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
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Slimming Jab UK — Your Complete Options Guide
A slimming jab is a prescription-only weight loss injection licensed in the UK to help adults manage excess weight alongside lifestyle changes. Three main options are currently available: Mounjaro, Wegovy, and Saxenda. Each works differently, requires a clinical assessment before prescription, and can be accessed through NHS specialist services or private online prescribing. This guide compares all three options so you can understand what's available and how to discuss them with your GP or healthcare provider.
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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.
UK-Licensed Weight Loss Injections — How They Compare
Mounjaro (tirzepatide) is a once-weekly injection that activates two hormone pathways — GIP and GLP-1. Clinical trials showed it helped participants lose an average of 15–21% of their body weight over 72 weeks. Maximum dose is 15 mg weekly.
Wegovy (semaglutide) is also a once-weekly injection but activates the GLP-1 pathway only. STEP clinical trials demonstrated average weight loss of around 15% body weight at the maximum dose of 2.4 mg. Dosing starts at 0.25 mg and escalates gradually.
Saxenda (liraglutide) is a daily injection (not weekly) and was the first GLP-1 weight loss injection approved in the UK. Maximum dose is 3 mg daily. Clinical evidence shows average weight loss of approximately 5–10% body weight.
All three are prescription-only medicines requiring a clinical assessment before being prescribed. Individual results vary significantly, and all work best alongside a reduced-calorie diet and increased physical activity.
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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
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NHS vs Private Access — How to Get a Slimming Jab
NHS access: NICE has approved tirzepatide (Mounjaro) and semaglutide (Wegovy) for NHS specialist tier 3/4 weight management services. You'll need a GP referral to a specialist weight management clinic. Availability varies by region (ICB), and waiting lists can be lengthy — sometimes months to over a year depending on your area.
Private access: All three options are available through UK-licensed online prescribers following a clinical assessment. No NHS referral needed. Available more quickly than NHS, but you pay the full cost (typically £129–£320 monthly depending on dose and provider).
Eligibility: Adults with BMI ≥30, or BMI 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, sleep apnoea, or cardiovascular disease), may qualify. Lower thresholds apply for South Asian, Chinese, Black African, and Caribbean adults.
- 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's the difference between a slimming jab and weight loss jab?
These terms are used interchangeably in the UK to describe GLP-1 and GIP/GLP-1 receptor agonist injections licensed for weight management. They're prescription-only medicines, not cosmetic injections. All require a clinical assessment before being prescribed by a licensed healthcare provider.
Which slimming jab is best — Mounjaro, Wegovy, or Saxenda?
Clinical trials show Mounjaro achieved the highest average weight loss (15–21%), followed by Wegovy (around 15%), then Saxenda (5–10%). However, individual results vary significantly. The "best" option depends on your personal health, preferences (weekly vs daily injection), and what your prescriber recommends after assessment.
Can I get a slimming jab on the NHS?
Yes, through tier 3 specialist weight management services after GP referral. NICE approved Mounjaro and Wegovy for NHS use, but availability varies by region and waiting lists can be very long. Private prescription is faster but involves full cost. Check with your local ICB for NHS availability in your area.
How quickly do slimming jabs work?
Weight loss typically begins within the first 2–4 weeks, with most significant changes occurring over 8–12 weeks. Doses are escalated gradually (every 4 weeks typically), so full effects aren't seen until you reach your maintenance dose, usually around 16–20 weeks. Progress varies between individuals.
Do slimming jabs have side effects?
Common side effects include nausea, constipation, diarrhoea, and stomach pain — usually mild and reducing over time as your body adjusts. Serious side effects are rare but can include severe stomach pain (pancreatitis) or gallbladder issues. Your prescriber will discuss all risks before you start. Track your progress using our free progress tracker to monitor how you respond.
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.