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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Which Is More Effective: Mounjaro or Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work slightly differently and clinical trial data shows different average outcomes. The answer to which is more effective depends on your individual circumstances, health profile, and how your body responds — but the evidence suggests Mounjaro may have a modest advantage on average. This guide compares both medications directly to help you understand the 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 Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. This dual mechanism may give it an edge in appetite suppression and blood sugar regulation.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. Both medications slow gastric emptying and help regulate appetite, but the dual action of Mounjaro may explain why clinical trial results show slightly higher average weight loss.
Your GP or prescriber can assess which mechanism might suit your health profile better. Both require a prescription following a clinical assessment 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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Clinical Trial Results: Average Weight Loss Compared
In the SURMOUNT-1 trial, Mounjaro at the maximum dose of 15 mg showed average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy at 2.4 mg showed average weight loss of approximately 14.9% at 68 weeks. However, these are averages — individual results vary widely. Factors like your starting weight, diet, exercise, adherence, and metabolism all influence real-world outcomes. Neither medication assures specific weight loss, and both work best alongside reduced-calorie diet and increased physical activity.
- 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 definitely more effective than Wegovy?
Clinical trials suggest Mounjaro may have a modest advantage on average, but individual results vary significantly. Some people respond better to Wegovy, others to Mounjaro. Effectiveness depends on your personal metabolism, adherence, lifestyle changes, and how your body responds. Your prescriber can help choose the best option for you based on your health history.
What's the key difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist (two mechanisms), while Wegovy is a GLP-1 agonist only (single mechanism). This difference in how they work may explain the variation in average trial outcomes, but both are effective for many people. The best choice depends on your individual health profile and prescriber recommendation.
Which medication is more readily available on the NHS in the UK?
Both Mounjaro and Wegovy access on the NHS varies significantly by region and specialist weight management service availability. Many areas have long waiting lists. Private prescribing is faster but costs vary. Ask your GP about local NHS options or a private online clinical assessment to compare timelines and costs.
How long does it take to see weight loss results with either medication?
Weight loss typically begins within 2–4 weeks of starting either medication, but most significant loss occurs over weeks 8–16 as doses are increased. Clinical trial results were measured at 68–72 weeks. Mounjaro requires dose titration over 5+ months to reach 15 mg; Wegovy requires titration to 2.4 mg over about 4 months. Patience and consistency matter.
Can I switch from Wegovy to Mounjaro if the first doesn't work for me?
Yes, switching between medications is possible and managed by your prescriber. There is no official conversion chart, but approximate equivalences exist (e.g., Wegovy 2.4 mg ≈ Mounjaro 15 mg). Any switch must be supervised by your prescriber to ensure safe dose transitions. If one medication isn't working, don't stop — discuss alternatives with your GP or prescriber.
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