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 Cause Kidney Stones?
Kidney stones are not listed as a common side effect of Mounjaro (tirzepatide) in UK clinical trials or prescribing information. However, weight loss medications can indirectly affect kidney health through dehydration or rapid weight changes. This guide compares kidney-related safety between Mounjaro and Wegovy, explains the actual risks, and helps you understand what to discuss with your GP before starting either medication.
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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.
Kidney Health and Mounjaro vs Wegovy — What Clinical Trials Show
Neither Mounjaro (tirzepatide) nor Wegovy (semaglutide) lists kidney stones as a direct side effect in their UK prescribing information or SURMOUNT and STEP clinical trials. However, both medications can affect kidney function indirectly through several mechanisms:
- Dehydration risk: Both GLP-1 and dual GIP/GLP-1 medications cause nausea, vomiting, and diarrhoea — common side effects that increase dehydration risk, which may increase kidney stone risk in susceptible individuals
- gradual weight management: Fast weight reduction can alter uric acid levels and electrolyte balance, potentially increasing stone formation risk
- Dose titration: Mounjaro's slower dose escalation (starting 2.5 mg, increasing weekly) compared to Wegovy may result in different dehydration patterns, though clinical data does not show significant kidney stone differences between the two
- Individual variation: Pre-existing kidney disease, family history of stones, or low fluid intake are stronger predictors of stone risk than the medication itself
If you have a history of kidney stones, your prescriber will assess suitability during your clinical assessment.
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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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Individual Risk Assessment — Which Medication Is Right for You?
Whether Mounjaro or Wegovy is more suitable depends on your individual health profile, not kidney stone risk alone. Both medications require a clinical assessment before prescription through either NHS specialist services or a private prescriber. Your prescriber will review your medical history, current medications, kidney function (via blood tests if needed), and weight-related health conditions to determine which option is appropriate.
If you have had kidney stones previously, inform your GP or prescriber. You can also use our free GLP-1 weight loss calculator to explore projected weight loss outcomes with either medication, and track your progress using our progress tracker once treatment begins. Staying well-hydrated and maintaining adequate fluid intake is important alongside either medication.
- 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 cause kidney stones directly?
Kidney stones are not listed as a direct side effect of Mounjaro in UK clinical trials or prescribing data. However, Mounjaro can cause nausea, vomiting, and diarrhoea — side effects that increase dehydration, which may increase kidney stone risk in susceptible people. This is an indirect risk, not a direct effect of the medication itself.
Is Wegovy safer than Mounjaro for kidney health?
No significant difference in kidney stone risk between Wegovy and Mounjaro has been reported in clinical trials. Both are GLP-1-based medications with similar gastrointestinal side effects. Individual risk depends more on your personal history, hydration habits, and pre-existing kidney conditions than on which medication you choose.
What should I tell my prescriber about kidney health?
Inform your GP or prescriber if you have a history of kidney stones, chronic kidney disease, recurrent urinary tract infections, or family history of kidney stones. Your prescriber may order blood tests to check kidney function before starting Mounjaro or Wegovy. This helps ensure the medication is safe for you individually.
How can I reduce kidney stone risk while taking Mounjaro?
Drink plenty of water — aim for at least 2–3 litres daily unless advised otherwise by your GP. Limit salt intake, maintain a balanced diet, and monitor your fluid intake carefully if you experience nausea or diarrhoea. Report persistent vomiting or dehydration symptoms to your prescriber immediately.
Does dose titration affect kidney stone risk differently between Mounjaro and Wegovy?
Mounjaro starts at 2.5 mg with slower escalation; Wegovy starts at 0.25 mg with faster escalation. The different titration schedules may affect nausea severity and dehydration patterns in early weeks, but clinical trials do not show significant kidney stone risk differences. Your individual response to either medication is more important than the dose schedule itself.
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