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 Slimming Injections? NHS Guide for UK Adults
Slimming injections are prescription-only weight loss medications now available in the UK through both NHS specialist services and private prescribers. Despite the colloquial name "fat jabs", they are not cosmetic injections—they are evidence-based pharmaceutical treatments that work by mimicking natural hormones to help regulate appetite and support weight management. Three medications are currently licensed for this use in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All require a clinical assessment before prescription and work best alongside dietary changes and physical activity.
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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.
Three UK-Licensed Weight Loss Injections Explained
Mounjaro (tirzepatide) is a once-weekly injection that works as a dual GIP and GLP-1 receptor agonist—activating two natural hormone pathways to help reduce appetite and support fullness. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary significantly.
Wegovy (semaglutide) is also a once-weekly injection but works via GLP-1 alone. In clinical trials, it helped participants lose approximately 15% of their body weight on average alongside lifestyle changes.
Saxenda (liraglutide) is a daily injection (not weekly) and was the first GLP-1 medication licensed for weight management in the UK. All three are prescription-only and require assessment by a licensed prescriber to determine suitability based on your individual health, BMI, and weight-related conditions. Use our free GLP-1 weight loss calculator above to explore potential outcomes at different doses.
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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 Slimming Injections Work—The GLP-1 Mechanism
GLP-1 receptor agonists mimic a natural hormone your gut produces after eating. This hormone signals to your brain that you are full, which may help reduce hunger and food cravings. The injections also slow how quickly food leaves your stomach, helping you feel satisfied for longer after meals. This is not about suppressing appetite artificially—it is about supporting your body's natural fullness signals. Weight loss injections work best as part of a comprehensive approach: they support reduced-calorie eating and increased physical activity, not replace them. Your GP or prescriber will assess whether this approach is appropriate for you. Check your personal BMI using our free BMI calculator to understand your starting point and eligibility.
- 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 slimming injections the same as cosmetic fat-dissolving injections?
No. Slimming injections like Mounjaro and Wegovy are prescription medications that work systemically by regulating appetite hormones. Cosmetic injections (like aqualyx) dissolve localised fat deposits. They are entirely different treatments. Prescription weight loss injections require a clinical assessment and are only available through licensed prescribers.
Can I get slimming injections through the NHS?
Yes, NHS specialist tier 3 weight management services can prescribe these medications. However, you need a GP referral and access varies significantly by region. Some areas have waiting lists of many months. You can also access them privately through GPhC-registered online prescribers following an online clinical assessment, typically without an NHS referral.
What weight loss can I expect from slimming injections?
Clinical trials show average weight loss of 15–21% of body weight depending on the medication and dose, though individual results vary considerably. Some people lose more, others less. Weight loss also depends on your diet, physical activity, and how your body responds. Results are not assured, and they work best alongside lifestyle changes, not instead of them.
Who is eligible for slimming injections on the NHS?
Generally, adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea, cardiovascular disease). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds may apply. Your GP will assess your individual eligibility.
What are the side effects of weight loss injections?
Common side effects include nausea, diarrhoea, constipation, and stomach discomfort—usually mild and temporary. Serious side effects are rare but can include pancreatitis (severe stomach pain) and gallbladder issues. Always discuss potential risks and benefits with your prescriber before starting treatment. They will monitor you throughout.
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.