), results begin to diverge. Early SURMOUNT data suggests Mounjaro users typically experience noticeable appetite reduction and initial weight loss within the first month, with momentum building over weeks 2–12. STEP trial participants on Wegovy show similar early appetite suppression, though the rate of loss may differ depending on the starting dose and individual metabolism. By week 12, both medications show meaningful progress, but Mounjaro's dual mechanism often produces slightly more substantial early results on average. Individual variation is significant — some people respond faster, others more gradually. Your prescriber will monitor your response and adjust your KwikPen dose accordingly during this critical early phase.
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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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Individual Factors and Suitability for 3-Month Success
Success in the first three months depends less on which medication you choose and more on your starting point, adherence, and lifestyle support. Both require a reduced-calorie diet and increased physical activity to work optimally — medication alone does not produce weight loss. Your GP or prescriber will assess your eligibility based on BMI and weight-related health conditions under NHS or private clinical assessment. Try our free GLP-1 weight loss calculator to estimate potential results at different doses for your height and current weight. If you have type 2 diabetes, certain medications, or gallbladder concerns, your prescriber may favour one over the other. Neither is assured to work equally for everyone. After three months, a review with your prescriber helps determine whether to continue, adjust doses, or consider alternatives — this is why consistent monitoring matters more than the choice between the two.
- 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.
Can I expect to see Mounjaro results within 3 months?
Many people notice appetite reduction within the first 2–4 weeks and measurable weight loss by 8–12 weeks when combined with dietary changes and activity. Clinical trials showed progressive loss throughout this period, but individual results vary significantly based on starting weight, dose, and lifestyle adherence.
Is Mounjaro faster than Wegovy in the first 3 months?
SURMOUNT trial data suggests Mounjaro may produce slightly more substantial average weight loss early on due to its dual GIP+GLP-1 mechanism, but individual responses vary. Some people respond faster to Wegovy. Your prescriber can discuss which may suit your situation.
What dose of Mounjaro would I be on after 3 months?
After 3 months (12 weeks), most people have progressed through the titration schedule and are on their maintenance dose, typically between 5–10mg depending on tolerability and response. Starting doses are lower (2.5mg) and increase by 2.5mg every 4 weeks to minimise side effects.
Do side effects in the first 3 months affect weight loss results?
Side effects like nausea and constipation are common early on but often reduce as your body adjusts. They may actually support weight loss initially by reducing appetite, but severe side effects should be reported to your prescriber who can adjust your dose or approach.
Should I choose Mounjaro or Wegovy based on 3-month results?
Your prescriber will recommend based on your clinical assessment, not on trial averages. Factors like your BMI, comorbidities, medication interactions, and tolerability matter more than headlines about which medication loses "more" weight on average.
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.