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 Jab Options Available in the UK
A "fat jab" is a colloquial UK term for prescription weight loss injections — specifically GLP-1 and GIP/GLP-1 receptor agonist medications. These are not cosmetic injections but licensed medicines prescribed by healthcare professionals. Currently, three weight loss injections are available in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works slightly differently and is administered at different frequencies. Understanding your options helps you and your GP or prescriber decide which might be appropriate for your circumstances.
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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.
UK-Licensed Weight Loss Injections Compared
Three glp-1 weight loss injections are currently licensed in the UK for weight management in adults. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist administered once weekly in doses ranging from 2.5 mg to 15 mg. Clinical trials showed an average weight loss of approximately 15–21% of body weight. Wegovy (semaglutide) is a GLP-1-only injection, also once weekly, with doses from 0.25 mg to 2.4 mg, showing average weight loss of around 15% in trials. Saxenda (liraglutide) is a daily injection (not weekly) with doses from 0.6 mg to 3 mg daily, and typically produces more modest weight loss results. All three are prescription-only medicines requiring clinical assessment before prescription. Individual results vary significantly, and all work best alongside reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses.
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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.
NHS and Private Access to Fat Jabs in the UK
NHS access: Weight loss injections are available through tier 3 specialist weight management services in England, Scotland, Wales, and Northern Ireland. Eligibility typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Access varies significantly by region and waiting times can be lengthy. Private access: All three injections are available through GPhC-registered online prescribing services following a consultation with a UK-licensed prescriber. This route does not require NHS referral. Both routes require clinical assessment to confirm eligibility and suitability. Your GP can advise whether NHS referral is appropriate, or you can explore private options directly. Check your BMI using our free BMI calculator to understand your eligibility.
- 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.
What is the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) activates GLP-1 only. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (15–21% vs 15%), but individual results vary. Both are once-weekly injections. Your prescriber will assess which suits your health profile.
Is Saxenda less effective than Mounjaro or Wegovy?
Saxenda typically produces more modest weight loss than Mounjaro or Wegovy in clinical trials. However, it remains an effective option for many people and is administered as a daily injection rather than weekly. Suitability depends on individual health factors, preferences, and prescriber assessment — not on drug superiority alone.
Can I switch between different weight loss injections?
Switching between weight loss injections should only be done under prescriber guidance. Approximate dose equivalences exist (e.g., Mounjaro 5 mg ≈ Wegovy 0.5 mg), but there is no official conversion chart. Your prescriber will manage any switch safely, potentially adjusting your dose and monitoring closely during transition.
How do I access weight loss injections on the NHS?
Ask your GP about referral to your local tier 3 specialist weight management service. Eligibility requires BMI ≥30, or 27–29.9 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea, etc.). Access and waiting times vary by region. Private routes are available without NHS referral.
Are fat jabs covered by NHS prescriptions?
Yes, if prescribed through an NHS tier 3 specialist weight management service, they are covered under NHS prescriptions (standard prescription charges apply in England, free in Scotland/Wales/Northern Ireland). Private prescriptions require payment at the dispensing pharmacy, typically £129–£320/month depending on dose and provider.
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.