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, Kidney Function, and Wegovy — What's the Difference?
If you're considering Mounjaro (tirzepatide) or Wegovy (semaglutide) for weight management and have concerns about kidney health, it's important to understand how these medications work and what clinical evidence shows. Both are prescription-only GLP-1-based weight loss injections available in the UK, but they work differently and have different clinical profiles. Your GP or prescriber will assess your individual kidney function before recommending either medication, as this is a key safety consideration.
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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.
Clinical Trial Data and Kidney Safety Profiles
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) were studied extensively in clinical trials — SURMOUNT for Mounjaro and STEP for Wegovy. Neither medication is contraindicated in mild to moderate kidney disease, but kidney function must be assessed beforehand.
- Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways and may affect how your body manages blood sugar and fluid balance differently than single-mechanism medications
- Wegovy is a GLP-1 agonist only — it activates one hormone pathway and has a longer clinical safety history in kidney disease populations
- Both require baseline kidney function testing (eGFR and creatinine) before starting treatment
- Dose adjustments may be necessary if you have moderate to severe kidney disease
- Dehydration can affect kidney function — both medications may cause nausea and vomiting, which increases dehydration risk
Your prescriber will review your kidney function results and determine which medication (if either) is appropriate for your personal circumstances.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Individual Assessment and Prescriber Guidance
Neither Mounjaro nor Wegovy are suitable for everyone, and kidney function is one of several factors your prescriber will consider during a clinical assessment. If you have a history of kidney disease, pancreatitis, diabetic retinopathy, or thyroid problems, you must disclose this when seeking a prescription.
The choice between Mounjaro and Wegovy (or another option) depends on your individual health profile, kidney function test results, other medications, and weight loss goals. Your GP or a licensed prescriber will explain which option is safest for you. Use our free GLP-1 weight loss calculator above to explore potential outcomes with either medication, and always discuss kidney function concerns directly with your healthcare provider before starting 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.
Does Mounjaro affect kidney function?
Mounjaro is not known to directly damage kidneys in people with normal kidney function. However, because it can cause nausea and vomiting, dehydration risk increases — and dehydration can temporarily affect kidney function. Clinical trials did not show significant kidney problems, but baseline kidney tests are required before starting, and your prescriber will monitor you.
Can I take Mounjaro if I have kidney disease?
This depends on how severe your kidney disease is. Mild kidney disease may not prevent Mounjaro use, but your GP will need to assess your eGFR (estimated glomerular filtration rate) and creatinine levels. Moderate to severe kidney disease typically requires dose adjustments or alternative medications. Always inform your prescriber of any kidney problems.
Is Wegovy safer than Mounjaro for kidney health?
Neither medication is inherently "safer" — both require baseline kidney function assessment. Wegovy has a longer clinical history in kidney disease populations because semaglutide was developed earlier. The choice between them should be based on your individual kidney function results and other health factors, determined by your prescriber.
What kidney tests do I need before starting Mounjaro or Wegovy?
Your prescriber will order blood tests to measure eGFR (estimated kidney function) and creatinine levels. You may also need urinalysis. These tests establish your baseline kidney function before starting either medication and help determine the safest dose. Results guide your prescriber's recommendation.
What should I do if I have kidney concerns and want to use a weight loss injection?
Contact your GP first to discuss your kidney history. They can order kidney function tests and refer you to a specialist if needed. Once your kidney function is assessed, you can explore Mounjaro, Wegovy, or other options with a qualified prescriber who will recommend the safest choice for you based on clinical evidence.
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