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
Free tracker included with every download this week
How Do Fat Jabs Work — A Clear UK Explanation
Fat jabs, also called slimming injections or weight loss jabs, are prescription-only medicines that help regulate appetite and support weight management in the UK. They work by mimicking natural hormones that control hunger and fullness signals. Despite the informal name, these are medical injections, not cosmetic treatments. Three prescription options are currently licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All require a clinical assessment before prescription and work best alongside a reduced-calorie diet and increased physical activity.
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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.
Three UK-Licensed Fat Jabs Options
Three prescription weight loss injections are currently licensed in the UK:
- Mounjaro (tirzepatide) — a dual GIP/GLP-1 receptor agonist. Once-weekly injection starting at 2.5 mg, escalating to a maximum of 15 mg. Clinical trials showed average weight loss of 15–21% body weight alongside lifestyle changes.
- Wegovy (semaglutide) — a GLP-1 receptor agonist. Once-weekly injection, starting at 0.25 mg and escalating to 2.4 mg maximum. Average weight loss in trials was approximately 15% body weight.
- Saxenda (liraglutide) — a GLP-1 receptor agonist. Daily injection, doses range from 0.6 mg to 3 mg. Each is prescription-only and requires assessment by a licensed prescriber to determine suitability for your individual circumstances.
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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.
How the GLP-1 Mechanism Works
Fat jabs work by activating GLP-1 (glucagon-like peptide-1), a natural hormone your gut produces after eating. This hormone signals your brain that you are full, reducing cravings and supporting smaller portion sizes. Mounjaro uniquely activates two pathways — both GIP and GLP-1 — which may enhance appetite regulation further. The injections also slow how quickly food leaves your stomach, prolonging the feeling of fullness. Results vary between individuals. Use our free GLP-1 weight loss calculator to estimate potential outcomes, and discuss eligibility with your GP or a licensed prescriber based on your BMI and health profile.
- 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 cosmetic injections?
No. Fat jabs are prescription medications, not cosmetic injections. They are pharmaceutical GLP-1 or GIP/GLP-1 receptor agonists that work internally to regulate appetite and blood sugar. They are completely different from cosmetic fat-dissolving injections and require a clinical prescription.
How quickly do fat jabs work?
Weight loss typically becomes noticeable after 2–4 weeks at therapeutic doses, though some people see changes earlier. Maximum effects are usually seen after 12–16 weeks on a stable dose. Individual timelines vary significantly. Consistent diet and exercise support better results.
Can I get fat jabs on the NHS in the UK?
Yes, but access is through specialist tier 3 weight management services following GP referral. Availability varies significantly by region and waiting lists can be lengthy in some areas. Private prescription is also available through licensed UK prescribers without NHS referral.
Who is eligible for prescription slimming injections?
Adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease, sleep apnoea) may be eligible. Lower thresholds apply for South Asian, Black African, and Black Caribbean adults. A prescriber will assess your individual suitability.
Do fat jabs work without diet and exercise changes?
Slimming injections are most effective alongside a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss occurred when participants combined the injection with lifestyle changes. Medication supports your efforts but does not replace healthy habits.
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.