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 Side Effects — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weekly weight loss injections available in the UK, but they work differently and may cause different side effects. Understanding these differences can help you and your GP decide which medication might be more suitable. This guide compares both medications factually, based on clinical trial data and NHS guidance, so you can have an informed conversation with your healthcare provider about which option may be right for you.
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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.
Mechanism Differences and Side Effect Profiles
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 mechanism in Mounjaro may explain why some side effects differ slightly between the two.
Both medications commonly cause nausea, diarrhoea, vomiting, and stomach pain, particularly when starting or increasing doses. However, clinical trial data suggests gastrointestinal side effects may be slightly more pronounced with Mounjaro due to its dual action. Very common side effects affect more than 1 in 10 people with both medications. Common side effects (up to 1 in 10) include indigestion, headache, fatigue, dizziness, and injection site reactions. Serious side effects are rare but require immediate medical attention — these include severe stomach pain (possible pancreatitis), gallbladder problems, and serious allergic reactions.
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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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Individual Suitability and NHS Assessment
Neither medication is inherently "safer" than the other — suitability depends on your individual health profile, medical history, and how your body responds. Both require a clinical assessment from a prescriber before being prescribed. Your GP or specialist can assess which medication aligns better with your health needs, existing conditions, and any medications you currently take.
The NHS supports both medications for eligible adults, though access through specialist weight management services varies by region. Some individuals tolerate one medication better than the other. If side effects become problematic on one medication, a prescriber may suggest switching — though this must be managed carefully with appropriate dose transitions. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your eligibility criteria.
- 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.
Are Mounjaro and Wegovy side effects the same?
Both cause similar common side effects like nausea and diarrhoea, but Mounjaro's dual mechanism may cause slightly different patterns. Clinical trials show comparable safety profiles overall, though individual experiences vary. Your prescriber can discuss which suits you best based on your health history.
Which has fewer side effects, Mounjaro or Wegovy?
Neither is definitively "safer" — both have similar very common and common side effect rates. SURMOUNT and STEP trials show comparable tolerability. Side effect severity varies individually. What matters is how your body responds, which your prescriber will assess during clinical review.
Can I switch from Mounjaro to Wegovy if I have side effects?
Yes, but switching requires prescriber guidance. Your dose will need careful management during transition. Not all side effects warrant switching — some improve as your body adjusts over 4–8 weeks. Always discuss concerns with your GP or prescriber before making changes.
How long do Mounjaro and Wegovy side effects last?
Most side effects peak in the first 2–4 weeks and improve as your body adjusts. Nausea typically reduces within 4–8 weeks. Some people experience persistent side effects requiring dose adjustment or medication change. Hydration and small, frequent meals help manage gastrointestinal symptoms.
Which weekly weight loss injection does the NHS prefer?
The NHS approves both Mounjaro and Wegovy for eligible adults through specialist weight management services. Neither has priority — your local NHS team assesses individual suitability. NICE guidance supports both as options for appropriate candidates. Availability and waiting lists vary by region. ```
Ready to take control of your weight loss journey?
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