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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What To Expect On Mounjaro vs Wegovy
If you're considering Mounjaro or Wegovy for weight management, understanding what to expect from each medication is essential. Both are prescription-only GLP-1 receptor agonist injections licensed in the UK, but they differ in mechanism, dosing schedule, and clinical trial outcomes. This guide compares the real-world experience of both medications, helping you have an informed conversation with your GP or prescriber about which might suit your situation. Individual experiences vary significantly, so your prescriber will assess your personal eligibility and suitability.
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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.
Mechanisms, Doses, and Clinical Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways to regulate appetite and blood sugar. The SURMOUNT-1 clinical trial showed participants lost an average of 15–21% of their body weight over 72 weeks at maximum dose (15 mg weekly), alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a single GLP-1 receptor agonist — activating one hormone pathway. The STEP-1 trial showed average weight loss of approximately 14.9% of body weight at 68 weeks on the maximum 2.4 mg weekly dose, under the same lifestyle conditions.
- Mounjaro: starts at 2.5 mg, increases over 20 weeks to 15 mg maximum
- Wegovy: starts at 0.25 mg, increases over 16 weeks to 2.4 mg maximum
- Both: once-weekly injections via pre-filled pen
Both medications slow gastric emptying (how quickly food leaves your stomach), though this effect typically reduces over time. The dual mechanism of Mounjaro may explain slightly higher average weight loss in trials, but individual results 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.
Free Weight Loss Tools & Guides
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What You'll Actually Experience
Side effects are similar between both medications — nausea, diarrhoea, constipation, and stomach pain are very common, especially during dose increases. Most people experience improvements as their body adjusts over 4–8 weeks. You may notice reduced appetite, feeling fuller faster, and less interest in foods you previously craved. Injection site reactions (minor redness or bruising) are common but usually mild.
Mounjaro's dual mechanism may feel slightly different — some users report a more pronounced appetite reduction. However, this is highly individual. Both medications require a prescription following clinical assessment by a GP or licensed prescriber. Your choice between them depends on factors your prescriber will evaluate: medical history, other medications, BMI, and weight-related health conditions. Try our free GLP-1 weight loss calculator to explore potential outcomes, and use our BMI calculator to check where you currently stand.
- 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.
How quickly will I lose weight on Mounjaro vs Wegovy?
Clinical trials showed Mounjaro users lost approximately 15–21% of body weight and Wegovy users lost approximately 14.9% over similar timeframes. However, individual results vary significantly based on starting weight, lifestyle changes, dose tolerance, and metabolism. Weight loss is gradual, typically becoming noticeable after 4–8 weeks.
Are the side effects different between Mounjaro and Wegovy?
Both medications cause similar side effects — nausea, diarrhoea, constipation, and stomach pain are very common. Side effects usually improve within 4–8 weeks as your body adjusts. Serious side effects (severe abdominal pain, allergic reactions) are rare. Your prescriber will monitor you throughout treatment.
Why is Mounjaro a dual GIP and GLP-1 agonist, and does it matter?
Mounjaro activates two hormone pathways (GIP and GLP-1) while Wegovy activates only GLP-1. This dual mechanism may explain Mounjaro's slightly higher average weight loss in clinical trials. However, both are effective, and the best choice depends on your individual health profile and prescriber's assessment.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber — there is no official conversion chart between the two. Your prescriber will assess whether a switch is clinically appropriate and manage the transition carefully. You cannot stop one and immediately start the other without professional guidance.
Which medication will my NHS GP prescribe?
Access to both Mounjaro and Wegovy through the NHS is limited and varies by region. Both are approved by NICE for specialist weight management services, but you must be referred to a tier 3 specialist service. Private prescription access is available through online clinical assessments with UK-licensed prescribers.
Ready to take control of your weight loss journey?
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