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 NHS Slimming Injections?
Slimming injections, often called "weight loss jabs" or "fat jabs" in the UK, are prescription-only medicines that help regulate appetite and support weight management. These are not cosmetic injections—they're clinically approved medications available through NHS specialist services or private prescription. Three UK-licensed options are currently available, all working through GLP-1 hormone pathways. This guide explains what they are, how they work, and how to access them through the NHS.
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 Available
Three prescription slimming injections are licensed for weight management in the UK:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, injected once weekly. Maximum dose 15 mg. Clinical trials showed average weight loss of 15–21% body weight alongside diet and exercise.
- Wegovy (semaglutide) — a GLP-1 receptor agonist, injected once weekly. Maximum dose 2.4 mg. Evidence suggests average weight loss around 15% body weight in clinical trials.
- Saxenda (liraglutide) — a GLP-1 receptor agonist, injected daily. Maximum dose 3 mg. An established option with proven efficacy in weight management.
All three are prescription-only medicines requiring clinical assessment before use. They must be combined with a reduced-calorie diet and increased physical activity for best results. Individual results vary significantly.
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 Injections Work
GLP-1 weight loss injections work by activating hormone pathways that regulate appetite and food intake. GLP-1 (glucagon-like peptide-1) is a natural hormone your body produces after eating. These medications mimic and amplify its effects:
- Signal to your brain that you feel full, reducing appetite
- Decrease food cravings and desire to eat between meals
- Slow how quickly food leaves your stomach (initially)
- Help regulate blood sugar and fat metabolism
Check your BMI using our free BMI calculator above to understand your weight category. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses. These are medical tools, not shortcuts—prescription slimming injections work best alongside lifestyle changes.
- 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 available on the NHS?
Yes, but access is limited. Tirzepatide (Mounjaro) and semaglutide (Wegovy) are approved by NICE for NHS specialist weight management services. However, availability varies by region and waiting lists can be long. Your GP can refer you to tier 3 specialist services in your area. Private access through UK-licensed online prescribers is also available.
What's the difference between these weight loss injections?
Mounjaro uses dual GIP and GLP-1 pathways (potentially more effective), while Wegovy and Saxenda use GLP-1 alone. Mounjaro and Wegovy are once-weekly injections; Saxenda is daily. All require prescription and clinical assessment. Individual suitability varies, and a prescriber will assess which is best for you.
Are slimming injections safe?
These are licensed medicines with established safety profiles. Common side effects include nausea, diarrhoea, and constipation—typically mild and reducing over time. Serious side effects are rare. Certain people (pregnant women, those with thyroid conditions, history of pancreatitis) cannot use them. Always inform your prescriber of all health conditions and medications.
Can I get slimming injections on the NHS if I'm overweight but not obese?
Yes, potentially. NHS criteria include BMI of 27–29.9 kg/m² with weight-related health conditions (diabetes, high blood pressure, heart disease). For South Asian, Chinese, and Black African populations, lower BMI thresholds apply. Your GP can assess your eligibility and refer you to specialist services if appropriate.
How quickly do slimming injections work?
Results vary. Some people notice reduced appetite within days; others take 2–4 weeks. Measurable weight loss typically appears within 4–8 weeks of starting. Clinical trials showed significant loss after 12–16 weeks. Weight loss continues as doses increase. Combining injections with diet and exercise produces better results than injections alone.
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.