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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What Is a Diet Jab? UK Weight Loss Injections Explained
A "diet jab" is a colloquial UK term for prescription weight loss injections, most commonly referring to GLP-1 receptor agonist medications like Wegovy (semaglutide) or Mounjaro (tirzepatide). These are licensed medications available through NHS specialist services and private prescribing in the UK. They work by mimicking natural hormones that regulate appetite and blood sugar, potentially helping you feel fuller for longer. This UK health guide explains how they work, eligibility criteria, and how to access them through the NHS or privately.
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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.
How Do Diet Jabs Work?
Diet jabs are GLP-1 or GIP/GLP-1 receptor agonist injections that activate natural hormone pathways in your body. Wegovy (semaglutide) is a once-weekly GLP-1 injection, whilst Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, also injected once weekly. Both may help reduce appetite, decrease food cravings, and support feeling fuller after smaller meals. They also slow how quickly food leaves your stomach, though this effect typically reduces over time.
- In clinical trials, Wegovy helped users lose an average of approximately 15% of body weight over 68 weeks
- Mounjaro clinical data showed average weight loss of 15–21% depending on dose
- Results were achieved alongside a reduced-calorie diet and increased physical activity
- Individual results vary significantly — weight loss is not assured
These are prescription-only medicines requiring clinical assessment before being prescribed. Use our free BMI calculator to check if you may be eligible based on your current BMI and health status.
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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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UK Eligibility and NHS Access
To access diet jabs in the UK, you must meet eligibility criteria: adults with a BMI of 30 kg/m² or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may have lower eligibility thresholds due to higher health risks at lower BMI levels.
- NHS access: Referral to a tier 3 specialist weight management service through your GP; availability varies significantly by region; waiting lists can be lengthy
- Private access: Online consultation with a UK-licensed prescriber; available without NHS referral; costs typically range from £129–£320 per month depending on dose
- Track your weight loss progress with our free progress tracker once you begin treatment
- 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 diet jabs safe?
Diet jabs are prescription-only medicines that have undergone clinical trials. Common side effects include nausea, diarrhoea, and constipation, particularly in early weeks. Serious side effects are rare but can include pancreatitis or gallbladder issues. Your prescriber will assess suitability based on your health history. Always report any concerns to your GP.
What's the difference between Wegovy and Mounjaro?
Both are once-weekly injections for weight loss. Wegovy (semaglutide) is a GLP-1 agonist; Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist. Clinical trials showed Mounjaro may produce slightly higher average weight loss (15–21%) compared to Wegovy (approximately 15%), though individual results vary. Both require prescription and clinical assessment.
Is Ozempic the same as a diet jab?
No. Ozempic contains semaglutide (like Wegovy) but is licensed for type 2 diabetes management, not weight loss. Wegovy is the UK-licensed weight management version of semaglutide. Using Ozempic for weight loss is off-label and not recommended. Always use the correctly licensed medication prescribed by your healthcare professional.
How do I get a diet jab on the NHS?
Ask your GP for a referral to your local tier 3 specialist weight management service. They will assess your BMI, health conditions, and suitability. NHS access varies by region — some areas have significant waiting lists. Private prescribing is faster but involves upfront costs. Your GP can discuss both options.
Can I use our weight converter and BMI calculator to check my eligibility?
Yes. Use our free weight converter to ensure you know your weight in kg, then use our BMI calculator to check where you fall on the NHS BMI scale. A BMI of 30 or above, or 27+ with weight-related health conditions, may make you eligible. Always discuss with your GP or prescriber for a formal assessment.
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.