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 Is a Slimming Injection and How Does It Work?
A slimming injection is a prescription-only medication that may help support weight management in UK adults. The most commonly discussed slimming injections are GLP-1 receptor agonist medications like Wegovy (semaglutide) and Mounjaro (tirzepatide), which work by reducing appetite and helping you feel fuller after eating. These are not cosmetic injections — they are licensed medicines that require a clinical assessment before prescription. Results vary between individuals, and injections work best alongside 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.
How Do Slimming Injections Work?
GLP-1 receptor agonist injections mimic a natural hormone produced in your gut after eating. They work by sending signals to your brain that reduce hunger and increase feelings of fullness after smaller meals. The medication also slows how quickly food leaves your stomach, helping you feel satisfied longer.
Key effects include:
- Reduced appetite and food cravings
- Increased satiety (feeling of fullness)
- Slower gastric emptying
- Better blood sugar control
In clinical trials, Wegovy (semaglutide) helped participants lose an average of approximately 15% of their body weight over 68 weeks — though individual results vary significantly. Mounjaro (tirzepatide) uses a dual mechanism, targeting both GIP and GLP-1 pathways, with clinical trials showing average weight loss of 15–21% depending on dose. Both are prescription-only medicines available in the UK through NHS specialist weight management services or private prescribing following a clinical assessment.
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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.
Eligibility and Access in the UK
You may be eligible for a slimming injection in the UK if you have a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. Lower BMI thresholds apply if you are of South Asian, Chinese, Black African, or Black Caribbean heritage, as these groups face higher health risks at lower BMI levels.
Access through the NHS involves referral to a specialist tier 3 or tier 4 weight management service — waiting times vary by region and can be lengthy. Alternatively, you can access these medications through private online prescribing with a UK-licensed prescriber. Whatever route you choose, a clinical assessment is required to confirm the injection is suitable for your individual health circumstances.
- 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 safe?
GLP-1 medications like Wegovy have been approved by the MHRA for weight management in UK adults and have extensive clinical trial data. Common side effects include nausea, diarrhoea, and constipation — usually mild and temporary. Serious side effects are rare but can include pancreatitis. Always discuss any health concerns with your GP or prescriber before starting.
Will I lose weight assured with a slimming injection?
No. Clinical trials show that on average, participants lose 15–21% of body weight, but results vary significantly between individuals. Weight loss injections work best alongside reduced-calorie diet and increased physical activity. Some people respond very well; others may see modest results. Your prescriber will monitor your progress.
What's the difference between Wegovy and Ozempic?
Both contain semaglutide, but Wegovy is licensed specifically for weight management at doses up to 2.4 mg. Ozempic is licensed for type 2 diabetes (doses up to 2 mg) and is not approved for weight loss in the UK. Using Ozempic for slimming is off-label. Wegovy is the correct licensed option for weight management.
How long do I need to use a slimming injection?
Slimming injections are typically long-term treatments. You continue using the injection as long as it is helping you lose weight and you tolerate it well. If you stop, weight regain can occur over time. Your prescriber will discuss how long treatment should continue based on your individual response and goals.
Can I use our BMI calculator to check if I'm eligible?
Yes. Use our free BMI calculator above to determine your current BMI and see where you fall in the NHS eligibility ranges. If your BMI is 27 or above with health conditions, or 30 and above, you may be eligible. For personalised advice about whether a slimming injection is right for you, contact your GP or a licensed prescriber for a clinical assessment.
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.