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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Blood Tests for Mounjaro — What You Need to Know
If you're taking Mounjaro (tirzepatide) in the UK, blood tests are an essential part of safe treatment. Your GP or prescriber will order specific blood tests before you start and during your treatment to monitor your health, check how your body is responding, and ensure the medication is safe for you. Understanding what these tests measure and why they're needed helps you get the most from your weight management journey.
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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.
Why Blood Tests Matter Before and During Mounjaro Treatment
Before starting Mounjaro, your prescriber needs a baseline picture of your health. Blood tests assess your liver function, kidney function, and blood sugar levels — all crucial because tirzepatide affects how your body processes nutrients and glucose. If you have type 2 diabetes or prediabetes, your fasting glucose and HbA1c (average blood sugar over 3 months) will be measured to establish your starting point.
During treatment, you'll typically have blood tests repeated at intervals — usually 3–6 months initially, then annually once you're stable on a dose. These check that your liver and kidneys are coping well with the medication, monitor your cholesterol levels, and track any changes in blood sugar control. Some prescribers also check your vitamin B12 levels, as GLP-1 medications can occasionally affect absorption.
- Liver function tests (LFTs) — ensure your liver can safely process tirzepatide
- Kidney function (creatinine and eGFR) — check your kidneys are filtering waste properly
- Fasting glucose and HbA1c — track blood sugar control if you have diabetes
- Lipid profile (cholesterol, triglycerides) — often improve with weight loss
- Full blood count — screen for anaemia or other blood abnormalities
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS vs Private: Blood Test Requirements and Costs
On the NHS, blood tests are free through your GP surgery as part of specialist tier 3 weight management services. Your prescriber will request tests and arrange them via your local NHS pathology service. This is straightforward but can add delays to your prescribing pathway, as results may take 1–2 weeks.
Privately, when obtaining Mounjaro through a registered online prescriber, blood tests are your responsibility. Some private weight management clinics include baseline testing in their service fee; others require you to arrange and pay for tests separately (typically £80–£150 for a comprehensive profile). Many high street pharmacies and online blood test services (such as Medichecks or LetsGetChecked) offer Mounjaro baseline packages.
Before starting any dose, ensure your prescriber has recent results — usually no older than 3 months. This protects your safety and ensures tirzepatide is appropriate for your individual health profile.
- 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 liver function tests, kidney function tests (creatinine and eGFR), fasting glucose and HbA1c (if diabetic), lipid profile, and a full blood count. These establish your baseline health and confirm Mounjaro is safe for you. Results typically take 1–2 weeks. If you have any pre-existing conditions, additional tests may be needed.
How often do I need blood tests whilst taking Mounjaro?
After starting Mounjaro, expect blood tests at 3–6 months, then annually once stable on your dose. Your prescriber may request more frequent testing if you have diabetes, liver or kidney concerns, or if your weight loss is very rapid. Always follow your individual prescriber's guidance.
Can I start Mounjaro without recent blood test results?
No. Your prescriber must have recent baseline blood tests (usually within the last 3 months) before issuing your first prescription. This is a safety requirement for both NHS and private prescribing. Starting without results risks missing contraindications or unsafe dosing.
Why do I need kidney and liver function tests for Mounjaro?
Mounjaro is processed by your liver and kidneys. If either organ isn't working properly, the medication may accumulate to unsafe levels. Kidney or liver disease can affect how your body handles tirzepatide, so baseline testing ensures the dose is safe for your individual function.
Are blood tests included in NHS Mounjaro treatment?
Yes. If prescribed through NHS tier 3 specialist services, blood tests are free and arranged by your GP or the specialist clinic. Privately, tests are usually your cost unless your chosen provider includes them. Costs range from £80–£150 for a comprehensive baseline profile at private providers.
Ready to take control of your weight loss journey?
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