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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Weight Loss vs Fat Loss — What's the Difference?
When you step on the scales, you're measuring total body weight — but that number doesn't tell you whether you've lost fat, muscle, water, or a combination. Understanding the difference between weight loss and fat loss is crucial when considering weight management in the UK, especially if you're exploring prescription options like Mounjaro or Wegovy. True fat loss is the health goal; simple weight loss can sometimes be misleading. This guide explains what's actually happening in your body and how prescription weight loss medication supports fat loss effectively.
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Calculate Your BMI Instantly
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.
How Weight Loss and Fat Loss Differ
Weight loss is any reduction in total body weight on the scales — this includes fat, muscle, water, and even digestive contents. Fat loss is specifically the reduction of stored body fat while ideally preserving muscle mass.
Why the distinction matters: You could lose 2kg on the scales but lose only 0.5kg of actual fat if the rest is water and muscle. Conversely, if you build muscle while losing fat, the scales might barely move — but your body composition improves significantly.
- gradual weight management often involves water loss and muscle loss (unhealthy)
- Sustainable fat loss preserves muscle and requires a modest calorie deficit alongside physical activity
- The NHS recommends losing 0.5–1kg per week, which typically reflects genuine fat loss rather than water fluctuation
Prescription weight management medications like Mounjaro (tirzepatide) and Wegovy (semaglutide) work by reducing appetite and supporting a calorie deficit. Clinical evidence shows these medications help achieve fat loss when combined with a reduced-calorie diet and increased physical activity — not just temporary weight fluctuation.
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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.
Weight Loss Medication Available in the UK
For eligible UK adults, three prescription-only weight loss medications are licensed: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All work as appetite suppressants and support fat loss when combined with lifestyle changes.
These are available through the NHS via specialist weight management services or through private prescribing following a clinical assessment. You're typically eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
In clinical trials, adults using these medications lost an average of 15–21% of their body weight over 68 weeks alongside reduced-calorie diet and exercise — with individual results varying significantly. Track your progress with our free progress tracker as you work towards your health goals, and use our GLP-1 weight loss calculator to understand potential outcomes at different medication doses.
- 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's the difference between losing weight and losing fat?
Weight loss is any reduction in total body weight (fat, muscle, water). Fat loss is specifically reducing stored body fat while preserving muscle. You could lose weight through water loss without losing fat. Sustainable fat loss requires a modest calorie deficit and exercise, which is why the NHS recommends losing 0.5–1kg per week.
Can prescription weight loss medication help me lose fat specifically?
Yes. Mounjaro, Wegovy, and Saxenda are designed to support fat loss by reducing appetite and helping you maintain a calorie deficit. Clinical trials show meaningful fat loss alongside reduced-calorie diet and increased physical activity. Results vary by individual, and you'll need a prescription from a UK-licensed prescriber following a clinical assessment.
How do I know if I'm losing fat rather than just water weight?
gradual weight management (more than 1kg per week) often includes water loss. Sustainable fat loss follows the NHS guidance of 0.5–1kg per week. Body measurements (waist, hips, chest) often show fat loss before scales do. Use our free progress tracker to log weight, measurements, and fitness progress together.
Am I eligible for prescription weight loss medication on the NHS?
You may be eligible if you have a BMI of 30 or above, or 27 or above with weight-related conditions like type 2 diabetes or high blood pressure. Eligibility is assessed through NHS specialist weight management services (tier 3). Your GP can refer you; availability varies by region and waiting lists can be lengthy.
What's the fastest I can safely lose fat with medication?
The NHS recommends losing 0.5–1kg per week as sustainable fat loss. Prescription medications support this rate by reducing hunger, not by forcing rapid loss. Faster loss risks muscle loss and metabolic damage. Your prescriber will monitor your progress and adjust your dose accordingly. Calculate your personal BMI to understand your starting point. ```
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.