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 Does The Fat Jab Work?
The term "fat jab" refers to prescription weight loss injections now widely discussed in the UK. These are GLP-1 and GIP receptor agonist medications that help regulate appetite and support weight management. They're not cosmetic injections—they're prescription-only medicines requiring clinical assessment. Understanding how they work can help you decide if weight loss injections might be suitable for you alongside diet and exercise changes.
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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 Slimming Injections
Three prescription slimming injections are currently licensed for weight management in the UK. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist administered once weekly, with doses ranging from 2.5 mg to 15 mg. Wegovy (semaglutide) is a GLP-1 agonist also given once weekly, escalating from 0.25 mg to a maximum of 2.4 mg. Saxenda (liraglutide), also a GLP-1 agonist, is the only daily injection option, with doses from 0.6 mg to 3 mg. All three are prescription-only medicines available through NHS specialist weight management services or private prescribing following a clinical assessment. Individual eligibility depends on BMI, weight-related health conditions, and overall health status. Try our free BMI calculator to check where you currently stand and understand your eligibility criteria.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 GLP-1 Weight Loss Works
GLP-1 (glucagon-like peptide-1) is a natural hormone your gut produces after eating. GLP-1 weight loss injections activate the body's GLP-1 receptors, which send fullness signals to the brain, reducing appetite and food cravings. This mechanism helps you feel satisfied with smaller meals and may reduce the urge to snack. The medication also slows how quickly food moves through your stomach, prolonging the feeling of fullness. For glp-1 weight loss to be effective, these injections work alongside a reduced-calorie diet and increased physical activity—they're not a standalone solution. Clinical trials show that weight loss injections uk licensed options may help participants lose 15–21% of body weight on average, though individual results vary significantly. Use our GLP-1 weight loss calculator to explore potential outcomes at different 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.
Are fat jabs the same as cosmetic injections?
No. Fat jabs are prescription-only medications (GLP-1 or GIP/GLP-1 receptor agonists), not cosmetic injections. They work by regulating appetite and blood sugar, not by dissolving fat. They require a prescription from a UK-licensed prescriber and a clinical assessment to ensure they're appropriate for your health.
How quickly do slimming injections start working?
Most people notice appetite changes within the first 2–4 weeks, though some take longer. Weight loss typically becomes visible after 4–8 weeks. Results build gradually as doses are increased over time. Everyone responds differently, and consistent diet and exercise changes are essential alongside the medication.
Can I get fat jabs on the NHS in the UK?
Yes, GLP-1 medications are available on the NHS through specialist tier 3 weight management services. You'll need a GP referral and must meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Availability and waiting times vary significantly by region.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. Both are once-weekly injections. Clinical trials suggest Mounjaro may produce slightly higher average weight loss (up to 21% vs 15%), but individual results vary. Your prescriber will assess which is most suitable.
Do I need to stay on slimming injections forever?
No. Treatment duration is determined by your prescriber based on your response and health goals. Some people use them for 6–12 months to establish new eating habits, while others continue longer. Stopping requires medical guidance, as weight regain is possible without ongoing lifestyle changes.
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.