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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The Difference Between Mounjaro and Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and have different strength profiles. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1-only medication. In clinical trials, both helped people lose significant amounts of weight alongside diet and exercise, though results vary considerably between individuals. This guide explains how they differ, what each can do, and how to access them in the UK.
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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) activates two hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite more comprehensively and support fat utilisation. Wegovy (semaglutide) activates only the GLP-1 pathway, which reduces appetite and helps you feel fuller after eating smaller portions.
In the SURMOUNT-1 clinical trial, Mounjaro at 15mg produced an average weight loss of approximately 20.9% of body weight over 72 weeks. The STEP-1 trial showed Wegovy at 2.4mg resulted in approximately clinically studied weight loss over 68 weeks. However, individual results vary significantly, and these figures represent averages alongside reduced-calorie diet and increased physical activity. Use our GLP-1 weight loss calculator to see projected results at different doses.
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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, Access, and Practical Differences
Dosing: Mounjaro starts at 2.5mg and increases up to a maximum of 15mg weekly. Wegovy starts at 0.25mg and increases to 2.4mg weekly. Both are once-weekly injections using a pre-filled pen — Mounjaro uses a KwikPen device.
UK access: Both medications are prescription-only and require clinical assessment. NHS access is through specialist tier 3 weight management services (though availability varies by region and waiting lists can be lengthy). Private access is available through GPhC-registered online pharmacies following an online prescription assessment. Important: Avoid confusion with Ozempic — it also contains semaglutide but is licensed only for type 2 diabetes, not weight management. Wegovy is the licensed weight loss version.
- 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 is more effective — Mounjaro or Wegovy?
Clinical trials show Mounjaro produced slightly higher average weight loss (20.9% at maximum dose vs 14.9% for Wegovy), but results vary significantly between individuals. Neither is universally "better" — effectiveness depends on your body's response. Your prescriber will assess which is more suitable for you based on your medical history and health conditions.
Can you switch from Wegovy to Mounjaro?
Yes, you can switch between medications, but this must be managed by your prescriber. There is no official conversion chart — approximate equivalence suggests Mounjaro 10mg is roughly comparable to Wegovy 1.7mg, though switching requires individualised clinical guidance and a new assessment.
Are the side effects the same for both?
Both share common side effects (nausea, diarrhoea, constipation, vomiting, stomach pain), but severity and individual tolerance vary. Some people tolerate one medication better than the other. Side effects typically ease as your body adjusts to each dose increase over 4 weeks.
Can I get either on the NHS in the UK?
Both Mounjaro and Wegovy are approved by NICE for NHS use within specialist weight management services (tier 3). However, NHS availability is limited and varies significantly by region. You can also access either privately through an online prescription assessment with a UK-licensed prescriber and GPhC-registered pharmacy.
How long do you take Mounjaro or Wegovy for?
Both are intended as ongoing treatments — you continue as long as the medication is helping you manage your weight alongside lifestyle changes. If you stop, weight may return over time. Your prescriber will review your progress regularly and discuss whether to continue, adjust dose, or try an alternative approach.
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.