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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Fat Jabs — UK Weight Loss Injection Options
Fat jabs, commonly known as slimming injections or weight loss jabs, are prescription-only medicines licensed in the UK to help adults manage their weight. Three main options are currently available: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works differently and is administered at different frequencies. Whether you're eligible depends on your BMI, health conditions, and individual circumstances. This guide explains how each works, how to access them through the NHS or privately, and what to expect.
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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.
How UK Fat Jabs Work — Mounjaro, Wegovy, and Saxenda Compared
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways to help regulate appetite and support feeling fuller after meals. It's a once-weekly injection with doses ranging from 2.5 mg to 15 mg. Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% of body weight alongside diet and exercise. Wegovy (semaglutide) is a GLP-1 receptor agonist available as a once-weekly injection, with doses from 0.25 mg to 2.4 mg. Clinical trials (STEP-1) showed average weight loss of approximately 15% of body weight. Saxenda (liraglutide) is also a GLP-1 receptor agonist but administered as a daily injection rather than weekly, with doses from 0.6 mg to 3 mg. All three work by affecting appetite signals, slowing stomach emptying, and supporting reduced food intake. Individual results vary significantly — these are averages from clinical trials, not assured outcomes. Use our free GLP-1 weight loss calculator to explore projected results at different doses based on current evidence.
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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.
Accessing Fat Jabs in the UK — NHS and Private Routes
NHS access: Fat jabs are available through NHS specialist weight management services (tier 3/4), but you must be referred by your GP and meet strict eligibility criteria: BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Waiting times vary significantly by region — some areas have multi-year waiting lists. Private access: All three medications are available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber. No NHS referral needed. You can access private prescribing within days rather than months. Both routes require a prescription — fat jabs are never available over-the-counter. Your prescriber will assess suitability based on your health history, medications, and individual circumstances. Check your eligibility using our free BMI calculator to see which category you fall into.
- 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.
Are fat jabs the same as cosmetic injections?
No. Fat jabs (slimming injections) are prescription medicines containing active pharmaceutical ingredients like tirzepatide or semaglutide. They work internally to affect appetite and metabolism. Cosmetic fat-dissolving injections (such as aqualyx) are entirely different treatments. Fat jabs are prescription-only medications requiring clinical assessment.
How much weight can I expect to lose with fat jabs?
Clinical trials show average weight loss of 15–21% of body weight over several months, but individual results vary significantly. Some people lose more, some less. Weight loss depends on your starting weight, adherence, diet, exercise, and individual metabolism. Your prescriber can discuss realistic expectations based on your circumstances.
Can I get fat jabs on the NHS?
Yes, through NHS specialist weight management services. You need a GP referral, a BMI of 30 or above (or 27–29.9 with health conditions), and to meet your local ICB's criteria. Waiting times vary from months to years depending on your region. Private access is typically faster.
What's the difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP+GLP-1 agonist with slightly higher average weight loss (15–21% vs 15%). Wegovy is GLP-1 only. Mounjaro doses weekly from 2.5–15 mg; Wegovy doses weekly from 0.25–2.4 mg. Both are once-weekly injections. Your prescriber will recommend based on your health profile.
Do I need to diet and exercise while taking fat jabs?
Yes. Clinical trial results were achieved alongside reduced-calorie diet and increased physical activity. Fat jabs support these changes but don't replace them. Weight loss medications work best when combined with sustainable lifestyle changes. Your prescriber will discuss this with you.
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.