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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Starting Mounjaro vs Wegovy — Which Is Right for You?
If you're considering starting Mounjaro or Wegovy for weight management in the UK, you'll want to understand the key differences between these two prescription injections. Both are licensed for weight loss in adults, but they work differently and have distinct trial outcomes. This guide explains the mechanisms, clinical trial data, doses, and how your GP or prescriber will help determine which might be more suitable for your individual situation.
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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 — Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways in the body. This dual mechanism means it may work on appetite regulation and blood sugar control in two complementary ways. The SURMOUNT-1 clinical trial showed average weight loss of approximately 15% at 5mg, 19.5% at 10mg, and 20.9% at 15mg over 72 weeks, alongside reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it targets a single hormone pathway. The STEP-1 clinical trial demonstrated average weight loss of approximately 14.9% body weight at 68 weeks on the maximum 2.4mg dose, also with lifestyle changes.
The key difference: Mounjaro's dual mechanism may produce slightly higher average weight loss in clinical trials, but individual results vary significantly. Your GLP-1 weight loss calculator can help you explore projected outcomes at different doses based on trial averages.
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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.
Doses, Side Effects, and Prescriber Choice — What Determines Suitability?
Mounjaro doses range from 2.5mg to 15mg once weekly, whilst Wegovy escalates from 0.25mg to 2.4mg weekly. Side effects are similar for both — nausea, diarrhoea, constipation, and stomach pain are common, particularly during dose escalation. Neither is superior; both require prescription and clinical assessment. Your prescriber will consider your medical history, other medications, BMI, and weight-related health conditions to recommend which is more appropriate for you. Both are available through NHS specialist weight management services (with potential waiting lists) or private online prescribing following an assessment. The choice is highly individual — there is no one-size-fits-all answer.
- 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 causes more side effects — Mounjaro or Wegovy?
Side effect profiles are very similar for both medications. Nausea and diarrhoea are common during the first few weeks and dose increases. Individual tolerance varies significantly. Your prescriber will discuss potential side effects and how to manage them before you start treatment.
Does Mounjaro cause faster weight loss than Wegovy?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss (around 20% vs 15% at maximum doses), but individual results vary widely. Some people respond better to Wegovy; others to Mounjaro. Results depend on your metabolism, lifestyle adherence, and individual biology — not assured outcomes.
Can I switch from Wegovy to Mounjaro if I'm not seeing results?
Yes, switches between medications are possible, but only under prescriber guidance. Your GP or specialist can discuss switching if one medication isn't working well for you. Any switch requires a new assessment and management plan — never stop and start without medical supervision.
How much does each cost privately in the UK?
Private costs vary by provider and dose but typically range from £129–£320 per month. Mounjaro and Wegovy are similar in price. NHS access is free but involves specialist referral and varies significantly by region due to commissioning differences.
How long does it take to see weight loss on either medication?
Most people notice appetite reduction within the first 2–4 weeks, but measurable weight loss typically appears after 4–8 weeks. You can track your progress using our free progress tracker. Remember that results take time — consistent effort over months is normal for sustainable weight loss.
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.