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
If you're considering weight loss medication or exploring NHS weight management options, blood tests are often an essential first step. Your GP or prescriber will use blood tests to assess your health, check for underlying conditions, and determine whether prescription weight loss treatment is safe and appropriate for you. This guide explains what tests are typically done, why they matter, and what results mean for your weight loss journey in the UK.
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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 Are Required Before Weight Loss Treatment
Blood tests serve several critical purposes before starting prescription weight loss medication or accessing NHS weight loss support. They help your GP or prescriber identify any underlying health conditions that might affect treatment eligibility or safety. Testing typically includes checks for type 2 diabetes or prediabetes, liver and kidney function, cholesterol levels, and thyroid function — all relevant to weight management and overall health.
- Baseline measurements: establish your starting point for blood sugar, cholesterol, and liver health
- Safety screening: identify conditions that might contraindicate certain medications like GLP-1 weight loss injections
- Medication eligibility: determine whether you meet criteria for NHS weight loss or prescription treatments
- Risk assessment: evaluate cardiovascular and metabolic health alongside your BMI
For those exploring weight management UK options like Mounjaro or Wegovy, blood tests are standard. They ensure the medication is appropriate for your individual health profile and help establish whether you have weight-related health conditions (like diabetes or high blood pressure) that increase your eligibility for treatment.
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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.
Common Blood Tests Before Weight Loss Treatment
Fasting blood glucose and HbA1c: measure blood sugar control and screen for type 2 diabetes or prediabetes — key conditions affecting weight loss eligibility.
Lipid panel: checks cholesterol and triglycerides, which often improve with weight loss. High cholesterol is a weight-related health condition that may increase your eligibility for prescription weight loss treatment.
Liver function tests (LFTs): assess kidney and liver health, important because some medications are processed through the liver.
Thyroid function (TSH): rules out thyroid disorders that can affect weight and metabolism. A GP may also order BMI-based assessments alongside blood work to determine your overall health profile and weight loss options available on the NHS.
- 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. Blood tests are essential before prescription weight loss medication. Your prescriber needs to assess your baseline health, check for diabetes or prediabetes, evaluate liver and kidney function, and confirm you don't have medical conditions that would make the medication unsafe. All prescription weight loss treatment in the UK requires a clinical assessment, which includes blood work.
What happens if my blood tests show I have prediabetes?
Having prediabetes actually strengthens your eligibility for prescription weight loss treatment in the UK. Prediabetes is classified as a weight-related health condition, which means you may qualify for GLP-1 weight loss medication or NHS weight loss support even if your BMI is slightly lower (27–29.9 instead of 30+). Blood sugar control often improves significantly with weight loss.
How long does it take to get blood test results?
Typically 3–7 working days for standard tests through the NHS. Private providers may have faster turnaround times (24–48 hours). Your GP or prescriber will discuss results with you and confirm whether you're eligible to proceed with weight loss treatment. Delays can happen during busy periods, so it's worth checking with your practice.
Will my cholesterol or blood pressure affect my eligibility for weight loss medication?
High cholesterol or high blood pressure are recognised weight-related health conditions. If your blood tests show either of these, it actually increases your eligibility for prescription weight loss treatment — you can qualify with a lower BMI (27–29.9 instead of 30+). These conditions often improve significantly with even modest weight loss (5–10% body weight reduction).
Can I start weight loss medication before blood tests come back?
No. Prescription weight loss medications require a full clinical assessment and clear blood test results before dispensing. This is a safety requirement in the UK, regardless of whether you're accessing treatment through the NHS or privately. Your prescriber cannot issue a prescription without this baseline health data, as it ensures the medication is 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.