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 — What Your GP Needs to Know
If your GP has discussed weight loss injections with you, you may have heard about both Mounjaro and Wegovy. Both are prescription-only GLP-1-based medications licensed in the UK for weight management, but they work slightly differently and have different evidence bases. Understanding the key differences can help you and your GP decide which might be suitable for your individual circumstances. This guide compares both medications factually, using data from clinical trials and UK licensing information.
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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 — The Key Difference
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one natural hormone pathway to regulate appetite and blood sugar. It's manufactured by Novo Nordisk and is given as a once-weekly injection, with doses ranging from 0.25 mg up to 2.4 mg. Clinical trial data from the STEP trials showed participants lost an average of approximately 15% of their body weight over 68 weeks when using the maximum 2.4 mg dose, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways instead of one. This dual mechanism may support greater appetite regulation and metabolic effects. Made by Eli Lilly, Mounjaro is also a once-weekly injection but uses a different dose titration schedule, starting at 2.5 mg and escalating up to 15 mg. The SURMOUNT-1 trial showed participants using the maximum 15 mg dose lost an average of approximately 21% of their body weight over 72 weeks, again alongside lifestyle changes.
- Wegovy: single mechanism (GLP-1 only)
- Mounjaro: dual mechanism (GIP + GLP-1)
- Both require clinical assessment before prescription
- Both are prescription-only in the UK
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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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NHS Access and Your GP's Role in Deciding
NICE has approved both tirzepatide (Mounjaro) and semaglutide (Wegovy) for use within NHS specialist weight management services. However, NHS access is typically through tier 3 specialist services, not standard GP prescription — your GP would refer you to a specialist weight management centre, and waiting times vary significantly by region. Private access through GPhC-registered online prescribing services is available without NHS referral.
Your GP or prescriber will assess which medication may suit you based on your individual health profile, including any existing conditions, other medications, and your medical history. Neither medication is assured to work, and individual results vary significantly. Your prescriber will also consider cost, availability in your area, and any contraindications specific to you. Both require a commitment to dietary changes and physical activity for best results. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to track your baseline.
- 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 is better — Mounjaro or Wegovy?
Neither is universally "better" — both are effective weight loss medications with strong clinical evidence. Mounjaro showed slightly greater average weight loss in trials (21% vs 15%), but both work best alongside lifestyle changes. Your GP will assess which is more suitable based on your individual health, other medications, and personal circumstances. Results vary between individuals.
Can I switch from Wegovy to Mounjaro on the NHS?
Switching between medications is possible but requires prescriber assessment. NHS access depends on your local tier 3 specialist service and regional availability. If you're accessing privately, you can discuss switching with your prescriber. Any switch must be managed medically — do not stop or change medication without guidance.
What's the difference between Mounjaro and Ozempic?
Both contain semaglutide but serve different purposes. Ozempic is licensed for type 2 diabetes (up to 2 mg doses) and is not approved for weight loss in the UK. Wegovy is the licensed weight management version (up to 2.4 mg). Using Ozempic for weight loss is off-label. Your GP will prescribe the appropriate licensed product for your specific need.
How does my GP decide between Mounjaro and Wegovy?
Your GP or specialist will consider your BMI, weight-related health conditions, other medications (especially oral contraceptives or diabetes drugs), allergies, medical history, and local availability. Both require clinical assessment. Cost and regional NHS provision also influence the decision. Individual suitability varies, so personal discussion with your healthcare provider is essential.
Are side effects different between Mounjaro and Wegovy?
Both share similar common side effects — nausea, diarrhoea, constipation, and stomach discomfort are most frequent. Individual tolerance varies. Mounjaro users in trials reported slightly different patterns due to its dual mechanism, but side effects are generally manageable and often reduce over time as the body adjusts. Report any concerns to your prescriber.
Ready to take control of your weight loss journey?
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