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 Shot vs Wegovy — Which Is Right for You?
Both Monjaro and Wegovy are prescription-only weight loss injections available in the UK, but they work differently and have different starting doses. Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. Both may help with weight management when used alongside a reduced-calorie diet and increased physical activity. Understanding the differences can help you and your prescriber decide which might be suitable for you. This guide compares both medications factually, using clinical trial data and UK prescribing information.
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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 (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may enhance appetite regulation and support feeling fuller after eating. In the SURMOUNT-1 clinical trial, participants taking 15 mg Monjaro lost an average of approximately 20.9% of their body weight over 72 weeks, alongside diet and exercise.
Wegovy (semaglutide) activates the GLP-1 receptor pathway only. In the STEP-1 clinical trial, participants taking 2.4 mg Wegovy lost an average of approximately 14.9% of their body weight over 68 weeks. Both weight loss injections work by reducing appetite, slowing gastric emptying, and supporting metabolic regulation.
- Monjaro: dual GIP + GLP-1 mechanism; SURMOUNT trial showed ~20.9% average loss at maximum dose
- Wegovy: single GLP-1 mechanism; STEP trial showed ~14.9% average loss at maximum dose
- Individual results vary significantly — trial data represents averages, not assured outcomes
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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, Administration, and Choosing Between Them
Monjaro starts at 2.5 mg once weekly and escalates every 4 weeks up to a maximum of 15 mg. Wegovy starts at 0.25 mg and escalates over weeks up to 2.4 mg. Both are once-weekly injections using a pre-filled pen. Choosing between them depends on individual factors including your BMI, any weight-related health conditions, previous medication responses, and prescriber clinical judgment. Neither medication is superior — suitability is personal. Your GP or a licensed prescriber will assess whether Monjaro or Wegovy is clinically appropriate for you based on your health history, current medications, and weight management goals. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses, then discuss options with your prescriber.
- 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.
What's the main difference between Monjaro and Wegovy?
Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) activates GLP-1 only. This dual mechanism in Monjaro may enhance appetite regulation, which is why trial data showed higher average weight loss (~20.9% vs ~14.9%). However, individual results vary — the "best" option depends on your personal health profile and prescriber assessment.
Which injection causes fewer side effects?
Both medications cause similar common side effects including nausea, diarrhoea, constipation, and stomach discomfort, particularly in the first few weeks. Side effect profiles are comparable, though individual tolerance varies. Common effects often reduce after a few weeks as your body adjusts. Always discuss side effect concerns with your prescriber.
How do I know which one my GP will prescribe?
Your GP or prescriber will consider your BMI, weight-related health conditions, previous treatment responses, and any contraindications. Both are prescription-only and require clinical assessment. Neither has automatic priority on the NHS — availability depends on your regional health service. Private prescribing offers access to both options without NHS waiting lists.
Can I switch from Wegovy to Monjaro or vice versa?
Yes, switching is possible but must be managed by your prescriber. There's no official established conversion chart between the doses, so switching typically involves stopping one medication and starting the other at its starting dose. Your prescriber will guide you safely through any transition.
Do I need a special prescription for Monjaro as a "shot"?
"Monjaro shot" refers to the weekly injection form of tirzepatide — it's the standard Monjaro delivery method (KwikPen device). You'll need a UK prescription from a licensed prescriber (GP or specialist). Private online prescribing is available if NHS access is delayed. Storage requires refrigeration until first use.
Ready to take control of your weight loss journey?
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