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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Can Mounjaro Affect Your Liver?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medicines, it can potentially affect different parts of your body, including liver function. This guide explains what NHS guidance and clinical evidence tell us about Mounjaro and liver health, what monitoring may be needed, and when to speak to your GP about your individual risk.
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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.
What NHS Guidance Says About Mounjaro and Liver Function
Mounjaro (tirzepatide) is not known to cause liver damage in healthy individuals, but monitoring is important for certain people. NHS prescribers follow MHRA guidance, which recommends baseline liver function tests before starting the medication, and periodic monitoring for patients with existing liver conditions.
Clinical trials for Mounjaro did not show significant liver injury, but patients with severe liver disease or cirrhosis should not take this medication. Your prescriber will assess your liver function through blood tests (liver function tests or LFTs) as part of the pre-treatment assessment. If you have a history of hepatitis, fatty liver disease, or cirrhosis, inform your GP or specialist before starting Mounjaro. The medication is metabolised in the liver, so baseline and ongoing monitoring is standard practice across NHS tier 3 weight management services.
- Baseline liver function tests required before starting
- Monitoring recommended if you have existing liver conditions
- Not suitable if you have severe liver disease or cirrhosis
- Inform your prescriber of any history of hepatitis or liver problems
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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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When to Seek Medical Advice About Liver Health
Contact your GP or weight management specialist immediately if you develop unexplained jaundice (yellowing of skin or eyes), persistent nausea, severe abdominal pain in the upper right area, or dark urine — these may suggest liver problems. Although rare, any sign of liver dysfunction should be assessed promptly. Your prescriber may recommend stopping Mounjaro temporarily and performing urgent liver function tests.
If you have a BMI indicating you may benefit from Mounjaro or another GLP-1 medication, use our BMI calculator to understand your starting point, then discuss liver monitoring with your GP. Those with existing conditions like non-alcoholic fatty liver disease (NAFLD) should have this conversation before starting any weight loss medication. Most people taking Mounjaro in NHS services have normal liver function throughout treatment.
- 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.
Will Mounjaro definitely damage my liver?
No. Mounjaro has not been shown to cause liver damage in healthy individuals in clinical trials. However, baseline liver function tests are recommended before starting, and ongoing monitoring is standard practice if you have existing liver conditions.
Do I need liver function tests before starting Mounjaro?
Yes. NHS prescribers require baseline liver function tests as part of the pre-treatment assessment. Your GP will arrange these blood tests, which measure how well your liver is working before you start the medication.
Can I take Mounjaro if I have fatty liver disease?
This depends on the severity of your liver condition. Non-alcoholic fatty liver disease (NAFLD) is not an automatic reason to avoid Mounjaro, but your prescriber must assess your baseline liver function. Discuss your liver health openly with your GP before starting.
What are the signs I should stop Mounjaro due to liver problems?
Seek urgent medical advice if you develop jaundice (yellowing of skin or eyes), persistent severe nausea, dark urine, or pain in the upper right abdomen. These are rare but may indicate liver dysfunction. Stop taking Mounjaro and contact your GP or NHS 111 immediately.
How does Mounjaro affect the liver compared to Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1-based medications requiring baseline and ongoing liver monitoring for certain patients. Both are metabolised by the liver but have not been shown to cause liver damage in healthy individuals. Your prescriber will assess which is appropriate for you.
Ready to take control of your weight loss journey?
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