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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Wegovy vs Mounjaro: Comparing Weight Loss Jabs
Wegovy and Mounjaro are both prescription-only weekly weight loss injections available in the UK, but they work differently and have distinct trial data. This guide compares these two popular semaglutide weight loss jabs side by side — covering how they work, expected results, doses, and which factors your prescriber considers when deciding which is right for you. Both are licensed for weight management in UK adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Individual suitability varies, and both require clinical assessment before prescription.
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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: Mechanism and Trial Data
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates a single hormone pathway that signals fullness to the brain, slows stomach emptying, and helps regulate blood sugar. In the STEP-1 clinical trial, participants on Wegovy 2.4 mg (the maximum dose) lost an average of approximately 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. This dual mechanism may help with appetite control in a different way. In the SURMOUNT-1 trial, participants on Mounjaro 15 mg (maximum dose) lost an average of approximately 20.9% of body weight over 72 weeks with the same lifestyle support.
Both injections are administered once weekly. Clinical trials showed different average weight loss percentages, though individual results vary significantly and depend on dose, adherence, diet, and activity level.
- Wegovy: Single GLP-1 mechanism
- Mounjaro: Dual GIP + GLP-1 mechanism
- Both: Once-weekly injectable, prescription-only, require clinical assessment
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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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Doses, Side Effects, and Prescriber Considerations
Wegovy escalates from 0.25 mg to a maximum of 2.4 mg over approximately 16 weeks. Mounjaro starts at 2.5 mg and escalates to 15 mg over approximately 20 weeks. Both can cause nausea, diarrhoea, constipation, and appetite loss — especially during dose increases. Your GP or prescriber will assess which medication suits your health profile, existing conditions, and any medications you take. Neither is automatically "better" — suitability depends on your individual circumstances. Both are available through NHS specialist weight management services (with potential waiting lists) or private prescribing following an online clinical assessment. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your starting point for eligibility context.
- 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 participants lost slightly more weight on average (20.9% vs 14.9%), but individual results vary widely. The "best" option depends on your health profile, not trial averages. Your prescriber will assess which is more suitable for you personally. Both are effective when combined with diet and activity changes.
What's the difference between the GLP-1 in Wegovy and the GIP+GLP-1 in Mounjaro?
Wegovy activates one hormone pathway (GLP-1) which signals fullness and slows digestion. Mounjaro activates two pathways (GIP and GLP-1) simultaneously, potentially offering a different appetite-regulation effect. Both approaches have strong trial evidence, but they work via different mechanisms.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between these medications is possible but must be managed by your prescriber. There is no official conversion chart — any change requires a new clinical assessment and prescription. Never switch without medical guidance.
How do I access Wegovy or Mounjaro on the NHS in the UK?
Both are available through NHS tier 3 specialist weight management services — you'll need a GP referral. Availability and waiting lists vary significantly by region (ICB). Private access is available through GPhC-registered online prescribing following a clinical assessment, though costs apply.
Which weekly weight loss injection should I choose?
You don't choose — your prescriber does, based on your health history, BMI, existing conditions, other medications, and clinical suitability. Both are prescription-only and require a full assessment. Discuss your concerns and health profile with your GP or prescriber to understand their recommendation.
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.