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: Liver Safety & Clinical Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK. This guide compares their mechanisms, clinical trial evidence, and safety profiles—including liver considerations. Both medications work through different hormone pathways and have been studied extensively in clinical trials. Neither medication is known to cause liver damage in clinical practice, but individual suitability depends on your health profile. Your GP or prescriber will assess which medication is appropriate for you following a thorough 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.
Mechanism of Action: Dual vs Single Hormone Pathway
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. This means it activates two natural hormone pathways in the body simultaneously—GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may support appetite regulation and blood sugar control through two complementary pathways.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—it activates a single hormone pathway. Both medications slow gastric emptying (how quickly food leaves the stomach) and signal fullness to the brain. In clinical trials, Mounjaro produced average weight loss of approximately 15–21% of body weight across the dose range (SURMOUNT trials), whilst Wegovy showed average weight loss of approximately 15% (STEP-1 trial). Results vary significantly between individuals, and both medications require a reduced-calorie diet and increased physical activity alongside their use.
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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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Liver Safety & Individual Suitability
Neither Mounjaro nor Wegovy are known to cause liver damage in clinical trials or post-marketing experience. However, both medications can cause nausea, vomiting, and stomach upset—particularly at higher doses—which may temporarily affect liver health indirectly through dehydration or electrolyte changes. If you have existing liver disease or take multiple medications, your prescriber will assess suitability individually. Your GP or a licensed prescriber will review your full health history, including liver function tests if appropriate, before prescribing either medication. Using our GLP-1 weight loss calculator can help you explore potential outcomes at different doses, whilst the BMI calculator provides context for your current health status and 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.
Is Mounjaro safe for your liver?
Clinical trials show no direct liver toxicity from Mounjaro. However, if you have existing liver disease, your prescriber may need to adjust your dose or recommend an alternative. Always disclose your full medical history during your clinical assessment.
What's the main difference between Mounjaro and Wegovy mechanisms?
Mounjaro activates two hormone pathways (GIP and GLP-1), whilst Wegovy activates one (GLP-1 only). This dual action in Mounjaro may offer different appetite control, though both achieve significant weight loss in clinical trials.
Which medication causes more liver problems?
Neither medication is known to cause liver damage. Both are considered safe for liver function in clinical practice. Your prescriber will assess your individual risk based on your health history and any existing liver conditions.
Can you take Mounjaro or Wegovy if you have fatty liver disease?
Both medications may be prescribed for fatty liver disease in some cases, as weight loss can improve liver health. However, your GP must assess your individual suitability and liver function before prescribing. Never self-prescribe either medication.
How do clinical trial results compare for these two medications?
Mounjaro (SURMOUNT trials) showed average weight loss of 15–21% body weight; Wegovy (STEP trials) showed approximately 15% at maximum dose. Both results occurred alongside dietary changes and exercise. Individual outcomes vary considerably.
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.