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: Week 4 Comparison
At week 4, you'll be on your Mounjaro starting dose or beginning to escalate. But how does Mounjaro compare to Wegovy, the other main GLP-1 weight loss injection available in the UK? Both are prescription-only medicines that work differently in your body. Understanding the key differences—mechanism, dosing schedule, and clinical trial results—can help you and your prescriber decide which is right for your weight management journey. Use our free GLP-1 weight loss calculator to explore projected outcomes for both medications.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. It activates two natural hormone pathways—GIP (glucose-dependent insulinotropic polypeptide) and GLP-1—which work together to regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. This dual mechanism is what sets it apart from traditional GLP-1 injections.
Wegovy (semaglutide) is a GLP-1 receptor agonist only. It activates the GLP-1 pathway, which signals fullness to the brain and slows how quickly food leaves your stomach. Both work through natural hormone systems, but Mounjaro's dual action may offer different results for some people.
- Mounjaro: dual GIP + GLP-1 mechanism; once-weekly injection
- Wegovy: GLP-1 only; once-weekly injection
- At week 4, Mounjaro users are typically on 2.5 mg (first escalation dose); Wegovy users may be on 0.5 mg or 1 mg depending on tolerance
Clinically BackedWeight Loss
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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.
Clinical Trial Results and Eligibility
In the SURMOUNT-1 trial, Mounjaro showed an average weight loss of approximately 15% body weight at 5 mg dose and 20.9% at the maximum 15 mg dose over 72 weeks. The STEP-1 trial showed Wegovy achieved approximately 14.9% average weight loss at the maximum 2.4 mg dose over 68 weeks. Both trials included diet and exercise changes alongside medication.
Individual results vary significantly—some people respond better to one medication than the other. Both require a UK-licensed prescriber's assessment to determine suitability. Your GP or private prescriber will consider your BMI, health conditions, and medication history. Neither is "better"—the right choice depends on your personal health profile and how your body responds.
- 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.
At week 4, what dose of Mounjaro should I be on?
At week 4, you'll typically be on the Mounjaro 2.5 mg dose (your starting/adjustment dose). Each dose is maintained for a minimum of 4 weeks before escalation. Your prescriber may adjust based on side effects or response. Week 5 onwards, you may escalate to 5 mg if tolerated.
Is Mounjaro or Wegovy more effective?
Clinical trials show Mounjaro achieved slightly higher average weight loss (20.9% vs 14.9%) but differences narrow when comparing equivalent therapeutic doses. Individual results vary substantially. The "best" medication depends on your personal health factors, not general superiority. Your prescriber will recommend based on your suitability.
What's the main difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 agonist; Wegovy is GLP-1 only. The dual mechanism in Mounjaro may help regulate appetite differently through two hormone pathways. Both activate natural systems in your body—the additional GIP action is what theoretically distinguishes Mounjaro's mechanism.
Can I switch from Mounjaro to Wegovy at week 4?
Switching medications is possible but should only be done under prescriber guidance. There's no official conversion chart, though approximate equivalences exist. Switching requires clinical assessment and new dosing from your prescriber—it's not a simple swap.
Which injection is available on the NHS at week 4?
Both Mounjaro and Wegovy are NHS-approved for specialist tier 3 weight management services, though regional availability varies significantly. Many NHS areas have lengthy waiting lists. Private access via UK-licensed online prescribers is faster but involves cost. Your GP can discuss both routes.
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.