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 Cause Kidney Problems?
Mounjaro (tirzepatide) does not directly damage healthy kidneys, but kidney function is carefully monitored before and during treatment. If you have existing kidney problems, your GP or prescriber must assess whether Mounjaro is suitable for you. The MHRA-approved patient information highlights kidney monitoring as part of NHS and private prescribing protocols. This guide explains what you need to know about Mounjaro, kidney safety, and UK prescribing practice.
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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 Does NHS Guidance Say About Mounjaro and Kidney Safety?
Mounjaro has no direct nephrotoxic (kidney-damaging) effect in people with normal kidney function. However, kidney health is assessed before treatment starts because:
- GLP-1 and dual GIP/GLP-1 receptor agonists can affect blood pressure and blood sugar, which may impact kidney function over time
- Dehydration (common during GI side effects like diarrhoea and vomiting) can stress the kidneys
- People with pre-existing kidney disease may be at higher risk of complications
The NHS tirzepatide prescribing pathway requires baseline kidney function tests (serum creatinine and estimated glomerular filtration rate — eGFR) before starting treatment. If your eGFR is below 30 mL/min/1.73m² (severe kidney disease), Mounjaro is generally not recommended. Regular monitoring continues during treatment, especially if you have diabetes or hypertension. Your GP or NHS specialist tier 3 weight management service will review kidney function at intervals and adjust treatment if needed.
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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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Who Needs Extra Kidney Monitoring Before Starting Mounjaro?
Certain groups require additional assessment and closer monitoring:
- Adults with type 2 diabetes: Diabetes is a common cause of kidney disease; your prescriber will check kidney function more carefully
- Adults with high blood pressure: Hypertension also damages kidneys over time; baseline and ongoing monitoring is essential
- Adults aged 65+: Kidney function naturally declines with age; age-adjusted risk assessment applies
- Adults with pre-existing CKD (chronic kidney disease): Your prescriber may recommend a lower starting dose or alternative treatment
Before starting Mounjaro, tell your GP or prescriber about any kidney problems, dialysis, or family history of kidney disease. Use our free BMI calculator to understand your starting point, and ask your healthcare team about monitoring frequency during your treatment journey.
- 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 damage kidneys in people with normal kidney function?
No. Mounjaro does not directly damage healthy kidneys. However, kidney function is monitored before and during treatment because dehydration from GI side effects, or changes in blood pressure and blood sugar, could theoretically affect kidney health over time. Baseline kidney tests are part of standard NHS prescribing protocol.
What kidney tests does the NHS do before starting Mounjaro?
Your GP or NHS tier 3 weight management service will order blood tests measuring serum creatinine and eGFR (estimated glomerular filtration rate). eGFR shows how well your kidneys filter waste. If eGFR is below 30 mL/min/1.73m², Mounjaro is not recommended. Results guide whether treatment can safely begin.
Can I take Mounjaro if I have kidney disease?
It depends on the severity. Mild to moderate kidney disease may not exclude you, but your prescriber must assess risk carefully. Severe kidney disease (eGFR <30) is generally a contraindication. Always disclose kidney problems to your prescriber before starting tirzepatide. Your GP can refer you to nephrology (kidney specialist) if needed.
What side effects of Mounjaro might affect my kidneys?
Diarrhoea, vomiting, and nausea can cause dehydration, which may temporarily stress kidney function. Staying well hydrated during Mounjaro treatment is important. If you experience severe or prolonged GI symptoms, contact your GP — they may adjust your dose or offer anti-nausea support.
How often does the NHS check kidney function during Mounjaro treatment?
Frequency varies based on your baseline kidney function and other health conditions. If you have normal kidney function and no diabetes or hypertension, annual monitoring is typical. If you have diabetes, hypertension, or pre-existing kidney disease, more frequent testing (every 3–6 months) may be recommended. Your prescriber will advise you on your specific monitoring schedule.
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