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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Week 3 Mounjaro vs Wegovy — What's the Difference?
If you're at week 3 of a weight loss injection, you might be wondering how Mounjaro compares to Wegovy. Both are prescription-only GLP-1-based medications licensed in the UK for weight management, but they work differently and have different dosing schedules. This guide explains the key differences at week 3, what clinical trials showed, and how to decide which might be suitable for you. Always discuss with your prescriber which option is right for your individual health 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.
How Mounjaro and Wegovy Differ at Week 3
At week 3, the two medications are at different stages of their dose titration schedules. Mounjaro uses a 2.5 mg starting dose for weeks 1–4, so at week 3 you'd still be on your initial 2.5 mg injection. Wegovy starts at a lower 0.25 mg dose for weeks 1–4, meaning you're also on your starting dose at week 3. The critical difference is their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways simultaneously, while Wegovy is a GLP-1 receptor agonist only. Clinical trials (SURMOUNT for Mounjaro, STEP trials for Wegovy) showed that Mounjaro achieved average weight loss of 15–21% of body weight, whilst Wegovy achieved approximately 15% average weight loss at the maximum dose. Individual results vary significantly, and both work best alongside a reduced-calorie diet and increased physical activity. Your prescriber will have chosen based on your medical history, preferences, and eligibility.
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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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Which Medication Might Be Right for You?
Both Mounjaro and Wegovy require a UK-licensed prescriber to assess your eligibility, so the choice isn't yours alone — it's a clinical decision. Your GP or specialist will consider your BMI, any weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease), your medical history, and any medications you take. Some people may be better suited to one over the other based on side effects, injection preferences, or contraindications. Neither medication is "better" — they work differently and suit different people. Track your progress with our free progress tracker to monitor how you respond. If you experience significant side effects or feel a medication isn't working for you at week 3, discuss this with your prescriber; dose adjustments or a switch may be possible.
- 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 difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1 receptor agonist only. Both are prescription-only, once-weekly injections. Mounjaro starts at 2.5 mg; Wegovy starts at 0.25 mg. Clinical trials showed different average weight loss outcomes, but individual results vary.
Which one loses more weight — Mounjaro or Wegovy?
Clinical trials showed Mounjaro achieved average weight loss of 15–21% of body weight, whilst Wegovy achieved approximately 15% at maximum dose. However, these are averages; individual results vary significantly. Your response depends on your metabolism, adherence, lifestyle changes, and other factors. Your prescriber can discuss realistic expectations based on your circumstances.
Can I switch from Wegovy to Mounjaro at week 3?
Theoretically possible, but not typically done at week 3 since you're still in the titration phase. Any medication switch must be managed by your prescriber. They would need to assess the reason (side effects, inadequate response, etc.) and plan a safe transition. Switching usually happens after several weeks on a stable dose.
Do Mounjaro and Wegovy have the same side effects?
Both share common side effects like nausea, diarrhoea, constipation, and stomach pain. Side effect profiles are similar because both activate GLP-1 pathways. Individual tolerance varies. Some people tolerate one better than the other. If you're experiencing difficult side effects at week 3, discuss with your prescriber before deciding to continue or switch.
How do I know which one my GP will prescribe?
Your GP or specialist will assess your eligibility, medical history, and suitability. Both are NHS-approved for weight management, but availability varies by region. Private prescribing also offers both options. Your prescriber will explain why they've recommended one over the other based on your individual health circumstances and needs.
Ready to take control of your weight loss journey?
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