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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How Do Fat Jabs Work — UK Weight Loss Options Explained
Fat jabs are prescription-only weight loss injections that work by mimicking natural hormones to help regulate appetite and support weight management. In the UK, these are also known as slimming injections or GLP-1 medications. Three main types are currently licensed: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works slightly differently and has different dosing schedules. All require a prescription and clinical assessment before being prescribed. Individual results vary significantly, and they work best when combined with lifestyle changes like a reduced-calorie diet and increased physical activity.
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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 Fat Jabs — How They Compare
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways simultaneously. It comes as a once-weekly injection with doses ranging from 2.5 mg to 15 mg. In clinical trials, Mounjaro helped participants lose an average of 15–21% of their body weight over 72 weeks, though results vary. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. It's also injected once weekly, with a maximum dose of 2.4 mg. Average weight loss in trials was approximately 15% of body weight. Saxenda (liraglutide) is also a GLP-1 agonist but requires a daily injection (not weekly) with a maximum dose of 3 mg. All three work by signalling to your brain that you're full, reducing food cravings, and slowing how quickly food leaves your stomach. Try our GLP-1 weight loss calculator to see estimated projections at different dose levels.
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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.
Accessing Fat Jabs in the UK — NHS and Private Routes
In the UK, fat jabs are prescription-only medicines available through two main routes. NHS access is through specialist tier 3/4 weight management services — usually requiring a GP referral. Eligibility typically requires a BMI of 30 or above, or 27 or above with a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. However, NHS availability varies significantly by region, and waiting lists can be lengthy. Private access is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber. This route does not require NHS referral. Before starting any weight loss injection, a prescriber will assess your individual health, medications, and suitability. Check your eligibility using our free BMI calculator to understand where you fall on NHS thresholds.
- 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's the difference between fat jabs and insulin injections?
Fat jabs are not insulin. They're hormone-mimicking medications that reduce appetite and support weight management. Insulin is used to manage blood sugar in diabetes. Some fat jabs (like Mounjaro) do help with blood sugar control as a side effect, but they work through a completely different mechanism to insulin injections.
How quickly do fat jabs start to work?
Most people notice reduced appetite within the first 1–2 weeks, though some take longer. Significant weight loss typically begins after 4–8 weeks as doses are gradually increased. Maximum weight loss usually occurs after 3–6 months on a stable dose. Results vary between individuals.
Can I use fat jabs without changing my diet?
While fat jabs do help reduce appetite, they work best alongside a reduced-calorie diet and increased physical activity. They support weight loss but are not a substitute for lifestyle changes. Clinical trials showed the best results when medications were used with diet and exercise adjustments.
Are fat jabs safe long-term?
Fat jabs have been used safely for many years in diabetes treatment. For weight management, the longest clinical trials ran for 68–72 weeks with monitored safety profiles. Long-term safety data beyond this is still being gathered. Your GP or prescriber will assess whether they're suitable for your individual health situation.
What happens when I stop using fat jabs?
Most people gradually regain weight when they stop, though not necessarily all of it. Many people maintain some weight loss through continued healthy eating and exercise habits. Some choose to continue long-term if they're tolerating the medication well. This should be discussed with your prescriber based on your individual circumstances.
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.