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 — Weight Loss Comparison for UK Adults
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weekly weight loss injections licensed in the UK. They work differently and produce different results in clinical trials. This guide compares the two medications factually — mechanism of action, average weight loss, doses, and how your prescriber will assess which may be suitable for you. Individual results vary significantly, and neither is "better" — suitability depends on your personal health profile and clinical assessment.
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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 Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. It activates two natural hormone pathways, which may explain why clinical trial data shows stronger average weight loss. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. Both slow gastric emptying and help regulate appetite, but Mounjaro's dual action on GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 may provide additional metabolic benefit.
- Mounjaro: dual mechanism, stronger trial results on average
- Wegovy: single mechanism, well-established evidence base
- Both require weekly injections via pre-filled pen
- Both are prescription-only and require clinical assessment
In the SURMOUNT-1 clinical trial, Mounjaro at 15 mg produced average weight loss of approximately 20.9% body weight. Wegovy in the STEP-1 trial achieved approximately 14.9% average weight loss. However, these are clinical averages — individual responses vary considerably.
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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, Trials, and Which May Suit You
Mounjaro ranges from 2.5 mg to 15 mg once weekly, with 4-week intervals between dose increases. Wegovy ranges from 0.25 mg to 2.4 mg, also escalated over 16 weeks. Your prescriber will assess your health, any weight-related conditions, and medical history to determine which medication, if any, is appropriate for you. Both are prescribed alongside a reduced-calorie diet and increased physical activity — medication alone is not sufficient for weight loss. Use our GLP-1 weight loss calculator to estimate potential results at different doses, and our BMI calculator to assess your current weight status relative to NHS guidelines.
- Both require prescription from a UK-licensed prescriber
- Access via NHS specialist weight management services or private prescribing
- Clinical assessment determines suitability, not patient preference
- Results depend on adherence, diet, exercise, and individual metabolism
- 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 weight loss injection is better, Mounjaro or Wegovy?
Neither is universally "better" — suitability is individual. Mounjaro shows stronger average trial results but Wegovy has longer established safety data. Your prescriber will assess your health profile, any medical conditions, and medications to determine which may be appropriate for you. Both are effective for many people; results vary significantly.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways. Wegovy is a GLP-1 receptor agonist only. This dual action in Mounjaro may contribute to its stronger average weight loss in clinical trials (approximately 20.9% vs 14.9%), though individual results vary and not everyone responds the same way.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber. There is no official conversion chart. Any change in medication requires a new clinical assessment. Your prescriber will discuss whether switching is appropriate based on your response, tolerance, and current health status.
How much weight can I expect to lose with Mounjaro or Wegovy?
Clinical trials show averages of approximately 20.9% body weight with Mounjaro and 14.9% with Wegovy — but individual results vary widely. Some people lose more, some less. Factors include your starting weight, adherence, diet, exercise, metabolism, and how your body responds to the medication. Your prescriber can give personalised expectations.
Do I need a prescription for Mounjaro or Wegovy in the UK?
Yes, both are prescription-only medicines in the UK. You cannot purchase them without a valid prescription from a UK-licensed prescriber. Access is through NHS specialist weight management services (following GP referral) or private online prescribing services following clinical assessment. Eligibility varies by route and region.
Ready to take control of your weight loss journey?
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