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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Semaglutide Wegovy vs Mounjaro — How Do They Compare?
If you're exploring prescription weight loss injections in the UK, you've probably encountered both Wegovy (semaglutide) and Mounjaro (tirzepatide). Both are licensed for weight management and work similarly — but they're not identical. This guide compares them factually, covering their mechanisms, clinical trial results, dosing, side effects, and what that means for your individual suitability. Neither is universally "better" — the right choice depends on your personal health profile and a prescriber's assessment.
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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 Wegovy and Mounjaro Work Differently
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one hormone pathway in your body. GLP-1 (glucagon-like peptide-1) is a natural hormone that helps regulate appetite, blood sugar, and how quickly food leaves your stomach. Semaglutide mimics and amplifies these signals, helping you feel fuller after smaller meals and reducing food cravings.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. In addition to GLP-1, it also activates GIP (glucose-dependent insulinotropic polypeptide), another appetite-regulating hormone. This dual mechanism is what makes Mounjaro different from other weight loss injections including semaglutide.
Clinically, this means Mounjaro works on two biological targets where Wegovy works on one. Whether this translates to a meaningful difference for you personally depends on how your body responds — and that's why individual results vary significantly between the two medications.
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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 Data and Dosing Comparison
In the STEP-1 clinical trial, Wegovy at its maximum dose of 2.4 mg resulted in an average weight loss of approximately 14.9% of body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity. In the SURMOUNT-1 trial, Mounjaro at 15 mg (maximum dose) achieved an average weight loss of approximately 20.9% over 72 weeks under the same lifestyle conditions.
However, these are averages — individual weight loss varies considerably with both medications. Wegovy escalates from 0.25 mg to 2.4 mg over 16 weeks. Mounjaro escalates from 2.5 mg to 15 mg over approximately 20 weeks. Both are once-weekly injections. Your prescriber will determine the appropriate dose for you based on your response and tolerance.
Both medications are prescription-only and require a clinical assessment before being prescribed. Your GP or a licensed prescriber will evaluate your BMI, weight-related health conditions, medical history, and current medications to determine which — if either — is suitable for you. Use our free GLP-1 weight loss calculator to estimate potential weight loss projections with different GLP-1 medications, and our BMI calculator to check where you currently stand against NHS healthy weight guidelines. The choice between Wegovy and Mounjaro is ultimately a clinical decision made with your prescriber, not a consumer choice based on trial averages.
- 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.
Is Mounjaro always more effective than Wegovy because the trial data shows higher average weight loss?
No. Clinical trial averages don't predict individual outcomes. Some people respond better to Wegovy, others to Mounjaro. Factors like your metabolism, adherence, lifestyle, and how your body responds to each medication matter more than trial averages. Your prescriber will assess which suits you.
Can I switch from Wegovy to Mounjaro if I'm not seeing results?
Possibly, but this is a prescriber decision. Don't switch medications without clinical guidance. It typically takes 12–16 weeks to assess response to a medication at therapeutic dose. If you're not progressing, discuss this with your GP or prescriber — they may adjust your dose, check for adherence issues, or discuss alternatives.
What's the main difference between semaglutide in Wegovy vs Ozempic?
Both contain semaglutide, but Wegovy is licensed for weight management and goes up to 2.4 mg. Ozempic is licensed for type 2 diabetes and goes up to 2 mg. Using Ozempic for weight loss is off-label in the UK. Wegovy is the licensed weight management version.
Do Wegovy and Mounjaro have different side effects?
Both cause similar common side effects: nausea, diarrhoea, constipation, vomiting, and stomach pain — especially early in treatment. Serious side effects (pancreatitis, gallbladder problems, severe allergic reactions) are possible but rare with both. Your prescriber will discuss individual risk.
How do I access Wegovy or Mounjaro on the NHS in the UK?
Through referral to a specialist tier 3 or tier 4 weight management service. Availability and waiting times vary significantly by region (ICB). Your GP can refer you if you meet eligibility criteria. Private access is available through GPhC-registered online prescribing services without NHS referral, but costs apply.
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.