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- 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 vs Wegovy: Blood Tests and Clinical Monitoring
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, and both require blood tests and clinical assessment before prescribing. Your GP or licensed prescriber will order baseline blood work to check your kidney function, liver health, and metabolic markers before you start either medication. This guide compares the clinical monitoring requirements for both medications and explains what tests you can expect as part of your assessment.
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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.
Blood Tests and Clinical Assessment: Mounjaro vs Wegovy
Both Mounjaro and Wegovy require baseline blood tests before starting treatment. Your prescriber will typically check:
- Kidney function — creatinine and eGFR (estimated glomerular filtration rate) to ensure safe medication use
- Liver function tests — AST, ALT, and bilirubin to assess liver health
- Fasting glucose or HbA1c — to check your current blood sugar control or diabetes status
- Lipid profile — cholesterol and triglycerides, which relate to cardiovascular health
Both medications have similar baseline requirements because they work on similar metabolic pathways. The tirzepatide (Mounjaro) mechanism is dual GIP+GLP-1, while semaglutide (Wegovy) is GLP-1-only, but both require the same safety screening. The main difference is in ongoing monitoring frequency — some prescribers monitor more closely at the start of Mounjaro due to its relative newness in the weight loss market, though this practice is still evolving in UK clinical settings.
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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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Who Gets Each Medication: Clinical Assessment Matters
Your prescriber will decide between Mounjaro and Wegovy based on your individual health profile, not based on blood test results alone. Eligibility criteria are the same for both: BMI of 30 kg/m² or above, or BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, heart disease history). If you have existing kidney or liver disease, your prescriber may prefer one medication over the other, or may adjust the starting dose. Some UK prescribers have more experience with Wegovy due to its longer availability, though Mounjaro is now increasingly prescribed. Use our GLP-1 weight loss calculator to explore potential outcomes, then discuss which medication suits your health profile with your GP or licensed prescriber.
- 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 blood tests do I need before starting Mounjaro?
Your prescriber will order baseline tests checking kidney function (eGFR and creatinine), liver function (ALT and AST), fasting glucose or HbA1c, and lipid profile (cholesterol and triglycerides). These tests ensure safe use and establish your baseline metabolic health before starting the medication.
Are the blood tests the same for Wegovy as for Mounjaro?
Yes, baseline blood tests are essentially the same for both medications because both require assessment of kidney and liver function, blood sugar, and cardiovascular markers. Individual prescribers may add additional tests based on your personal health history.
How often do I need blood tests while taking Mounjaro or Wegovy?
This varies by prescriber and setting. Some UK clinics check blood work every 3–6 months during dose titration, particularly to monitor kidney function and blood sugar changes if you have diabetes. Once stable on a maintenance dose, testing frequency typically reduces. Your prescriber will advise you.
Can I start Mounjaro or Wegovy if I have kidney disease?
Possibly, but it depends on the severity of your kidney disease. Both medications are processed by the kidneys, so your prescriber will assess your eGFR carefully. Some prescribers may still offer these medications at lower doses if your kidney function is moderately reduced. Severe kidney disease may be a contraindication — your GP will advise.
Do these medications affect liver function?
Both Mounjaro and Wegovy are generally safe for the liver, though baseline and periodic liver function tests are part of standard monitoring. If you have existing liver disease, your prescriber will assess suitability carefully. Report any signs of liver problems (yellowing of skin or eyes, abdominal pain) immediately.
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