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 Weight Loss — What You Need to Know
Before starting any weight loss medication in the UK — including GLP-1 injections like Wegovy or Mounjaro — your GP or prescriber will typically arrange blood tests to assess your general health and suitability for treatment. These tests help identify any underlying health conditions that might affect your eligibility or require monitoring. Understanding what to expect from your pre-treatment screening can help you prepare for 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.
What Blood Tests Are Required Before Weight Loss Treatment?
If you're pursuing prescription weight loss medication through the NHS or private prescribers, blood tests are a routine part of the clinical assessment process. Your prescriber will typically check:
- Full blood count (FBC) — measures red cells, white cells, and platelets to identify anaemia or infection risk
- Urea and electrolytes (U&E) — checks kidney function, which is important as GLP-1 weight loss medications affect how your body processes water and nutrients
- Liver function tests (LFTs) — assesses liver health, particularly important for sustained weight loss
- Fasting blood glucose and HbA1c — measures blood sugar control and screens for type 2 diabetes or prediabetes, common reasons for weight loss medication eligibility
- Lipid profile (cholesterol) — checks total, LDL, and HDL cholesterol, often improved by weight loss and GLP-1 medication
- Thyroid function (TSH) — rules out thyroid disorders that can affect weight and weight loss medication suitability
These tests provide a baseline to compare against during your weight management UK treatment course, allowing your prescriber to monitor any changes in your health markers.
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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.
When Are Blood Tests Done and What Happens Next?
Timing: Blood tests are typically arranged before you start any GLP-1 weight loss injection. If you're accessing weight loss medication through NHS specialist services, your GP may perform initial screening; private prescribers usually request tests as part of their online assessment process. Results typically take 3–7 days.
What comes next: Your prescriber reviews the results to confirm you're fit to start treatment. If results are abnormal — such as significantly reduced kidney function or uncontrolled diabetes — your dose may be adjusted or alternative approaches discussed. Once cleared, you'll receive your prescription. Many people also find it helpful to use our free BMI calculator and progress tracker to monitor your starting point and track improvements in weight and health markers over time.
- 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.
Do I need blood tests before starting Wegovy or Mounjaro?
Yes. Before prescription weight loss medication, your prescriber will arrange blood tests to check kidney function, blood sugar, cholesterol, and liver health. These ensure the medication is safe for you and provide a baseline to monitor changes during treatment.
What if my blood test results are abnormal?
Abnormal results don't automatically disqualify you from treatment. Your prescriber will review results individually. Some conditions may require dose adjustments, additional monitoring, or alternative treatments. Always discuss any concerns with your GP or prescriber.
Can I start weight loss medication without blood tests?
No. UK-licensed prescribers — both NHS and private — are required to assess your health before prescribing GLP-1 medication. Blood tests are a standard part of this clinical assessment and cannot be skipped.
How often are blood tests repeated during weight loss treatment?
Frequency varies by prescriber and your health status. Many recommend baseline tests, then repeat testing at 3–6 months to monitor kidney function, blood sugar, and cholesterol changes. Your prescriber will advise on your specific monitoring schedule.
Do blood tests affect my weight loss medication eligibility?
Tests inform eligibility rather than automatically exclude you. Results help your prescriber determine the right dose, identify any conditions requiring additional monitoring, and ensure GLP-1 weight loss injections are safe for your individual health profile. ```
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.