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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What's the Difference Between Wegovy and Mounjaro?
Both Mounjaro and Wegovy are prescription-only weight loss injections licensed in the UK, but they work differently and come from different manufacturers. Understanding the key differences — mechanism, dosing, clinical trial results, and how they're prescribed — helps you have an informed conversation with your GP or prescriber. This guide breaks down the main distinctions so you can understand what each medication does and why one might be more suitable for you than the other.
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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 Differently
The fundamental difference between these two medications is their mechanism of action. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways in your body simultaneously. This dual action may help regulate appetite, reduce food cravings, and support feeling fuller after eating.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates just the GLP-1 hormone pathway. Both medications slow how quickly food leaves your stomach and signal fullness to your brain, but Mounjaro's dual mechanism means it works on two pathways rather than one.
- Mounjaro: GIP + GLP-1 agonist (dual mechanism)
- Wegovy: GLP-1 agonist only (single mechanism)
In clinical practice, this means Mounjaro and Wegovy may have slightly different effects on appetite, blood sugar control, and weight loss outcomes — though individual responses vary significantly.
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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, Clinical Trial Results, and Prescribing
Mounjaro starts at 2.5 mg once weekly and escalates up to a maximum of 15 mg. In the SURMOUNT-1 clinical trial, participants on the highest dose lost an average of approximately 20.9% of their body weight over 72 weeks alongside diet and exercise changes.
Wegovy starts at 0.25 mg and escalates up to a maximum of 2.4 mg once weekly. The STEP-1 clinical trial showed average weight loss of around 14.9% of body weight at 68 weeks on the maximum dose.
These are trial averages — your GP will help determine which dose and medication is appropriate for your individual circumstances. Both require a prescription following clinical assessment, and both work best alongside reduced-calorie diet and increased physical activity. Your prescriber considers your medical history, other medications, and personal health goals when deciding which is right for you.
- 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 medication causes fewer side effects, Mounjaro or Wegovy?
Both have similar common side effects: nausea, diarrhoea, constipation, and stomach pain. Individual experiences vary widely. Your GP can discuss which medication might suit your tolerance profile based on your health history.
Can I switch between Mounjaro and Wegovy?
Switching between these medications must always be managed by your prescriber. There is no official conversion chart — your GP or prescriber will adjust doses appropriately if a switch is needed. Any change requires clinical guidance.
Which is available on the NHS in the UK?
NICE has approved both tirzepatide (Mounjaro) and semaglutide (Wegovy) for NHS use within specialist tier 3 weight management services. However, NHS access varies significantly by region and waiting times can be substantial. Private prescribing is also available through UK-licensed online prescribers.
Do I need a GP referral for either medication?
For NHS access, you typically need a GP referral to a specialist weight management service. For private prescribing, you can access an online clinical assessment directly without a referral. Either way, both medications require a prescription from a UK-licensed prescriber following assessment.
How do I know which one my prescriber will recommend?
Your prescriber will consider your BMI, medical history, other medications, weight-related health conditions, and personal preferences. Both are suitable for adults with BMI ≥30, or BMI 27–29.9 with weight-related health complications. They'll discuss which is more appropriate for your individual situation.
Ready to take control of your weight loss journey?
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