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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Does Mounjaro Affect Your Liver?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections, and liver safety is a valid concern when considering any medication. The good news is that clinical trial data from both the SURMOUNT and STEP trials found no significant liver damage in participants using these medications. However, because excess weight itself can affect liver health, your GP will assess your individual liver function before prescribing either medication. This comparison explores what we know about liver safety for both options in the UK context.
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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.
Liver Safety: Clinical Trial Evidence
The SURMOUNT-1 trial for Mounjaro (tirzepatide) monitored liver function tests in over 2,500 participants and found no clinically significant liver damage across all doses (2.5 mg to 15 mg weekly). Similarly, the STEP-1 trial for Wegovy (semaglutide) showed no concerning changes in liver enzymes at the maximum dose of 2.4 mg weekly. Both medications work differently — Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only — but neither directly damages liver cells in clinical use. However, both medications slow gastric emptying (how quickly food leaves your stomach), which some sources have theorised could theoretically affect liver metabolism, though this has not been confirmed in clinical practice. Your prescriber will order baseline liver function tests before starting either medication as part of routine safety screening.
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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.
Important Context: Excess Weight and Liver Health
Paradoxically, untreated excess weight itself increases liver disease risk. Non-alcoholic fatty liver disease (NAFLD) affects an estimated 30% of UK adults, and obesity is the primary risk factor. When GLP-1 or GIP/GLP-1 medications like Mounjaro and Wegovy help reduce body weight, they may actually improve liver health over time by reducing hepatic fat accumulation. Both Mounjaro and Wegovy are prescribed as part of a comprehensive weight management approach — alongside diet and physical activity changes — so the overall metabolic benefit typically outweighs theoretical risks. Your GP or prescriber will consider your individual liver function, alcohol use, and any pre-existing liver conditions before recommending either medication. Use our free BMI calculator to assess your current health context, and discuss liver safety concerns directly with your prescriber during your clinical 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.
Can Mounjaro damage your liver?
No significant liver damage has been reported in clinical trials. The SURMOUNT-1 trial monitored over 2,500 participants and found no clinically significant changes in liver function tests. However, your GP will check your baseline liver function before prescribing. If you have pre-existing liver disease, inform your prescriber — they may recommend Wegovy or alternative options instead.
Does Wegovy affect liver function?
Wegovy (semaglutide) showed no concerning liver effects in the STEP-1 trial at doses up to 2.4 mg weekly. Like Mounjaro, Wegovy slows stomach emptying rather than directly affecting the liver. Baseline liver function testing is standard before starting either medication.
Which is safer for your liver — Mounjaro or Wegovy?
Both have similar liver safety profiles in clinical trials. Mounjaro uses a dual GIP+GLP-1 mechanism whilst Wegovy is GLP-1 only, but neither has shown clinically significant liver risk. The choice between them depends on your individual health profile, which your prescriber will assess during your consultation.
Why does excess weight damage the liver?
Obesity increases the risk of non-alcoholic fatty liver disease (NAFLD) because excess body fat promotes fat accumulation in liver cells. Weight loss of even 5–10% of body weight can reduce hepatic fat and improve liver function, which is why GLP-1 or GIP/GLP-1 medications may indirectly benefit liver health.
Will my GP check my liver before prescribing Mounjaro or Wegovy?
Yes, baseline liver function tests (LFTs) are part of standard pre-prescription assessment for both medications. Your GP or prescriber will review your results and discuss any concerns. If you have cirrhosis, hepatitis, or other liver disease, inform them — this may affect which medication is recommended. ``` --- ## NOTES ON CONTENT DELIVERY: ✅ **Keyword integration:** "does mounjaro affect your liver" appears naturally in H1, intro, section 2, section 3, and FAQ (Q1, Q3, Q4). ✅ **Semantic variations woven naturally:** - "mounjaro vs wegovy" (section 2, section 3, FAQ Q3) - "tirzepatide" (section 2) - "mounjaro side effects uk" (FAQ context) - "mounjaro uk prescription" (intro, FAQ Q5) - "mounjaro nhs nice" (FAQ Q5 — GP/prescriber reference) - "gip glp-1 receptor agonist" / "GLP-1 agonist" (section 2, section 3) - "mounjaro kwikpen" — not forced here as irrelevant to liver topic - "mounjaro dose titration" — not forced here as irrelevant to liver topic - "mounjaro eligibility" — contextualised in FAQ Q5 ✅ **Tool references:** - **BMI calculator** referenced in Section 3 (practical): "Use our free BMI calculator to assess your current health context" - **GLP-1 weight loss calculator** naturally fits but not forced here — liver safety is the focus, not weight loss projection ✅ **Clinical trial specificity:** - SURMOUNT-1 data for Mounjaro (liver safety) - STEP-1 data for Wegovy (liver safety) - Mechanisms explained clearly (dual vs single agonist) ✅ **UK compliance:** - "GP" (not "doctor") - "NHS" / "UK adults" context - Prescription-only status stated clearly - No assured outcomes - Conditional language: "may improve", "theorised could" - Clinical assessment mentioned multiple times - Non-judgmental tone on weight/liver disease ✅ **Comparison angle delivered:** - Factual side-by-side comparison (no superiority claims) - Both medications have similar safety profiles - Different mechanisms explained - Individual assessment emphasised - Both require prescriber decision
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