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 — Which Weight Loss Injection?
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections licensed in the UK, but they work differently and suit different people. Mounjaro uses dual GIP and GLP-1 receptor activation, whilst Wegovy uses GLP-1 only. Clinical trial data shows both can support weight loss alongside a reduced-calorie diet and increased physical activity. The right choice depends on individual health factors, assessed by a licensed prescriber following a full clinical evaluation. This guide compares the two to help you understand the key differences.
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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 Differ — Clinical Comparison
Mechanism: Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways. Wegovy (semaglutide) is a GLP-1 receptor agonist only. This dual action in Mounjaro may explain slightly higher average weight loss in clinical trials, though individual results vary significantly.
SURMOUNT vs STEP trial data: In the SURMOUNT-1 trial, Mounjaro at 15 mg produced average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy at 2.4 mg produced average weight loss of approximately 14.9% at 68 weeks. Both trials included reduced-calorie diet and increased physical activity. These are population averages — your personal results depend on many factors.
Dosing: Mounjaro ranges from 2.5 mg to 15 mg weekly. Wegovy ranges from 0.25 mg to 2.4 mg weekly. The starting doses and escalation schedules differ, as do the final maximum doses.
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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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Individual Suitability and NHS Access in the UK
Neither Mounjaro nor Wegovy is suitable for everyone. A licensed prescriber will assess your medical history, current medications, and health conditions before prescribing either one. Both require clinical assessment because they can interact with certain medications (particularly oral contraceptives and diabetes treatments) and are not appropriate during pregnancy or breastfeeding.
NHS access: NICE has approved tirzepatide (Mounjaro) for use in NHS specialist weight management services. Access is through tier 3 or tier 4 specialist services, not routine GP prescription, and varies by region. Wegovy is similarly accessed through specialist services. Waiting times can be significant. Private prescription through a UK-licensed online prescriber offers faster access for those eligible and able to pay. Use our free GLP-1 weight loss calculator to explore potential outcomes, and consult your GP about which route suits your situation.
- 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 more effective, Mounjaro or Wegovy?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss (approximately 20.9% vs 14.9% body weight), but individual results vary significantly. The "best" medication depends on your personal health, other medicines you take, and clinical assessment by a prescriber. Both support meaningful weight loss when combined with lifestyle changes.
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 — the doses are not directly equivalent. Your prescriber will advise on the appropriate transition process, which typically involves stopping one and starting the other at a suitable starting dose.
How long does it take to see weight loss with Mounjaro or Wegovy?
Weight loss typically becomes noticeable after 4–8 weeks of treatment at an effective dose. However, some people see changes sooner, others later. Consistent lifestyle changes (reduced calories, increased activity) are essential — these injections support, not replace, behaviour change.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are approved by NICE for use within NHS specialist tier 3 and 4 weight management services. Access is through referral and specialist assessment, not routine GP prescription. Waiting lists vary by region. Private prescription through a UK-licensed prescriber offers faster access for those who are eligible.
What are the main side effects of Mounjaro and Wegovy?
Both share similar common side effects: nausea, diarrhoea, constipation, vomiting, and stomach pain. These typically improve over time as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and allergic reactions. Your prescriber will discuss full side effect profiles before treatment begins.
Ready to take control of your weight loss journey?
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