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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Mounjaro vs Wegovy — How Do These Injections Compare?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only injectable medications licensed in the UK for weight management. While they work in similar ways — helping reduce appetite and support weight loss — they have key differences in their mechanism, dosing, and trial outcomes. Understanding these differences helps you have an informed conversation with your GP or prescriber about which might suit your situation. This guide compares both options based on UK clinical guidance and evidence.
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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.
Clinical Mechanism and Trial Data Compared
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and blood sugar control. In the SURMOUNT-1 clinical trial, participants on Mounjaro 15 mg lost an average of 20.9% of their body weight over 72 weeks alongside dietary changes and increased physical activity.
Wegovy (semaglutide) is a single GLP-1 receptor agonist, activating one hormone pathway. In the STEP-1 clinical trial, participants on Wegovy 2.4 mg lost an average of 14.9% of their body weight over 68 weeks with the same lifestyle support.
The dual mechanism in Mounjaro may offer slightly greater average weight loss in trials, though individual results vary significantly. Both medications are used alongside a reduced-calorie diet and increased physical activity — they are not standalone treatments. Your prescriber will assess which approach suits your health profile, as both are equally valid options.
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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, Suitability, and Prescriber Assessment
Mounjaro starts at 2.5 mg weekly and escalates to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg weekly and escalates to a maximum of 2.4 mg weekly. The different dose ranges reflect different molecular concentrations, not superiority — they are not directly interchangeable.
Both medications require a clinical assessment by a UK-licensed prescriber to determine eligibility and suitability. Your prescriber will consider your BMI, any weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease), your medical history, and any other medications you take. Neither medication is "better" — the choice depends on your individual health circumstances. Use our free GLP-1 weight loss calculator above to estimate potential outcomes with either medication, and 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.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials show Mounjaro produced slightly higher average weight loss (20.9% vs 14.9%), but individual results vary widely. Both medications are effective. Your prescriber will choose based on your health profile, not trial averages, as effectiveness differs between people.
Can I switch from Wegovy to Mounjaro or vice versa?
Any switch between medications must be managed by your prescriber. There is no official conversion chart between the two — doses are not directly equivalent. Your prescriber will assess whether switching is appropriate and adjust doses accordingly.
Which injection requires more frequent dosing — Mounjaro or Wegovy?
Both Mounjaro and Wegovy are once-weekly injections. Liraglutide (Saxenda) is the only UK-licensed weight management injection requiring daily dosing. Once-weekly injections tend to be easier to remember and fit into routines.
What are the main differences in side effects between Mounjaro and Wegovy?
Both share similar common side effects: nausea, diarrhoea, constipation, and stomach pain. Mounjaro reports slightly higher nausea in trials. Side effect severity varies between individuals. Your prescriber will discuss what to expect and how to manage side effects.
How do I know which injection my GP will prescribe if I'm eligible?
Your GP or prescriber will assess your individual health circumstances, including medical history, other medications, and any contraindications. Both are equally valid NHS choices. Private prescribers also assess suitability individually. Neither is automatically preferred — suitability is personal.
Ready to take control of your weight loss journey?
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