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? A UK Guide
Slimming injections are prescription-only medications that help regulate appetite and support weight loss in UK adults. Often called "fat jabs" or weight loss injections, they're GLP-1 and GLP-1/GIP receptor agonist medications—not cosmetic injections. Three options are currently licensed in the UK for weight management. They work by mimicking natural hormones that signal fullness to your brain, typically used alongside a reduced-calorie diet and increased physical activity. Access is available through NHS specialist services or private prescribing with online clinical assessment. MyWeightCheck's GLP-1 weight loss calculator can help you explore potential outcomes with these treatments.
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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 Slimming Injections Explained
Three prescription slimming injections are currently licensed in the UK for weight management. Mounjaro (tirzepatide) is a once-weekly injection using a dual GIP and GLP-1 mechanism—the newest option. Clinical trials showed participants lost an average of 15–21% body weight at the highest dose. Wegovy (semaglutide) is also injected once weekly and works as a GLP-1 agonist alone, with average weight loss around 15% in clinical trials. Saxenda (liraglutide) is a daily injection, also GLP-1, approved earlier than the other two. All three are prescription-only and require a clinical assessment before dispensing. Individual results vary significantly, and all are most effective when combined with lifestyle changes. None are available on the NHS as routine prescriptions—access is through specialist tier 3 weight management services following GP referral, though waiting lists vary by region and many people access them privately.
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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
Slimming injections work by activating GLP-1 receptors in your brain and gut. GLP-1 is a natural hormone released after eating that signals fullness and reduces appetite. These medications amplify that signal, helping you feel fuller on smaller portions and reducing cravings. They also slow how quickly food leaves your stomach, extending the feeling of satiety. Mounjaro adds a second mechanism by activating GIP receptors as well, which may enhance fat regulation. The medications do not work like insulin and are not stimulants—they support your body's own appetite control systems. Weight loss injections uk eligibility depends on BMI and health factors. Use our free BMI calculator to check if you might qualify, and consult your GP about whether weight loss injection eligibility applies to your 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 the same as cosmetic fat-dissolving injections?
No. Prescription weight loss injections (Mounjaro, Wegovy, Saxenda) are systemic medications injected weekly or daily that regulate appetite via the GLP-1 pathway. Cosmetic fat-dissolving injections (like Aqualyx) are localized injectable products with a different mechanism entirely. Slimming injections are medications requiring prescription and clinical assessment; cosmetic injections are aesthetic treatments. Slimming injections cause body-wide weight loss; cosmetic injections target specific small areas.
Can I get slimming injections through the NHS?
Access to NHS slimming injections is limited and varies by region. You must be referred to a tier 3 specialist weight management service by your GP. Eligibility requires BMI ≥30, or ≥27 with weight-related health conditions. Many areas have long waiting lists. Private prescribing via online consultation is more readily available if you meet clinical criteria. Your GP can advise on NHS availability in your local area and discuss private options.
What's the difference between Mounjaro and Wegovy for weight loss?
Both are once-weekly injections, but Mounjaro (tirzepatide) uses dual GIP+GLP-1 activation, while Wegovy (semaglutide) uses GLP-1 alone. Clinical trial data suggests Mounjaro may produce slightly higher average weight loss (15–21% vs ~15%), though individual results vary considerably. Maximum doses differ: Mounjaro reaches 15mg weekly; Wegovy reaches 2.4mg weekly. Both require prescription. Your prescriber will assess which is most suitable for you.
Do I need to diet and exercise while using slimming injections?
Yes. Prescription weight loss injections are most effective alongside a reduced-calorie diet and increased physical activity. Clinical trials showing weight loss results were conducted with participants following dietary guidance and exercise recommendations. The injection supports appetite regulation, but sustainable weight loss requires lifestyle change. Your prescriber will discuss dietary and physical activity goals as part of your treatment plan.
How quickly will I lose weight with slimming injections?
Weight loss is gradual and varies significantly between individuals. Most people see initial results within the first 2–4 weeks, with continued loss over months. NHS recommends aiming for 0.5–1kg loss per week as a safe, sustainable rate. Clinical trials measured results over 68–72 weeks. Never expect assured outcomes—factors including starting weight, diet adherence, activity level, and individual metabolism all affect results. Track your progress over months, not days or weeks.
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.