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 And Liver Damage — Wegovy Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, and questions about liver safety are important when considering either medication. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Clinical trial data and post-market monitoring provide key insights into how each medication affects liver function. This guide compares the evidence on liver safety, mechanisms of action, and what this means for your individual suitability — which your GP or prescriber will assess during a clinical consultation.
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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 Evidence And Liver Safety Profiles
Mounjaro (tirzepatide) was assessed in the SURMOUNT clinical trials involving thousands of participants over 72 weeks. Liver function tests (ALT and AST enzymes) were monitored, and no significant liver injury or hepatotoxicity was identified in trial data. Most participants showed stable or improved liver function markers, particularly those with non-alcoholic fatty liver disease (NAFLD), which often improves with weight loss.
Wegovy (semaglutide) underwent similar monitoring in the STEP trials. Like Mounjaro, Wegovy showed no pattern of liver damage in clinical trials. Liver enzyme elevations were rare and similar to placebo groups.
Both medications may actually benefit liver health indirectly through weight loss itself — reducing excess weight often improves fatty liver disease and metabolic function. The GLP-1 mechanism in both drugs supports insulin sensitivity and metabolic health, which are protective for the liver. Use our free GLP-1 weight loss calculator to understand potential weight loss outcomes with either medication.
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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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Mechanism Differences And Individual Suitability
The key difference is mechanism: Mounjaro activates both GIP and GLP-1 pathways (dual action), whilst Wegovy activates GLP-1 only. This does not mean one is safer than the other for liver health — clinical evidence supports both. Individual suitability depends on your personal health profile, any existing liver conditions, other medications you take, and how your body responds.
If you have existing liver disease (fatty liver, hepatitis, cirrhosis, or elevated liver enzymes), your GP or prescriber must assess whether either medication is appropriate and monitor liver function more closely. Some patients with liver conditions may require dose adjustments or alternative approaches. Your prescriber will order baseline liver function tests before starting and monitor periodically during treatment.
Calculate your BMI using our free tool above to understand your weight management baseline and discuss prescription options with your GP.
- 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.
Can Mounjaro damage your liver?
No significant liver damage has been identified in clinical trials of Mounjaro. Liver function tests remained stable or improved during the SURMOUNT trials. In fact, weight loss from Mounjaro often improves liver health, especially in people with fatty liver disease. However, if you have existing liver conditions, your GP must assess suitability and monitor liver function during treatment.
Is Wegovy safer for the liver than Mounjaro?
Both Mounjaro and Wegovy show similar safety profiles regarding liver function in clinical trials. Neither medication showed patterns of liver injury. The choice between them depends on individual factors — mechanism of action (GLP-1 only vs dual GIP+GLP-1), your medical history, and prescriber assessment. Your GP will recommend the most suitable option for you.
Will either injection help if I have fatty liver disease?
Yes. Both GLP-1 and dual GIP+GLP-1 medications support weight loss, which directly benefits non-alcoholic fatty liver disease (NAFLD). Weight loss and improved metabolic health often reverse fatty liver. However, if you have diagnosed NAFLD or other liver conditions, inform your prescriber before starting treatment so they can monitor appropriately and adjust doses if needed.
What liver monitoring happens when taking Mounjaro or Wegovy?
Your GP or prescriber will typically order baseline liver function tests (ALT, AST, bilirubin) before starting either medication. Periodic monitoring during treatment depends on your risk factors — more frequent if you have existing liver disease or take other hepatotoxic drugs. Standard monitoring is part of safe prescribing practice for all weight loss medications.
Which injection should I choose — Mounjaro or Wegovy?
This is a clinical decision made with your GP or prescriber based on your individual health profile, medical history, existing conditions, and medication interactions. Both are prescription-only and require a clinical assessment. Mounjaro uses a dual mechanism (GIP+GLP-1), whilst Wegovy uses GLP-1 only. Neither is inherently "better" — suitability varies by person. Your prescriber will guide you.
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