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
Fat Jabs UK — Your Guide to Weight Loss Injections
Fat jabs (also called slimming injections or weight loss injections UK) are prescription-only medicines that help regulate appetite and support sustainable weight loss. Three medications are currently licensed in the UK for weight management: Mounjaro, Wegovy, and Saxenda. Each works differently and suits different people. This guide compares all available options, how to access them through the NHS or privately, and what to expect from each.
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.
Comparing UK-Licensed Weight Loss Injections
Mounjaro (tirzepatide) is the newest option, working as a dual GIP and GLP-1 receptor agonist. It's injected once weekly at doses from 2.5 mg to 15 mg. Clinical trials showed participants lost an average of 15–21% of their body weight over 72 weeks alongside diet and exercise changes. Wegovy (semaglutide) is a GLP-1 receptor agonist injected once weekly, with a maximum dose of 2.4 mg. STEP trials demonstrated approximately 15% average weight loss. Saxenda (liraglutide) is an older GLP-1 option but requires daily injections rather than weekly, making it less convenient for many patients. Use our free GLP-1 weight loss calculator to estimate potential results at different doses and compare projected outcomes across the three options.
- Mounjaro: once weekly, dual hormone mechanism, fastest escalation
- Wegovy: once weekly, single hormone pathway, extensive trial data
- Saxenda: daily injections, longest track record, less convenient dosing
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.
NHS Access and Private Prescription Routes
In the UK, all weight loss injections are prescription-only medicines requiring clinical assessment. NHS access is through tier 3 specialist weight management services — you'll need a GP referral, though waiting lists vary significantly by region and can be lengthy. Private access is available through GPhC-registered online prescribers following an online consultation; results are usually available within days. Eligibility for either route includes 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. Check your BMI using our free BMI calculator above to see if you might be eligible, and discuss options with your GP or a licensed prescriber to find the right approach for your situation.
- 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 slimming injections?
Yes — "fat jabs", "slimming injections", and "weight loss injections" all refer to the same prescription-only medicines. They're GLP-1 or GIP/GLP-1 receptor agonists, not cosmetic injections. They work by regulating appetite hormones and require a clinical assessment before being prescribed.
How much weight will I lose with fat jabs?
Clinical trials show average weight loss of 15–21% of body weight over several months, depending on the medication and dose. However, individual results vary significantly — some people lose more, some less. Weight loss is achieved alongside reduced-calorie diet and increased physical activity, not from the injection alone.
Can I get weight loss injections on the NHS?
Yes, but access is through specialist tier 3 weight management services requiring a GP referral. Waiting lists are often long (months to years in some areas). Private online prescribing is faster but costs approximately £130–£320 per month depending on dose. Both routes require clinical eligibility assessment.
What are the side effects of fat jabs?
Common side effects include nausea, diarrhoea, constipation, and stomach pain — usually mild and temporary. Serious side effects are rare but include pancreatitis (severe stomach pain) and gallbladder problems. Most people tolerate these injections well, especially once their body adjusts. Always discuss any concerns with your prescriber.
Which fat jab is best — Mounjaro or Wegovy?
Both are effective; the "best" depends on your individual health profile, medical history, and prescriber recommendation. Mounjaro uses a dual hormone mechanism and may work slightly faster, while Wegovy has longer trial evidence. A prescriber will assess which is more suitable for you after a clinical consultation.
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.