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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Day 3 Mounjaro vs Wegovy — What's the Difference?
By day 3 of starting Mounjaro, you've completed your first dose and may be wondering how it compares to Wegovy, the other main GLP-1 weight loss injection available in the UK. While both are prescription-only medications used for weight management, they work slightly differently and have distinct clinical profiles. Understanding the key differences — including mechanism, trial outcomes, and eligibility — can help you and your prescriber make an informed decision. Our free GLP-1 weight loss calculator can help you explore potential outcomes with either medication based on your individual circumstances.
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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.
Mounjaro vs Wegovy: Key Mechanism and Trial Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. In the SURMOUNT-1 clinical trial, participants on the maximum 15 mg dose lost an average of 20.9% of their body weight over 72 weeks. Wegovy (semaglutide) is a GLP-1-only receptor agonist, and the STEP-1 trial showed average weight loss of approximately 14.9% at the maximum 2.4 mg dose over 68 weeks.
- Mounjaro's dual mechanism may offer additional appetite regulation through the GIP pathway
- Wegovy has a longer clinical track record in the UK market
- Both require a reduced-calorie diet and increased physical activity for best results
- Individual responses vary significantly — some people respond better to one than the other
Neither medication assures weight loss, and trial averages do not predict individual outcomes. Your response to Mounjaro at day 3 may look completely different from someone else's.
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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.
Eligibility, Access, and Prescriber Assessment
Both Mounjaro and Wegovy require a UK-licensed prescriber to assess your individual health and confirm eligibility. You must have a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply due to higher metabolic risk at lower weights.
Access in the UK is through NHS specialist tier 3 weight management services or private online prescribing following clinical consultation. Your GP or prescriber will assess which medication suits your health profile best — there is no "better" option universally, only what works best for you. Use our free BMI calculator to check your starting point, then discuss your options with a healthcare professional.
- 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.
Why does Mounjaro seem more effective than Wegovy in trials?
Mounjaro's dual GIP+GLP-1 mechanism may offer additional appetite suppression compared to Wegovy's single GLP-1 pathway. However, trial averages do not predict individual outcomes. Some people respond equally well to Wegovy, and personal factors like diet adherence and activity level matter more than the medication alone.
Is Mounjaro available on the NHS in the UK?
Yes. NICE has approved tirzepatide (Mounjaro) for use within NHS specialist weight management services. Access is through tier 3 or 4 services and typically requires a GP referral. Availability and waiting times vary significantly by region and NHS integrated care board.
Can I switch from Mounjaro to Wegovy if I'm not seeing results?
A switch is possible but must be managed by your prescriber. There is no official conversion chart between the two — any change requires a clinical reassessment. Your prescriber will consider your response to Mounjaro, any side effects, and your overall health before recommending a switch.
What happens if I stop Mounjaro on day 3?
Day 3 is just three days after your first 2.5 mg starter dose. Weight loss medications typically require weeks to months to assess effectiveness. If you're experiencing side effects, contact your prescriber immediately — they may adjust your approach rather than stopping altogether.
Which injection is best for weight loss in the UK?
Neither is universally "best" — suitability depends on your health profile, medical history, and how your body responds. Your prescriber assesses whether Mounjaro (tirzepatide), Wegovy (semaglutide), or another option is most appropriate for you personally. This is why clinical assessment is essential before any prescription.
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.