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 and Creatinine — What You Need to Know
Creatinine is a waste product your kidneys filter from your blood, and it's measured as part of kidney function tests. If you're considering Mounjaro (tirzepatide), your kidney function matters because this GLP-1 medication affects how your body processes glucose and fluid. Your GP or prescriber will check your creatinine levels before starting treatment to ensure it's safe for you. This guide explains the connection between Mounjaro and kidney health in the UK context, plus how to access this prescription-only injection through NHS or private routes.
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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.
Creatinine, Kidney Function, and Mounjaro Safety
Creatinine is a breakdown product of muscle metabolism filtered by your kidneys. A blood test measures creatinine levels to estimate your glomerular filtration rate (GFR) — essentially how well your kidneys are working. When considering Mounjaro, your prescriber needs to know your kidney function because:
- Mounjaro is processed by your kidneys, so impaired kidney function may affect how the medication works
- Severe kidney disease may mean you're not eligible for this tirzepatide injection
- Your dose may need adjusting if your kidney function is reduced
- Mounjaro can occasionally cause changes in kidney function, so baseline testing is essential
The UK's MHRA and NHS guidance recommend kidney function assessment before starting any GLP-1 medication, including Mounjaro's KwikPen device. If your creatinine is elevated or your GFR is low, your GP can discuss whether this medication is suitable or if alternatives are better. Regular monitoring may continue while you're on Mounjaro to track kidney function alongside weight loss.
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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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Accessing Mounjaro in the UK — NHS vs Private Routes
In the UK, you can access Mounjaro through NHS specialist weight management services or via private prescription. NHS route: Your GP refers you to tier 3 specialist services. They'll perform full health checks including kidney function tests and creatinine measurement before prescribing. This route is free but waiting lists vary by region (ICB). Private route: You can have an online consultation with a UK-licensed prescriber following a clinical assessment. They'll ask about your health history and may request recent kidney function results. If approved, the prescription is dispensed by a GPhC-registered pharmacy. Doses range from 2.5 mg to 15 mg in the KwikPen, with cost varying by dose and provider. Either route requires ongoing monitoring, especially if you have any kidney concerns.
- 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.
What exactly is creatinine and why does it matter for Mounjaro?
Creatinine is a waste product from muscle breakdown filtered by your kidneys. It's measured in a blood test to check kidney function (GFR). Mounjaro is processed by your kidneys, so good kidney function is important for safe use of this tirzepatide injection. Your prescriber will check your baseline creatinine before starting.
Will Mounjaro affect my creatinine levels or kidney function?
Most people tolerate Mounjaro well, but any medication affecting fluid balance can occasionally impact kidney function. This is why baseline kidney testing is essential and why some GPs monitor creatinine during treatment. If you have existing kidney disease, discuss this with your GP — you may need dose adjustments or alternative options.
Can I take Mounjaro if I have high creatinine or reduced kidney function?
It depends on the degree of kidney impairment. Mild reductions may not prevent Mounjaro use, but moderate to severe kidney disease typically means this medication isn't suitable. Your GP or specialist will assess your individual situation. If Mounjaro isn't appropriate, other weight management options including Wegovy may be considered based on your kidney function.
What's the difference between Mounjaro and Wegovy regarding kidney function?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1-based medications requiring kidney function assessment. Mounjaro is a dual GIP and GLP-1 agonist; Wegovy is GLP-1 only. Neither is absolutely contraindicated by kidney issues alone, but your prescriber must evaluate your individual creatinine levels and GFR to determine suitability and appropriate dosing.
How do I find out my creatinine level before requesting Mounjaro?
Ask your GP for a kidney function test (U&Es or urea and electrolytes), which includes creatinine and GFR measurement. This is usually a simple blood test. Private prescribers may also request recent results or arrange testing as part of their online consultation. Some private clinics can arrange blood tests through partner labs, ensuring you have current kidney function data before assessment.
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