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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Monjaro vs Wegovy Side Effects — What's the Difference?
Both Monjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work slightly differently and may cause different side effects. This guide compares the two medications side-by-side, explains how they differ as a GIP receptor agonist versus a GLP-1 receptor agonist, and helps you understand what your prescriber might consider when assessing which is more suitable for you. Both require a clinical assessment before prescription, and individual experiences vary significantly.
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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 Monjaro and Wegovy Work Differently
Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways simultaneously. This dual mechanism may enhance appetite suppression and support weight loss by working on two different appetite-regulating systems in the brain and gut.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. In clinical trials, Monjaro showed average weight loss of around 20.9% of body weight at the highest dose, whilst Wegovy showed approximately 14.9%. However, these are trial averages — individual results vary widely. Both are once-weekly injections administered via a pre-filled pen or KwikPen. Your prescriber will assess which mechanism and dose strength is most appropriate for your personal health profile during your clinical assessment.
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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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Comparing Side Effects: What to Expect
Both medications share very common side effects including nausea, diarrhoea, vomiting, constipation, and stomach pain — these are typical of GLP-1 class medications and often ease as your body adjusts. Monjaro users may experience slightly different tolerability due to its dual mechanism, though the most common side effects remain similar. Injection site reactions, headache, fatigue, and dizziness occur in both groups. Serious side effects are rare but can include severe stomach pain (possible pancreatitis) or gallbladder issues. Neither medication is suitable if you have a history of medullary thyroid cancer or multiple endocrine neoplasia. Your GP or prescriber will discuss your full medical history to ensure safe use. Try our free GLP-1 weight loss calculator to understand potential outcomes, and use our BMI calculator to assess your current health status.
- 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's the main difference between Monjaro and Wegovy?
Monjaro is a dual GIP and GLP-1 receptor agonist (tirzepatide), whilst Wegovy is a GLP-1 receptor agonist only (semaglutide). Monjaro's dual mechanism activates two appetite-regulating pathways, which may contribute to its slightly higher average weight loss in clinical trials. Both are once-weekly injections licensed for UK weight management.
Do Monjaro and Wegovy cause the same side effects?
Both share common side effects like nausea, diarrhoea, and stomach pain. Individual tolerability varies. Because Monjaro works on two hormone pathways, some people may experience different side effect patterns, but the most frequent side effects remain similar across both medications. Your prescriber will discuss what to expect based on your health.
Which causes more nausea — Monjaro or Wegovy?
Clinical data doesn't show one as significantly more nausea-inducing than the other. Nausea is common with both but typically improves within the first few weeks. Individual responses vary widely. Starting at the lowest dose and increasing gradually helps minimise early nausea. Your prescriber will tailor your dosing schedule.
Can I switch from Wegovy to Monjaro or vice versa?
Yes, switching is possible, but it must be managed by your prescriber. Approximate dose equivalences exist (e.g., Wegovy 1.7 mg ≈ Monjaro 10 mg), but there's no official conversion chart. Any switch requires a clinical review to ensure safety and appropriateness for your individual health needs.
How do I know which injection is right for me?
Your GP or prescriber will assess your medical history, current health conditions, medications, and weight management goals. Both require a clinical assessment before prescription. Factors like tolerability, mechanism preference, and dosing frequency may influence the choice. Neither is inherently "better" — suitability is individual.
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