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
How Does The Fat Jab Work?
Fat jabs, or slimming injections, are prescription-only weight loss medications now available in the UK. The most popular options — Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) — work by activating natural hormone pathways that help regulate appetite and support feeling fuller after smaller meals. These are not cosmetic injections but licensed medicines that require a clinical assessment before being prescribed. Understanding how each one works can help you make an informed decision with your GP or healthcare provider.
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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.
How the Three Main Weight Loss Injections Work in the UK
All three NHS-approved weight loss injections work by mimicking natural hormones that control appetite and hunger signals. Mounjaro (tirzepatide) is a dual mechanism medication — it activates both GIP and GLP-1 receptor pathways, which may offer a more potent effect on appetite suppression and fat regulation. Wegovy (semaglutide) activates the GLP-1 pathway alone and is administered once weekly at doses up to 2.4 mg. Saxenda (liraglutide) is the oldest option, also a GLP-1 medication, but requires daily injections rather than weekly ones.
In clinical trials, Mounjaro showed average weight loss of around 15–21% of body weight, whilst Wegovy showed approximately 14.9% at the maximum dose — though individual results vary significantly. All three slow gastric emptying (how quickly food leaves the stomach), reduce cravings, and support eating smaller portions. Each requires a reduced-calorie diet and increased physical activity for best results.
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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.
NHS Access vs Private — How to Get Slimming Injections in the UK
In the UK, all three weight loss injections are prescription-only medicines that can be accessed through NHS specialist weight management services or privately through online prescribing. NHS access: Referral from your GP to a tier 3 specialist weight management service; availability and waiting times vary significantly by region. Private access: Online consultation with a UK-licensed prescriber, assessment, and prescription dispensed by a GPhC-registered pharmacy if clinically appropriate.
Eligibility typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. People of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds due to higher health risks at lower weights. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your personal BMI.
- 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 weight loss tablets?
Weight loss injections like Mounjaro, Wegovy, and Saxenda work by activating hormone pathways that reduce appetite. Tablets work through different mechanisms — some increase metabolism, others reduce fat absorption. Injections may be more effective for some people. Both are prescription-only. Your GP can advise which is more suitable for you.
Do I need to diet and exercise if I take a slimming injection?
Yes. Weight loss injections work best alongside a reduced-calorie diet and increased physical activity. They support appetite control and help you eat smaller portions, but they're not a replacement for lifestyle changes. Clinical trial results were achieved with both medication and behaviour change combined.
How quickly do fat jabs start working?
Most people notice appetite reduction within the first 1–2 weeks, though gastric effects (slower digestion) may be more noticeable initially. Weight loss typically becomes measurable after 4–6 weeks. Full benefits take several months as doses are gradually increased. Individual timelines vary.
Can I switch between Mounjaro, Wegovy, and Saxenda?
Switching between weight loss injections should only be done under medical supervision. Whilst all three are GLP-1 or GIP/GLP-1 medications, they have different potencies and dosing schedules. Your prescriber will manage any switch carefully to avoid gaps in treatment.
Are slimming injections available on the NHS for everyone?
NHS access is through specialist tier 3 weight management services — not standard GP prescription. Eligibility requires BMI ≥30 (or ≥27.5 with comorbidities), and availability varies significantly by region. Many areas have long waiting lists. Private prescribing is available faster without NHS referral.
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.