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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Weight Loss Jab Checks in the UK
A weight loss jab (often called a "slimming injection" or "fat jab") is a prescription-only medicine that may help with weight management in eligible UK adults. Several medications are now available through both NHS specialist services and private prescribing. This guide explains which weight management options exist in the UK, who may be eligible, and how to find out if prescription weight loss treatment is right for you. Individual results vary, and all treatment requires a clinical assessment from a licensed prescriber.
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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.
UK-Licensed Weight Loss Injections
Three prescription-only medications are currently licensed in the UK for weight management in adults:
- Mounjaro (tirzepatide) — a once-weekly injection. Clinical trials showed participants lost an average of 15–21% of their body weight alongside diet and exercise changes. It is a dual GIP and GLP-1 receptor agonist.
- Wegovy (semaglutide) — a once-weekly injection. Trial data showed average weight loss of approximately 15% of body weight. It is a GLP-1 receptor agonist only.
- Saxenda (liraglutide) — a daily injection for weight management. Evidence supports meaningful weight loss when combined with lifestyle changes.
All three require a prescription and clinical assessment. Eligibility depends on your BMI, age, and any weight-related health conditions you may have. Use our free BMI calculator above to check your current BMI and understand where you fall on NHS weight classifications.
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.
Eligibility and NHS vs Private Access
You may be eligible for prescription weight loss medication if you are an adult with a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Lower thresholds may apply if you are of South Asian, Chinese, Black African, or Black Caribbean heritage. NHS access is through specialist tier 3 weight management services — your GP can refer you, though waiting times vary by region. Private access is available through UK-licensed online prescribers following a clinical consultation, without an NHS referral. Either route requires a licensed prescriber to assess whether treatment is suitable for you. Your GP can discuss which option may be best for 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.
What is a weight loss jab and how does it work?
Weight loss jabs are prescription medications that activate natural hormone pathways to help regulate appetite and food cravings. Mounjaro and Wegovy are once-weekly injections. They may help you feel fuller after smaller meals and reduce hunger signals. Individual results vary. They are not clinically supported solutions — they work alongside diet and exercise changes for best results.
How do I know if I'm eligible for weight loss medication?
Adults with a BMI of 30 or above are generally eligible. Those with a BMI of 27–29.9 may be eligible if they have weight-related health conditions like diabetes or hypertension. Your prescriber will assess your individual health and medical history. Use our free BMI calculator on this page to check your starting point, then discuss options with your GP.
Can I get weight loss jabs on the NHS?
Yes. NICE has approved tirzepatide (Mounjaro) and semaglutide (Wegovy) for NHS use through specialist weight management services. Your GP can refer you to your local tier 3 service. Waiting times and availability vary significantly by region. Some areas have long waiting lists. Private prescribing is another option if NHS waiting times are prohibitive.
What side effects should I expect from weight loss injections?
Common side effects include nausea, diarrhoea, constipation, and stomach discomfort — usually mild and temporary. Most people adjust within the first few weeks. Serious side effects are rare but can include pancreatitis or gallbladder problems. Your prescriber will discuss all risks with you. Always report unexpected symptoms to your GP or healthcare provider immediately.
Will I keep the weight off after I stop the injection?
Weight loss medications work while you are taking them. To maintain weight loss long-term, you will need to sustain the diet and exercise habits you develop during treatment. Some people continue on a maintenance dose. Your prescriber will create a plan that works for your goals. Regular progress tracking can help — try our free progress tracker to log your journey.
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.