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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GLP-1 vs Mounjaro: Understanding the Difference
Both GLP-1 medications and Mounjaro (tirzepatide) are prescription-only weight loss injections licensed in the UK, but they work differently. GLP-1 receptor agonists like Wegovy activate one hormone pathway, whilst Mounjaro uses a dual GIP and GLP-1 mechanism. This guide explains the key differences, clinical trial results, dosing schedules, and how your prescriber decides which is right for you. Both require a clinical assessment before prescription. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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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 GLP-1 and Mounjaro Work Differently
GLP-1 receptor agonists (such as Wegovy, containing semaglutide) activate the GLP-1 hormone pathway in the body. This helps reduce appetite, increase feelings of fullness, and slow how quickly food leaves the stomach.
Mounjaro (tirzepatide) is a dual mechanism medication — it activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 pathways simultaneously. This dual action may provide stronger appetite suppression and support for weight loss compared to GLP-1 alone.
- GLP-1 only: proven effective, single-mechanism approach
- Mounjaro dual GIP+GLP-1: newer mechanism, addresses two hormone pathways at once
- Both require once-weekly injections (Wegovy as GLP-1 example; Mounjaro available as the KwikPen)
Clinical trials suggest Mounjaro may deliver slightly greater average weight loss, though individual results vary significantly. Your prescriber will assess which mechanism suits your health profile.
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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 Dosing Comparison
The SURMOUNT-1 trial (Mounjaro) showed average weight loss of approximately 20.9% of body weight at the maximum 15 mg dose over 72 weeks. The STEP-1 trial (Wegovy, a GLP-1) demonstrated average weight loss of approximately 14.9% at the maximum 2.4 mg dose over 68 weeks. Both used reduced-calorie diet and increased physical activity alongside medication.
Mounjaro dosing: 2.5 mg up to 15 mg weekly. GLP-1 dosing: 0.25 mg up to 2.4 mg weekly. Dose escalation takes 16–20 weeks for Mounjaro versus 16 weeks for Wegovy. Both are prescription-only and require a prescriber's assessment to determine your starting dose and escalation pathway.
- 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, GLP-1 or Mounjaro?
Clinical trials show Mounjaro (tirzepatide) resulted in slightly higher average weight loss (~21%) than GLP-1 alone (~15%) in their respective studies. However, individual results vary significantly, and effectiveness depends on your body's response, adherence, diet, and exercise. Your prescriber will recommend based on your personal health profile, not trial averages.
Can I switch from GLP-1 to Mounjaro or vice versa?
Yes, switching is possible and sometimes done in clinical practice. However, any medication change must be managed by your prescriber. There is no official NHS conversion chart — your prescriber will assess your response on the current medication and adjust accordingly. Never switch without medical guidance.
Is Mounjaro available on the NHS in the UK?
Mounjaro (tirzepatide) has been approved by NICE for NHS use within specialist tier 3 weight management services. However, NHS availability varies significantly by region and waiting lists can be substantial. Private prescription is available through online prescribers following a clinical assessment.
What are the main side effects of GLP-1 and Mounjaro?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain — especially when starting or increasing doses. These typically improve over time as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and allergic reactions. Your prescriber will discuss side effect management and when to seek urgent help.
How do I know which medication my GP or prescriber will recommend?
Your prescriber will assess your BMI, weight-related health conditions, previous weight loss attempts, medication allergies, and other health factors. Both GLP-1 and Mounjaro require BMI ≥30 or BMI 27–29.9 with weight-related conditions. Individual suitability, local availability, and your health profile determine which is recommended. Always discuss options with your prescriber.
Ready to take control of your weight loss journey?
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