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
How Do Slimming Injections Work in the UK?
Slimming injections, commonly called "fat jabs" in the UK, are prescription-only weight loss medications that work by mimicking natural hormones in your body. These GLP-1 weight loss injections help regulate appetite and food intake, supporting weight management alongside lifestyle changes. Three options are currently licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). This guide explains how each works, how to access them, and whether they might be suitable for you.
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.
UK-Licensed Slimming Injections: How They Work
All licensed weight loss injections in the UK work by activating hormone pathways that regulate hunger and fullness. Here's how the three main options function:
- Mounjaro (tirzepatide): A dual GIP and GLP-1 receptor agonist. It activates two natural hormone pathways, enhancing appetite suppression and food cravings reduction. Injected once weekly at doses 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 that signals fullness to the brain and slows stomach emptying. Once-weekly injection at doses from 0.25 mg to 2.4 mg. STEP trial data showed average weight loss of approximately 15% of body weight.
- Saxenda (liraglutide): Also a GLP-1 agonist but administered as a daily injection at doses from 0.6 mg to 3 mg. Mechanism similar to Wegovy but requires daily dosing.
All three are prescription slimming injections — only available with a prescription following clinical assessment. They work best alongside reduced-calorie diet and increased physical activity. Individual results vary considerably.
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.
Accessing Slimming Injections: NHS vs Private Routes
In the UK, prescription weight loss injections are available through two routes. NHS access: Referral to a tier 3 specialist weight management service from your GP is required. Eligibility typically requires a BMI of 30 or above, or 27+ with weight-related health conditions. Availability varies significantly by region — waiting lists can be lengthy. Private access: Available through online consultations with UK-licensed prescribers. No NHS referral needed, but costs range from approximately £129–£320 monthly depending on dose. Both routes require a clinical assessment to ensure the medication is appropriate. Use our free GLP-1 weight loss calculator to explore projected outcomes, and our BMI calculator to check your personal BMI and eligibility context.
- 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.
What do slimming injections actually do to your body?
Slimming injections activate natural hormone pathways (GLP-1 and/or GIP) that reduce appetite, increase feelings of fullness, and slow stomach emptying. This helps you eat smaller portions and feel satisfied longer. They work on the brain's hunger centres, not through metabolism changes. Results require lifestyle changes too.
Are slimming injections safe in the UK?
Yes, when prescribed appropriately. All three UK-licensed options (Mounjaro, Wegovy, Saxenda) have undergone rigorous clinical trials. Common side effects are nausea and digestive changes, usually temporary. Serious side effects are rare. Your prescriber assesses suitability based on your health history. Always discuss concerns with your GP or prescriber.
How quickly do slimming injections work?
Weight loss typically begins within 2–4 weeks but varies individually. Most people see noticeable results by 8–12 weeks. Clinical trials measured results over 68–72 weeks. Doses are gradually increased over months, so full effects take time. Consistency with diet and activity matters significantly.
Can I get slimming injections on the NHS in the UK?
Yes, but only through specialist tier 3 weight management services after GP referral. Eligibility requires BMI 30+ or 27+ with weight-related health conditions. Availability and waiting times vary greatly by region. Many areas have long waiting lists. Private prescribing may be faster but involves cost.
Which slimming injection is most effective — Mounjaro, Wegovy, or Saxenda?
Clinical trials show similar weight loss averages (15–21%), but Mounjaro's dual mechanism showed slightly higher average results. However, individual response varies significantly. The "best" option depends on your personal health, injection preference (once weekly vs daily), and access route. Your prescriber will recommend based on your circumstances.
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.