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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What's the Difference Between Wegovy and Mounjaro?
Both Wegovy and Mounjaro are prescription-only weight loss injections licensed in the UK, but they work in different ways and have distinct dosing schedules. Wegovy (semaglutide) is a GLP-1 receptor agonist, while Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — this fundamental difference affects how they work and their average weight loss results. If you're considering either medication, understanding these differences is important for discussions with your GP or 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 They Work — The Key Mechanism Difference
Wegovy (semaglutide) activates a single hormone pathway — GLP-1 — which signals fullness to the brain, reduces appetite, and slows how food moves through the stomach. Mounjaro (tirzepatide) activates two pathways — both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 — creating a dual action that may provide stronger appetite suppression and additional metabolic effects.
In clinical trials, both medications helped participants lose weight when combined with a reduced-calorie diet and increased physical activity. On average, Wegovy (semaglutide) produced approximately clinically studied weight loss at the maximum dose of 2.4 mg in the STEP-1 trial. Mounjaro (tirzepatide) showed average weight loss of around 15–20.9% depending on the dose (between 5 mg and 15 mg) in the SURMOUNT-1 trial. However, individual results vary significantly — these are trial averages, not assured outcomes for everyone.
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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.
Dosing, Side Effects, and Practical Differences
Dosing: Wegovy starts at 0.25 mg weekly and escalates to a maximum of 2.4 mg over 16+ weeks. Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg over 20+ weeks. Both are once-weekly injections delivered via a pre-filled KwikPen device. Side effects are similar — both commonly cause nausea, diarrhoea, constipation, and stomach pain, especially during dose escalation. Cost and access: Both are available through NHS specialist tier 3 weight management services (with significant regional variation and waiting lists) or through private online prescribing following a clinical assessment. Both are prescription-only — eligibility is assessed individually by a licensed prescriber based on BMI and health conditions.
- 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 more effective than Wegovy for weight loss?
Clinical trials showed Mounjaro produced slightly higher average weight loss percentages (15–21%) compared to Wegovy (14.9%), but this may reflect trial design differences. Both medications work well for many people. Individual results vary significantly, and what works best depends on your personal response, health conditions, and lifestyle. Your prescriber can advise which is more suitable for you.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible and some prescribers do manage transitions between the two medications. However, there is no official established conversion between Wegovy and Mounjaro doses — any switch must be managed by your prescriber who will adjust doses carefully. Never switch medications without medical guidance.
Is Ozempic the same as Wegovy?
Both contain semaglutide, but Ozempic is licensed in the UK for type 2 diabetes management (up to 2 mg dose), while Wegovy is licensed specifically for weight management (up to 2.4 mg). Ozempic is not licensed for weight loss in the UK — using it for weight loss would be off-label prescribing. Wegovy is the approved weight management version.
How do I know which injection is right for me — Wegovy or Mounjaro?
Your GP or a licensed prescriber will assess your individual health profile, BMI, comorbidities, and medical history to determine which medication (if any) is appropriate for you. Both require a clinical assessment. Factors include your health conditions, other medications, allergies, and personal circumstances. Use our free BMI calculator and GLP-1 weight loss calculator to understand your starting point and projected outcomes.
Are Wegovy and Mounjaro available on the NHS?
Both are approved by NICE for use within NHS specialist weight management services (tier 3), but availability varies significantly by region and ICB. Many areas have long waiting lists. Private access is available through online prescribing following clinical assessment. Your GP can advise on NHS access in your area or refer you to specialist services if eligible.
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.