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: Risk and Safety Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work through different mechanisms and carry different safety profiles. Understanding the risks of each can help inform discussions with your prescriber about which medication might be suitable for you. This guide compares the evidence on side effects, tolerability, and safety considerations for both medications, helping you make an informed decision. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication alongside your clinical assessment.
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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.
Clinical Trial Data and Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. Wegovy (semaglutide) is a GLP-1 only agonist. This difference in mechanism may affect how each medication is tolerated.
The SURMOUNT-1 trial showed Mounjaro produced average weight loss of approximately 20.9% at the maximum 15 mg dose over 72 weeks. The STEP-1 trial showed Wegovy produced average weight loss of approximately 14.9% at 2.4 mg over 68 weeks. However, both trials were conducted in controlled conditions alongside reduced-calorie diet and exercise — real-world results vary.
Importantly, higher efficacy does not automatically mean lower risk. Both medications carry similar common side effects, with nausea, diarrhoea, vomiting, and constipation reported in clinical trials. The difference lies in frequency, timing, and individual tolerability rather than fundamentally different safety profiles.
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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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Side Effects and Individual Suitability
Both Mounjaro and Wegovy carry the same contraindications: pregnancy, breastfeeding, personal or family history of medullary thyroid carcinoma, and allergy to the active ingredient. Both require careful prescriber assessment before dispensing.
Mounjaro's dual mechanism may cause more pronounced gastrointestinal effects initially, particularly nausea. Wegovy, being a single-mechanism GLP-1 agonist, may have slightly fewer cumulative effects but both are still commonly associated with nausea in early weeks. Hair loss (alopecia) has been reported with both medications, likely linked to gradual weight management rather than the medication itself.
Your GP or licensed prescriber will assess which medication is more appropriate based on your medical history, other medications, and individual health factors. Neither medication is universally "safer" — suitability is personal. Use our BMI calculator to understand your baseline health context before your assessment.
- 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 riskier than Wegovy in the UK?
Both are prescription-only medications with similar safety profiles. Mounjaro may cause more initial nausea due to its dual mechanism, but neither is objectively "riskier." Your prescriber will assess individual suitability based on your medical history, allergies, and other medications. Results and tolerability vary between individuals.
What side effects are common to both medications?
Nausea, diarrhoea, vomiting, constipation, and stomach pain are very common with both. Injection site reactions, headache, fatigue, and dizziness occur less frequently. These typically improve as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems — seek immediate medical attention if these occur.
Why is Mounjaro sometimes prescribed instead of Wegovy?
Mounjaro's dual GIP+GLP-1 mechanism may produce greater weight loss in clinical trials. Some prescribers may recommend it if GLP-1 alone (Wegovy) hasn't been effective, or if you have specific medical conditions. However, both are first-line options — prescriber choice depends on individual assessment, not medication superiority.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber. There is no official established conversion, though approximate equivalences exist (Wegovy 2.4 mg roughly equates to Mounjaro 10 mg). Any switch requires clinical reassessment and careful dose titration to minimise side effects.
How do I know which medication is right for me?
Your GP or a licensed prescriber will assess your BMI, health conditions, medication interactions, and personal tolerance to gastrointestinal effects. Use our GLP-1 weight loss calculator to explore potential outcomes, but the final decision on which medication is appropriate must be based on a professional clinical assessment of your individual circumstances.
Ready to take control of your weight loss journey?
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