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 in the UK?
Slimming injections are prescription-only weight loss medications available in the UK that work by regulating appetite and helping you feel fuller after eating smaller meals. Often called "weight loss jabs" or "fat jabs," these are licensed pharmaceutical injections—not cosmetic treatments. Three prescription options are currently available on the NHS and privately: Mounjaro, Wegovy, and Saxenda. All require a clinical assessment before being prescribed and are typically used alongside a reduced-calorie diet and increased physical activity for best results.
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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.
Three UK-Licensed Weight Loss Injections
Mounjaro (tirzepatide) is a once-weekly dual GIP and GLP-1 receptor agonist injection manufactured by Eli Lilly. Clinical trials showed participants lost an average of 15–21% of their body weight. Wegovy (semaglutide) is a once-weekly GLP-1 receptor agonist made by Novo Nordisk, with trial data showing average weight loss of around 15% body weight. Saxenda (liraglutide) is a daily GLP-1 injection also from Novo Nordisk, typically prescribed at doses up to 3 mg per day.
All three are prescription-only medicines in the UK. Access is available through NHS specialist weight management services (tier 3) or privately via a UK-licensed prescriber following an online or in-person clinical assessment. Each medication requires individual eligibility assessment, and a prescriber will determine which—if any—is suitable for you based on your health profile, current medications, and weight management goals.
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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 Weight Loss Injections Work
Weight loss injections work by mimicking or enhancing natural gut hormones that regulate appetite and blood sugar. When you eat, your gut naturally releases GLP-1 (and in the case of Mounjaro, GIP as well), which signals to your brain that you're full and satisfied. These injections amplify that signal, meaning you feel fuller sooner, eat smaller portions, and experience fewer food cravings throughout the day.
The mechanism also slows how quickly food leaves your stomach, giving your body more time to absorb nutrients and maintain that feeling of fullness. Individual responses vary significantly—some people experience dramatic appetite reduction, whilst others notice a more subtle effect. Combined with a reduced-calorie diet tracked using our free progress tracker and regular physical activity, these injections may support meaningful weight loss 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 slimming injections the same as cosmetic fat-dissolving injections?
No. Prescription weight loss injections like Mounjaro and Wegovy are pharmaceutical medicines that work internally by regulating appetite hormones. Cosmetic fat-dissolving injections are a completely different treatment. Weight loss injections are prescription-only and require clinical assessment; cosmetic injections are not pharmaceutical treatments.
How do I know if I'm eligible for prescription weight loss injections in the UK?
Generally, you may be eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Use our free BMI calculator above to check your BMI, then speak to your GP or a licensed prescriber about whether prescription weight loss medication is appropriate for you.
Can I get weight loss injections on the NHS?
Yes, but availability varies significantly by region. You'd need to be referred to a tier 3 specialist weight management service by your GP. Waiting times can be lengthy in many areas. Private prescribing via a licensed UK prescriber is also available without an NHS referral, though this involves a cost.
How quickly will I lose weight with slimming injections?
Weight loss varies between individuals. Clinical trials showed average losses ranging from 15–21% of body weight over several months, achieved alongside a reduced-calorie diet and exercise. Results are not assured, and some people lose less than the average. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
What are the common side effects of weight loss injections?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach discomfort—often most noticeable when starting treatment. These typically improve over time as your body adjusts. Serious side effects are rare but can include pancreatitis. Always discuss potential side effects with your prescriber before starting treatment.
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.