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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Risks of Mounjaro vs Wegovy — What You Need to Know
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and carry distinct risk profiles. This guide compares their safety, side effects, and clinical evidence to help you understand which medication might be suitable for you — a decision that should always be made with your GP following a full 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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways simultaneously. This dual mechanism may explain why clinical trials showed slightly higher average weight loss compared to single-mechanism medications. Wegovy (semaglutide) is a GLP-1-only receptor agonist, working through a single pathway.
This mechanistic difference is important because it can influence both effectiveness and how your body responds. The SURMOUNT-1 trial for Mounjaro showed average weight loss of approximately 20.9% at the maximum 15 mg dose, whilst the STEP-1 trial for Wegovy showed approximately 14.9% at 2.4 mg. However, individual results vary significantly, and trial averages do not predict personal outcomes. Use our free GLP-1 weight loss calculator to estimate potential weight loss at different doses based on clinical trial data.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Side Effects and Safety Comparison
Both medications share similar common side effects — nausea, diarrhoea, vomiting, and stomach pain are reported in clinical trials for both drugs. Gastrointestinal side effects are typically dose-related and often improve over time as your body adjusts. Serious side effects are rare but include pancreatitis (severe stomach pain), gallbladder complications, and allergic reactions — both require urgent medical attention.
Because Mounjaro has a dual mechanism, some patients report different tolerability profiles compared to Wegovy. A prescriber assessing you will consider your medical history, current medications, and personal risk factors to determine which carries lower risk for your specific situation. Neither medication is risk-free, and prescription decisions are always individualised.
[Keeping this section as COL_J per template — no additional COL section needed]- 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 safer than Wegovy?
Both are similarly safe when prescribed appropriately after clinical assessment. Mounjaro has slightly higher average weight loss in trials but is not inherently "safer" or "riskier" — it depends on your individual health, medications, and medical history. Your GP will assess which carries less risk for you personally.
What are the most common side effects of Mounjaro compared to Wegovy?
Both cause nausea, diarrhoea, vomiting, and stomach pain as very common side effects. Mounjaro users sometimes report slightly different GI tolerability due to its dual mechanism. Most side effects are dose-related and improve as your body adjusts during dose titration.
Can I switch from Wegovy to Mounjaro if I'm not tolerating it well?
Yes, switching between medications is possible, but it must be managed by your prescriber. Approximate dose equivalences exist (0.5 mg Wegovy ≈ 5 mg Mounjaro, 1.7 mg ≈ 10 mg, 2.4 mg ≈ 15 mg), but any switch should involve clinical assessment and careful transition planning.
Are there serious risks with either Mounjaro or Wegovy?
Serious risks are rare but include pancreatitis (severe stomach pain spreading to the back), gallbladder problems, and severe allergic reactions. Both medications carry a Black Triangle warning requiring close monitoring. Seek immediate medical attention if you experience severe symptoms.
How do I know which medication is lower risk for me?
Your GP or prescriber will assess your medical history, current medications, comorbidities, and contraindications. Neither is universally "better" — suitability is entirely individual. A UK-licensed prescriber must complete a clinical assessment before either medication can be prescribed.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.