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
What Are Fat Jabs and How Do They Work on the NHS?
"Fat jabs" is the colloquial UK term for prescription weight loss injections — specifically GLP-1 and GIP receptor agonist medications licensed for weight management in the UK. These are not cosmetic injections. They are evidence-based prescription medications that may help regulate appetite and support weight loss when combined with diet and lifestyle changes. Access to fat jabs in the UK is available through both NHS specialist services and private prescribing following clinical assessment.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Weight Loss Injections
Three prescription-only medications are currently licensed in the UK for weight management, often referred to as fat jabs or slimming injections:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, once-weekly injection, maximum dose 15 mg. In clinical trials, helped participants lose an average of 15–21% of body weight.
- Wegovy (semaglutide) — a GLP-1 receptor agonist, once-weekly injection, maximum dose 2.4 mg. Clinical trial data showed average weight loss of approximately 15% of body weight.
- Saxenda (liraglutide) — a GLP-1 receptor agonist, daily injection, maximum dose 3 mg. Typically produces weight loss of around 5–8% of body weight.
All three are prescription-only and require a clinical assessment before being prescribed. Individual results vary significantly.
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 Fat Jabs Work — The GLP-1 Mechanism
Fat jabs work by activating GLP-1 (glucagon-like peptide-1), a natural hormone that regulates appetite and blood sugar. When you take these medications, they signal your brain that you feel fuller after eating smaller meals, may reduce food cravings, and slow how quickly food leaves your stomach. This combination of effects supports weight loss alongside a reduced-calorie diet and increased physical activity. The effect is not immediate — most people need to be on the medication for several weeks before noticeable appetite changes occur. Try our free GLP-1 weight loss calculator to estimate 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 available on the NHS?
Fat jabs are available through NHS specialist weight management services (tier 3 and 4 services), but access is limited and varies significantly by region. Most areas have long waiting lists. Private access is available through GPhC-registered online pharmacies following an online clinical assessment. Your GP can advise on NHS availability in your area.
What's the difference between fat jabs and cosmetic injections?
Fat jabs are prescription medications that work internally to regulate appetite hormones. They are not cosmetic injections. Cosmetic fat-dissolving injections (like aqualyx) are a different treatment entirely and work differently. Fat jabs require a prescription and clinical assessment; cosmetic injections do not.
Who is eligible for fat jabs on the NHS?
NHS eligibility typically requires a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Lower BMI thresholds (from 27.5) may apply for people of South Asian, Chinese, Black African, or Black Caribbean heritage. Your GP or a prescriber will assess individual eligibility.
How much weight can I expect to lose with fat jabs?
Clinical trials show average weight loss of 5–21% of body weight depending on the medication and dose, but individual results vary significantly. Some people lose more, others less. Weight loss is most effective when combined with a reduced-calorie diet and increased physical activity. Use our BMI calculator to check your starting point and discuss realistic goals with your prescriber.
Are fat jabs safe long-term?
Fat jabs have been used safely in clinical trials and real-world practice, but they do carry potential side effects (nausea, constipation, stomach pain are common). Long-term safety data is still being gathered. Any concerns should be discussed with your GP or prescriber. They are not suitable for everyone — pregnancy, certain medical conditions, and specific medications are contraindications.
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.