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 Blood Tests vs Wegovy: What You Need to Know
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, but they work through different mechanisms and may have different monitoring requirements. Understanding the blood test and clinical assessment differences can help you and your GP decide which medication might be suitable. Both require initial clinical assessment and ongoing health monitoring to ensure safety and effectiveness.
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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.
Clinical Monitoring and Blood Test Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the body. Wegovy (semaglutide) is a GLP-1-only agonist with a single mechanism of action. This difference means prescribers may monitor certain markers differently. Before starting either medication, your GP or prescriber will arrange baseline blood tests to check: fasting blood glucose, HbA1c (average blood sugar over 3 months), kidney function (urea and creatinine), liver function tests, and lipid profile (cholesterol and triglycerides). During treatment, both medications typically require repeat blood tests every 3–6 months to monitor how your body is responding, particularly if you have type 2 diabetes or prediabetes. Your prescriber may also check pancreatic enzymes (amylase and lipase) if you experience severe abdominal pain, as pancreatitis is a rare but serious side effect.
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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.
Eligibility, Suitability, and Individual Assessment
Both Mounjaro and Wegovy require a thorough clinical assessment before prescription—blood tests are just one part of this. Your prescriber will review your full medical history, current medications, BMI, and any weight-related health conditions like type 2 diabetes or high blood pressure. For UK adults, eligibility typically begins at BMI 30 or above, or BMI 27–29.9 with a weight-related health condition. South Asian, Chinese, Black African, or Black Caribbean adults may qualify at lower BMI thresholds due to higher cardiovascular risk. The choice between Mounjaro and Wegovy depends on individual factors—your prescriber will assess which is more suitable based on your health profile, not on blood test results alone. Both medications require ongoing monitoring and work best alongside reduced-calorie diet and increased physical activity.
- 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 Mounjaro or Wegovy?
Yes. Both medications require baseline blood tests before prescription, including glucose, HbA1c, kidney function, liver function, and cholesterol checks. These ensure you're fit to take the medication safely. Your prescriber will arrange these as part of your clinical assessment.
What's the main difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1-only agonist with a single pathway. This difference may influence how your body responds, but both are effective for weight management in clinical trials.
How often will I need blood tests while taking these injections?
Most prescribers arrange repeat blood tests every 3–6 months during treatment to monitor kidney function, liver function, glucose control, and cholesterol. Frequency may increase if you have diabetes or if your prescriber needs to check specific markers.
Can I get Mounjaro or Wegovy on the NHS?
Both are available through NHS specialist tier 3 weight management services following referral from your GP, though availability and waiting times vary significantly by region. Private prescribing through GPhC-registered online pharmacies is also available without NHS referral.
Which injection is better—Mounjaro or Wegovy?
Both are effective. Choice depends on individual suitability, assessed by your prescriber during clinical evaluation. Clinical trials show Mounjaro averaged ~15–clinically studied weight loss and Wegovy ~clinically studied weight loss, but individual results vary significantly. Your GP will recommend based on your 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.