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
Are Fat Injections Safe? What You Need to Know
Fat injections, commonly known as slimming injections or weight loss jabs in the UK, are prescription-only medications that help regulate appetite and support weight management. These are not cosmetic injections—they're licensed medicines administered under medical supervision. Three options are available on the NHS and privately in the UK, all approved by the MHRA for weight management. Safety data comes from extensive clinical trials, though like all medications, they carry potential side effects. Understanding how they work and who they're suitable for helps you make an informed decision with your GP.
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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 Explained
Three prescription-only weight loss injections are currently licensed in the UK for weight management:
- Mounjaro (tirzepatide) — A dual GIP and GLP-1 receptor agonist manufactured by Eli Lilly. Doses range from 2.5 mg to 15 mg, administered once weekly. Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks when combined with diet and exercise.
- Wegovy (semaglutide) — A GLP-1 receptor agonist made by Novo Nordisk. Maximum dose is 2.4 mg once weekly. STEP trials demonstrated average weight loss of approximately 15% of body weight at 68 weeks alongside lifestyle changes.
- Saxenda (liraglutide) — Also a GLP-1 agonist, requiring daily injections (0.6 mg to 3 mg). The oldest option in this class, with well-established safety data over many years of use for both diabetes and weight management.
All three require a prescription from a UK-licensed healthcare professional and a clinical assessment before use. They're not available over-the-counter and must be prescribed following an evaluation of your health history and suitability.
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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 These Injections Work and Safety Considerations
GLP-1 receptor agonists (and tirzepatide's dual mechanism) work by mimicking natural hormones that regulate appetite and blood sugar. They signal to your brain that you're full, reduce food cravings, and slow how quickly food leaves your stomach. This helps you eat less and feel satisfied with smaller portions.
Safety data is robust: these medications have been used for type 2 diabetes management for years, and weight management-specific trials involved thousands of participants. Common side effects include nausea, diarrhoea, and constipation—usually mild and improving over time. Serious side effects are rare but can include pancreatitis or gallbladder problems. Your GP assesses your individual risk before prescribing. Use our free GLP-1 weight loss calculator to explore potential outcomes, or check your BMI with our calculator to understand your starting point.
- 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 injections?
No. Weight loss injections are prescription medicines that work on appetite hormones—they're not cosmetic procedures. Cosmetic fat-dissolving injections (like aqualyx) are different treatments altogether. Weight loss injections are medical treatments requiring clinical assessment and prescription.
What does "fat jabs" mean in the UK?
"Fat jabs" or "slimming jabs" are colloquial UK terms for GLP-1 and GIP/GLP-1 receptor agonist injections used for weight management. They're not a medical term, but they refer to prescription weight loss medications like Mounjaro, Wegovy, or Saxenda—not cosmetic procedures.
Can I get weight loss injections on the NHS in the UK?
Yes, through specialist tier 3 weight management services. Your GP can refer you, but NHS access varies by region and availability. Many areas have long waiting lists. Private prescription is also available through UK-licensed online prescribers without NHS referral.
What are the most common side effects?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain. These typically improve as your body adjusts. Less common but more serious side effects can include pancreatitis or gallbladder problems. Your prescriber discusses all risks before starting treatment.
How long do you stay on weight loss injections?
Duration depends on your individual response and goals. Some people use them for a set period (e.g., 12–24 months), while others continue longer-term for sustained weight management. Your prescriber will discuss the right approach for you and whether maintenance therapy is appropriate.
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.