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 Jabs? UK Guide to Weight Loss Injections
Slimming jabs are prescription-only weight loss injections that help regulate appetite and support weight management in UK adults. Also known as fat jabs or weight loss injections, these are licensed medications — not cosmetic treatments — that work by mimicking natural hormones. Three main options are currently available through the NHS and private prescribing in the UK. They're used alongside diet and exercise, not as a replacement, and require a clinical assessment before being prescribed. This guide explains what they are, how they work, and what side effects to expect.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Injection Options
Three prescription slimming injections are licensed for weight management in the UK:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist. Once-weekly injection with doses ranging from 2.5 mg to 15 mg. Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks.
- Wegovy (semaglutide) — a GLP-1 receptor agonist. Once-weekly injection with a maximum dose of 2.4 mg. STEP trial data showed average weight loss of approximately 15% of body weight.
- Saxenda (liraglutide) — a GLP-1 receptor agonist given as a daily injection (not weekly). Maximum dose 3 mg per day.
All three are prescription-only medicines that require a clinical assessment from a UK-licensed prescriber before being issued. Individual results vary significantly, and these medications are most effective when used alongside reduced-calorie diet and increased physical activity.
Clinically BackedWeight Loss
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 Slimming Jabs Work — The GLP-1 Mechanism
Slimming injections work by activating GLP-1 (glucagon-like peptide-1), a natural hormone your gut produces after eating. When activated, GLP-1 signals your brain that you're full, helping to reduce appetite and food cravings. These medications also slow how quickly food leaves your stomach, which extends the feeling of fullness after meals. Mounjaro works differently — it activates both GLP-1 and GIP pathways, offering a dual mechanism. Try our free GLP-1 weight loss calculator to explore how these medications may affect potential weight loss at different doses. Use our BMI calculator to understand your current weight category and whether you might be eligible for prescription weight loss treatment according to NHS guidelines.
- 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 jabs the same as cosmetic fat-dissolving injections?
No. Slimming jabs like Mounjaro and Wegovy are prescription medications — pharmaceutical injections — not cosmetic treatments. Cosmetic fat-dissolving injections (such as aqualyx) are unregulated treatments. Prescription weight loss injections work systemically through hormone pathways and require a clinical assessment before being prescribed.
What are the most common side effects of slimming injections?
The most common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain. These are typically mild to moderate and often improve over time as your body adjusts. Rarer side effects include gallbladder problems, pancreatitis, and changes in heart rate. Always discuss any persistent side effects with your GP or prescriber.
How do I access slimming injections in the UK?
You can access prescription slimming injections through two main routes: NHS specialist weight management services (by GP referral to tier 3 services) or private online prescribing (through GPhC-registered pharmacies following an online clinical assessment). NHS availability varies by region and may involve waiting lists. Private access is faster but involves a cost.
Are slimming jabs suitable for everyone?
No. You must meet NHS eligibility criteria: 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). You cannot take them if you're under 18, pregnant, breastfeeding, or have a history of certain medical conditions. A prescriber will assess your individual suitability.
What's the difference between slimming injections and diet alone?
Slimming injections work alongside diet and exercise — they're not a replacement. They help regulate appetite and reduce cravings, making it easier to maintain a calorie deficit. However, they only work effectively when combined with healthy eating and physical activity. Weight loss typically ranges from 0.5–1 kg per week on average, though individual results vary significantly.
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.