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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Monjaro Pens vs Wegovy — What's the Difference?
Both Monjaro and Wegovy are prescription-only weekly weight loss injections available in the UK, but they work differently. Monjaro (tirzepatide) uses a dual GIP and GLP-1 receptor agonist mechanism, whilst Wegovy (semaglutide) is a GLP-1 receptor agonist only. Clinical trial data shows different average weight loss outcomes, and eligibility criteria are identical. Your GP or prescriber will assess which is more suitable for you based on your individual health profile. This guide explains the key differences to help you understand your options.
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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 Monjaro and Wegovy Work — Mechanism and Trial Data
Monjaro pens contain tirzepatide, a dual GIP and GLP-1 receptor agonist. This dual mechanism activates two natural hormone pathways, potentially offering a different approach to appetite regulation and weight management compared to single-mechanism medications. The SURMOUNT-1 clinical trial showed average weight loss of approximately 15% to 21% body weight over 72 weeks, depending on dose (5 mg to 15 mg weekly).
Wegovy pens contain semaglutide, a GLP-1 receptor agonist only. It activates the GLP-1 hormone pathway, signalling fullness to the brain and slowing gastric emptying. The STEP-1 clinical trial showed average weight loss of approximately 14.9% body weight over 68 weeks at the maximum dose of 2.4 mg weekly.
- Monjaro: dual mechanism, more escalation doses available (up to 15 mg)
- Wegovy: single mechanism, maximum dose 2.4 mg
- Both require reduced-calorie diet and increased physical activity for best results
- Individual results vary significantly between users
Neither medication assures specific weight loss outcomes — clinical trial data represents averages alongside lifestyle changes.
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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.
Which Pen Is Right for You? — NHS Access and Individual Assessment
Both Monjaro and Wegovy have identical UK eligibility criteria: BMI of 30 kg/m² or above, or BMI of 27 to 29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease, or sleep apnoea). Neither medication should be chosen based on marketing or preference alone — your prescriber will assess clinical suitability, medical history, contraindications, and medication interactions.
Access via NHS specialist weight management services is through tier 3 services following GP referral, though waiting times vary by region. Private online prescribing is also available through UK-licensed prescribers and GPhC-registered pharmacies. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses, and our BMI calculator to confirm your starting point. Your prescriber will take your complete health picture into account before recommending either option.
- 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 injection causes less nausea — Monjaro or Wegovy?
Both can cause nausea, especially when starting or increasing doses. Nausea is listed as "very common" for both medications. Individual tolerance varies significantly. Your prescriber can discuss side effect profiles and dose escalation strategies to minimise nausea. Starting at the lowest dose and increasing gradually typically helps.
Can I switch from Monjaro pens to Wegovy or vice versa?
Yes, switching between medications is possible but must be managed by your prescriber. Approximate dose equivalences exist (Monjaro 5 mg ≈ Wegovy 0.5 mg; Monjaro 15 mg ≈ Wegovy 2.4 mg), but there is no official conversion chart. Any switch requires clinical assessment and individualised guidance.
How much weight will I lose with Monjaro or Wegovy pens?
Clinical trials show average weight loss of 14.9% to 21% body weight, depending on the medication and dose. However, individual results vary substantially. Some people lose more, some less. Weight loss depends on adherence, diet, physical activity, genetics, and medical factors. Your prescriber cannot assures specific outcomes.
Are Monjaro pens or Wegovy available on the NHS in the UK?
Yes, both are NHS-eligible through specialist tier 3 weight management services. Access requires GP referral and assessment by specialist services. Availability and waiting times vary significantly by region and ICB. NICE has approved both for NHS use, but local services determine prescribing decisions.
What is the main difference between tirzepatide and semaglutide?
Tirzepatide (Monjaro) is a dual GIP and GLP-1 receptor agonist, whilst semaglutide (Wegovy) is a GLP-1 receptor agonist only. The dual mechanism in Monjaro may provide a different approach to appetite regulation. Both require prescription and clinical assessment before use.
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.