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 Are Fat Jabs? UK Slimming Injections Explained
"Fat jabs" is a colloquial UK term for prescription weight loss injections — medically known as GLP-1 receptor agonists (and dual GIP/GLP-1 agonists). These are not cosmetic injections but licensed medications that can help regulate appetite and support weight management alongside dietary changes and physical activity. Three medications are currently licensed in the UK for weight management: Mounjaro, Wegovy, and Saxenda. All are prescription-only and require a clinical assessment before being prescribed. Understanding what these injections are, how they work, and their costs can help you decide if they're right for you.
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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
Mounjaro (tirzepatide) is a once-weekly injection manufactured by Eli Lilly. It's a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. Doses range from 2.5 mg to 15 mg weekly. In clinical trials, participants lost an average of 15–21% of their body weight over 72 weeks — though individual results vary significantly.
Wegovy (semaglutide) is also a once-weekly injection, made by Novo Nordisk. It's a GLP-1 receptor agonist (single mechanism). Maximum dose is 2.4 mg weekly. Clinical trials showed average weight loss of approximately 15% of body weight over 68 weeks alongside reduced-calorie diet and increased physical activity.
Saxenda (liraglutide) is a daily injection at doses up to 3 mg. It's also a GLP-1 agonist. All three are prescription-only medicines requiring assessment by a UK-licensed prescriber before being dispensed.
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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 Slimming Injections Work — The GLP-1 Mechanism
GLP-1 receptor agonists mimic a natural hormone your gut produces after eating. They work by helping your brain recognise when you're full, reducing cravings for food, and supporting you to feel satisfied after smaller meals. They also slow how quickly food leaves your stomach, which can extend feelings of fullness. The result is that many people naturally eat less without feeling deprived — making a calorie deficit easier to maintain. These injections don't work like clinically supported; they're designed to support sustainable weight loss when combined with healthier eating patterns and physical activity. Try our GLP-1 weight loss calculator to estimate potential weight loss at different doses, and use our BMI calculator to check where you currently stand on the healthy weight scale.
- 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 and slimming injections the same thing?
Yes. "Fat jabs" and "slimming injections" are colloquial UK terms for the same prescription medications — GLP-1 and GLP-1/GIP receptor agonist injections used for weight management. They're different from cosmetic fat-dissolving injections. All three UK-licensed options (Mounjaro, Wegovy, Saxenda) are prescription-only medicines, not cosmetic treatments.
How much do fat jabs cost in the UK?
Costs vary by dose and supplier. Private prescribing ranges approximately £129–£320 per month depending on the medication and dose strength. NHS access is available through specialist tier 3 weight management services, though availability and waiting times vary significantly by region. Your GP can advise on NHS eligibility in your area.
Can I get fat jabs on the NHS?
Yes, slimming injections are available through NHS specialist weight management services for adults meeting specific criteria: BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions like type 2 diabetes or high blood pressure. However, NHS availability varies by region, and waiting lists can be lengthy. Your GP can refer you to your local tier 3 service.
What's the difference between Mounjaro and Wegovy?
Both are once-weekly injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist — activating two hormone pathways — while Wegovy (semaglutide) activates GLP-1 only. Clinical trials showed Mounjaro achieved slightly higher average weight loss (15–21% vs 15%), but results vary between individuals. Your prescriber will assess which is more appropriate for you.
Do I need a prescription for fat jabs?
Yes, absolutely. All UK-licensed weight loss injections (Mounjaro, Wegovy, Saxenda) are prescription-only medicines. You cannot buy them without a valid prescription from a UK-licensed prescriber. Private prescribers offer online consultations; NHS prescriptions come through specialist weight management services following referral from your GP.
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.