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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What Is a Fat Jab? UK Guide to Slimming Injections
A "fat jab" is a colloquial UK term for prescription weight loss injections that contain GLP-1 or GIP/GLP-1 medications. These are licensed prescription-only medicines used to support weight management in adults with obesity or overweight with health complications. Fat jabs are not cosmetic injections—they're clinically approved treatments that work by regulating appetite and helping you feel fuller after eating. In the UK, three main options are available, each requiring a clinical assessment before prescription. They're used alongside a reduced-calorie diet and increased physical activity, not as a standalone treatment.
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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.
Three UK-Licensed Fat Jab Options
Three prescription weight loss injections are currently licensed in the UK for weight management:
- Mounjaro (tirzepatide) — A once-weekly injection with a dual GIP and GLP-1 mechanism. Doses range from 2.5mg to 15mg. Clinical trials showed participants lost an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly.
- Wegovy (semaglutide) — A once-weekly GLP-1 injection with doses from 0.25mg to 2.4mg. In clinical trials, participants achieved an average weight loss of approximately 15% of body weight at the maximum dose.
- Saxenda (liraglutide) — A daily GLP-1 injection with doses from 0.6mg to 3mg. This was the first weight loss injection licensed in the UK and requires daily administration rather than weekly.
All three are prescription-only medicines requiring assessment by a licensed prescriber. Access is available through NHS specialist weight management services or private online prescribing following clinical evaluation.
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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
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How Fat Jabs Work—The GLP-1 Mechanism
Fat jabs work by mimicking GLP-1, a natural hormone your gut produces when you eat. GLP-1 signals to your brain that you're satisfied, reduces cravings, and slows how quickly food leaves your stomach. This mechanism helps you eat smaller portions and feel fuller for longer. Mounjaro adds an additional GIP signal, which may enhance results. These effects typically develop gradually over weeks as your dose increases. Side effects are common initially—nausea, diarrhoea, and reduced appetite—but often improve as your body adjusts. Fat jabs are not clinically supported solutions; they work best combined with dietary changes and physical activity. Results vary between individuals based on genetics, lifestyle, and dose tolerance.
- 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.
Is a fat jab the same as a cosmetic injection?
No. Fat jabs are prescription weight loss medicines, not cosmetic procedures. They contain active pharmaceutical ingredients (GLP-1 or GIP/GLP-1 agonists) that regulate appetite internally. Cosmetic fat-dissolving injections are different products used for localised fat reduction and work through a completely different mechanism.
Who can get a fat jab in the UK?
Adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea) may be eligible. South Asian, Chinese, Black African, or Black Caribbean adults may qualify at lower BMI thresholds. A licensed prescriber assesses eligibility based on your individual health.
How quickly do fat jabs work?
Weight loss typically begins within the first few weeks but becomes more noticeable after 8–12 weeks as your dose increases. Most clinical trials measured results over 68–72 weeks. Sustainable weight loss happens gradually—the NHS recommends aiming for 0.5–1kg per week. Use our free GLP-1 weight loss calculator to project potential outcomes at different doses.
Are fat jabs available on the NHS?
NICE has approved tirzepatide and semaglutide for NHS use within specialist tier 3 weight management services. However, NHS availability varies significantly by region, and waiting lists can be lengthy in many areas. Private prescription is available through online assessment with a licensed prescriber.
What's the difference between fat jabs and weight loss tablets?
Fat jabs (injections) and weight loss tablets (like orlistat) work differently. Injections regulate appetite through GLP-1 mechanisms. Tablets like orlistat reduce fat absorption in the digestive system. Injections typically produce greater weight loss in clinical trials, but individual response varies. Your GP can advise which option suits your situation.
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.