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 Kidneys?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. If you're considering this medication or already taking it, you may be wondering whether it could affect your kidney function. The short answer: current evidence suggests Mounjaro itself does not directly damage kidneys in people with normal kidney function. However, kidney health matters when taking any medication, and your prescriber will assess this during your clinical assessment. This guide explains what the evidence shows and what you need to know.
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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 Does the Evidence Show About Mounjaro and Kidney Safety?
In the SURMOUNT clinical trials — the main studies that led to Mounjaro's approval in the UK — kidney function was monitored in participants. The trials found no direct kidney damage caused by tirzepatide itself in people with normal baseline kidney function. However, several important points apply: First, Mounjaro can cause nausea, vomiting, and diarrhoea, which may lead to dehydration if fluid intake is not maintained. Dehydration can temporarily affect kidney function, so staying hydrated is essential. Second, if you have existing kidney disease or reduced kidney function (eGFR below 30), Mounjaro requires careful prescriber assessment and may not be suitable. Third, Mounjaro may interact with how your kidneys handle certain other medications — your prescriber will review all your medicines during assessment.
- Mounjaro does not directly damage healthy kidneys
- Dehydration from side effects can temporarily affect kidney function
- People with existing kidney disease need specialist assessment
- Always inform your prescriber of any kidney condition
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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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What You Should Do Before and During Mounjaro Treatment
Before starting Mounjaro, your prescriber will ask about your medical history, including any kidney problems. They may request a baseline blood test to check your kidney function (measured as eGFR). During treatment, maintain good hydration, especially if you experience nausea or diarrhoea — aim for 6–8 glasses of water daily. If you have pre-existing kidney disease, high blood pressure, or type 2 diabetes, these conditions can affect kidney health independently, so regular GP check-ups remain important. If you develop symptoms like unusual fatigue, swelling in legs or ankles, changes in urination, or persistent nausea, contact your GP. Use our free BMI calculator and GLP-1 weight loss calculator to track your progress alongside your prescriber's guidance.
- 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.
Does Mounjaro cause kidney damage in healthy people?
No. Clinical trials found no direct kidney damage in people with normal kidney function taking Mounjaro. However, side effects like vomiting or diarrhoea can cause dehydration, which may temporarily affect kidney function, so staying hydrated is important.
Can I take Mounjaro if I have kidney disease?
If you have existing kidney disease or reduced kidney function, you need specialist assessment before starting Mounjaro. Some people with moderate to severe kidney disease may not be suitable candidates. Always tell your prescriber about any kidney conditions.
What kidney tests might I need before starting Mounjaro?
Your prescriber may request a blood test to check your baseline kidney function, measured as eGFR (estimated glomerular filtration rate). This helps ensure Mounjaro is safe for you and establishes a baseline for comparison during treatment.
How does dehydration from Mounjaro side effects affect kidneys?
Nausea, vomiting, or diarrhoea can reduce fluid intake and cause dehydration, which may temporarily lower kidney function. This is not direct kidney damage but a secondary effect. Drinking plenty of water and managing side effects helps prevent this.
Should I have kidney function monitored while taking Mounjaro?
If you have existing kidney disease or risk factors, your GP may recommend periodic kidney function checks. People with normal kidney function typically don't need routine monitoring, but always discuss this with your prescriber based on your individual health.
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