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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How Does Mounjaro Work vs Wegovy?
Mounjaro and Wegovy are both prescription-only weight loss injections available in the UK, but they work through different mechanisms. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. Understanding how each medication works can help you and your prescriber decide which might be most suitable for your individual health needs. Both require a clinical assessment before prescription.
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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.
Mechanism Differences: Dual Action vs Single Action
Mounjaro (tirzepatide) activates two natural hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism means it works on two different appetite control systems in your body, potentially offering a more comprehensive effect on hunger and food intake.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. It signals fullness, reduces appetite, and slows gastric emptying, but does not engage the GIP system.
The clinical difference: in the SURMOUNT-1 trial, Mounjaro at 15 mg showed average weight loss of approximately 20.9% of body weight, whilst in the STEP-1 trial, Wegovy at 2.4 mg (the maximum dose) showed average weight loss of approximately 14.9% of body weight. However, individual results vary significantly, and both work alongside a reduced-calorie diet and increased physical activity.
- Mounjaro: dual GIP+GLP-1 action
- Wegovy: GLP-1 action only
- Both require weekly injections
- Both slow stomach emptying
Clinically BackedWeight Loss
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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 Is Right for You? Individual Suitability
Choosing between Mounjaro and Wegovy is not about one being "better" — it's about which is more suitable for your individual health profile, medical history, and prescriber assessment. Both are licensed for UK adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
Your prescriber will consider factors including your current medications, any previous weight loss attempts, stomach or digestion issues, and your personal health goals. Some people respond better to one mechanism than another, and this cannot be predicted in advance. Access is available through NHS specialist weight management services or private online prescribing following a clinical assessment. Try our free GLP-1 weight loss calculator to explore potential outcomes at different dose levels.
- 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.
Does Mounjaro work better than Wegovy?
Clinical trials show different average weight loss percentages, but "better" depends on individual factors. Mounjaro showed ~20.9% average loss (SURMOUNT-1) vs Wegovy's ~14.9% (STEP-1), but individual results vary widely. Your prescriber will assess which suits your health profile best. Both are effective when used alongside lifestyle changes.
What does the dual GIP and GLP-1 action in Mounjaro mean?
Mounjaro activates two separate appetite control systems in your body simultaneously. GIP and GLP-1 are natural hormones that signal fullness and regulate hunger. This dual action may create a more comprehensive effect on appetite than single-pathway medications like Wegovy, though individual responses differ.
How often do you inject Mounjaro versus Wegovy?
Both are once-weekly injections. Mounjaro is administered as a pre-filled KwikPen device, as is Wegovy. The injection frequency is the same; the difference is in the mechanism and dosing schedule. Both require consistent weekly dosing for effectiveness.
Can you switch from Wegovy to Mounjaro?
Switching between medications must be managed by your prescriber. There is no official established conversion between them. Your GP or weight management clinic will assess whether a switch is appropriate and how to do it safely, including any transition period needed.
How long does it take to see results with Mounjaro?
Most people notice changes in appetite within the first few weeks, though weight loss typically becomes visible after 4–8 weeks of consistent use. Full effects develop over weeks 13–20 as doses are gradually increased. Individual timelines vary; some see faster results than others.
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.