The UK's most complete GLP-1 guide
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- 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 for Weight Loss in the UK
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weekly weight loss injections licensed in the UK, but they work differently and have different clinical trial results. This guide compares the two medications side-by-side to help you understand the key differences — though only a prescriber can assess which, if either, is suitable for your individual health. Both require a clinical assessment before prescription and work best alongside reduced-calorie diet and increased physical activity.
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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 — Key Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways simultaneously. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) together help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. This dual action is what makes Mounjaro distinct from earlier weight loss injections.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates only the GLP-1 pathway. This single mechanism also reduces appetite and supports fullness, but works through one hormone system rather than two. Wegovy has been in clinical use longer, with established STEP trial evidence.
In practical terms, the dual mechanism means Mounjaro may work differently in your body compared to Wegovy, but individual responses vary significantly. Your prescriber will assess which mechanism suits your health profile.
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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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Clinical Trial Results — SURMOUNT vs STEP Data
The SURMOUNT-1 trial for Mounjaro showed average weight loss of approximately 15–21% of body weight over 72 weeks at the maximum 15 mg dose, depending on the specific dosing group studied.
The STEP-1 trial for Wegovy showed average weight loss of approximately 15% of body weight over 68 weeks at the 2.4 mg dose.
Both trials included participants following a reduced-calorie diet and increased physical activity. Results are clinical averages — individual outcomes vary considerably. Weight loss depends on adherence, diet, exercise, genetics, and other health factors. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, though actual results differ between individuals.
- 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.
What is the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP + GLP-1 agonist (two hormone pathways), while Wegovy is a GLP-1 agonist only (one pathway). Both reduce appetite and support weight loss, but through different mechanisms. Only a prescriber can assess which is suitable for you.
Which one causes more weight loss — Mounjaro or Wegovy?
Clinical trials showed Mounjaro achieved slightly higher average weight loss (up to 21% body weight) compared to Wegovy (approximately 15%), but these are averages from specific trials. Individual results vary significantly based on genetics, lifestyle, adherence, and health factors. Both are effective for many people.
Are Mounjaro and Wegovy both available on the NHS in the UK?
Both are licenced in the UK and available through NHS specialist tier 3 weight management services, though regional availability varies. NHS access typically requires referral from your GP and assessment by a specialist service. Waiting times can be long in many areas. Both are also available privately through UK-registered online prescribing services.
What are the starting doses for Mounjaro and Wegovy?
Mounjaro starts at 2.5 mg weekly (escalated every 4 weeks up to maximum 15 mg). Wegovy starts at 0.25 mg weekly (escalated every week to maximum 2.4 mg). Mounjaro reaches higher doses; Wegovy requires more gradual escalation steps. A prescriber will determine your starting dose and escalation schedule.
Can you switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by a prescriber. There is no official conversion chart — approximate equivalence suggests Mounjaro 5 mg ≈ Wegovy 0.5 mg, though these are not precise conversions. Any switch requires clinical reassessment and must be prescribed by your GP or specialist.
Ready to take control of your weight loss journey?
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