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 — How Do They Compare?
If you're exploring prescription weight loss options in the UK, you've likely encountered both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are licensed for weight management and work via similar pathways, but there are meaningful differences in how they function, their dosing schedules, and what clinical trials showed. This guide compares both medications factually so you can understand the key distinctions — though any decision between them requires a clinical assessment from a licensed prescriber.
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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 Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. This means it activates two hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). The dual mechanism may help regulate appetite, reduce food cravings, and support feelings of fullness after meals.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates the GLP-1 pathway alone. Both approaches slow gastric emptying and signal satiety to the brain, but the additional GIP action in Mounjaro is theoretically why some clinical evidence suggests it may produce slightly higher average weight loss, though individual results vary significantly.
- Mounjaro: once-weekly injection, starting dose 2.5 mg, maximum 15 mg
- Wegovy: once-weekly injection, starting dose 0.25 mg, maximum 2.4 mg
- Both are prescription-only and require clinical assessment before prescribing
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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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Clinical Trial Results and Individual Suitability
In the SURMOUNT-1 clinical trial, Mounjaro at maximum dose (15 mg) showed average weight loss of approximately 20.9% of body weight over 72 weeks. The STEP-1 trial for Wegovy demonstrated average weight loss of approximately 14.9% at the 2.4 mg dose over 68 weeks. Both studies included participants following a reduced-calorie diet and increased physical activity — results are not achieved by injection alone.
These are clinical trial averages. In real-world use, outcomes vary based on individual metabolism, adherence, diet, and exercise. A prescriber will assess which medication (if either) suits your specific health profile, medical history, and weight management goals. Neither medication is universally "better" — suitability is personal.
Use our free GLP-1 weight loss calculator above to explore potential weight loss projections at different doses of either medication. This can give you a personalised estimate to discuss with your prescriber. Our BMI calculator also helps you understand your starting point and target healthy weight ranges according to NHS guidelines.
- 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.
Which medication causes more side effects — Mounjaro or Wegovy?
Both cause similar common side effects: nausea, diarrhoea, constipation, and stomach pain. These are most pronounced during dose escalation and often improve over time. Severity varies between individuals. Your prescriber will monitor you and adjust dosing if side effects are problematic. No evidence suggests one is universally better tolerated than the other.
Can I switch from Mounjaro to Wegovy or vice versa?
Yes, switching is possible and sometimes done in clinical practice. However, there is no official NHS conversion chart — any switch must be managed by your prescriber. Approximate equivalences exist (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg), but individual response varies. Discuss with your prescriber if you're considering a switch.
Is Mounjaro available on the NHS in the UK?
Mounjaro is approved for NHS use within specialist tier 3 weight management services. However, availability varies significantly by region and ICB. Waiting lists can be lengthy. Private access via UK-licensed prescribers and GPhC-registered pharmacies is available without an NHS referral, though there is a cost.
Do I need to take Mounjaro or Wegovy forever?
Both medications are typically continued long-term if weight loss goals are met and side effects are manageable. Some evidence suggests weight regains if treatment is stopped. Your prescriber will discuss duration, review progress regularly, and help you make informed decisions about continuing, pausing, or stopping treatment.
What's the difference between Mounjaro and Ozempic?
Both contain semaglutide but are different formulations. Mounjaro is tirzepatide (not semaglutide). Ozempic is approved for type 2 diabetes and contains semaglutide at lower doses (up to 2 mg). Wegovy is also semaglutide but at the weight management dose (up to 2.4 mg). In the UK, Wegovy is the licensed semaglutide product for weight loss; using Ozempic for weight management is off-label.
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