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 Injections?
Fat injections—commonly called slimming injections, weight loss injections, or fat jabs—are prescription-only medicines designed to help adults manage excess weight. Despite the name, they don't dissolve fat directly. Instead, they work by regulating appetite and helping you feel fuller after smaller meals. In the UK, three licensed options are available through prescription: Mounjaro, Wegovy, and Saxenda. All are based on GLP-1 or dual GIP/GLP-1 hormone-mimicking technology, not cosmetic procedures. They're part of the NHS weight management pathway and available privately through 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.
UK-Licensed Weight Loss Injections
Three prescription-only weight loss injections are currently licensed in the UK:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist given once weekly. Maximum dose is 15mg. Clinical trials showed participants lost an average of 15–21% of body weight over 72 weeks alongside dietary changes and increased physical activity.
- Wegovy (semaglutide) — a GLP-1 receptor agonist injected once weekly up to 2.4mg maximum. Average weight loss in trials was approximately 15% of body weight at the maintenance dose.
- Saxenda (liraglutide) — a GLP-1 receptor agonist given by daily injection up to 3mg. The oldest of the three, with well-established safety data.
All three require a prescription from a UK-licensed prescriber and a clinical assessment to confirm eligibility. Individual results vary significantly. You can estimate potential outcomes using our GLP-1 weight loss calculator, which helps visualise projected weight loss at different doses based on your starting weight.
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 Slimming Injections Work
These medicines work by mimicking natural hormones your gut produces when you eat. GLP-1 receptor agonists activate the GLP-1 hormone pathway, which signals to your brain that you're full, reducing appetite and cravings. Mounjaro's dual mechanism activates both GLP-1 and GIP pathways, which may enhance appetite suppression and fat utilisation. The injections also slow how quickly food leaves your stomach, helping you feel satisfied longer after meals. Results depend on dose, individual metabolism, lifestyle changes, and dietary adherence. Weight loss typically takes several weeks to become noticeable. Use our free BMI calculator and weight converter to track your baseline measurements and monitor progress over time.
- 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 injections the same as cosmetic fat-dissolving injections?
No. Fat injections for weight loss are prescription medicines (Mounjaro, Wegovy, Saxenda) that work systemically to regulate appetite. Cosmetic fat-dissolving injections are different products and are not weight loss treatments. Weight loss injections require clinical assessment and prescription from a licensed UK prescriber.
How much weight can I expect to lose with fat injections?
Clinical trials show average weight loss of 15–21% of body weight depending on the medication and dose. However, individual results vary significantly based on diet, physical activity, metabolism, and how long you continue the treatment. Your GP or prescriber can discuss realistic expectations for your situation.
Can I get weight loss injections on the NHS?
Yes, but availability varies by region. NHS access is through tier 3 specialist weight management services, usually requiring referral from your GP. Many areas have long waiting lists. Private prescription options are also available through online clinics following clinical assessment, typically costing £129–£320 monthly depending on dose.
What are the main side effects of fat injections?
Common side effects include nausea, diarrhoea, constipation, and stomach pain—usually mild and temporary. Some people experience headache, fatigue, or injection site reactions. Serious side effects are rare but can include pancreatitis or gallbladder problems. Your prescriber will discuss full safety information before starting treatment.
Am I eligible for weight loss injections in the UK?
Generally, you may be eligible if your BMI is 30 or above, or 27–29 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea). For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply. A GP or licensed prescriber will assess your individual suitability.
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.