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 Injections — How They Work in the UK
Weight loss injections are prescription-only medicines that help reduce appetite and support weight management in UK adults. Also known as "slimming injections" or "weight loss jabs", these GLP-1 and GIP/GLP-1 receptor agonist medications are licensed by the MHRA for people with a BMI of 30 or above, or 27 or above with weight-related health conditions. They work alongside diet and exercise, not instead of them. Access is available through NHS specialist weight management services or private prescribing following a clinical assessment. This guide explains how they work, eligibility, and what to expect.
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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 Weight Loss Injections Work
Weight loss injections contain medicines that mimic natural gut hormones. GLP-1 receptor agonists (like semaglutide in Wegovy) activate the GLP-1 hormone pathway, which signals to your brain that you are full, reduces appetite and food cravings, and slows how quickly food leaves your stomach. GIP/GLP-1 dual agonists (like tirzepatide in Mounjaro) activate two hormone pathways simultaneously, which may provide additional appetite suppression.
These effects typically develop gradually over weeks as your dose increases. Clinical trials show that on average, people using GLP-1 weight loss injections lose 15–21% of their body weight over 68–72 weeks, though individual results vary significantly. Weight loss occurs because you naturally eat less due to reduced hunger and increased fullness—not through toxins or dangerous mechanisms.
Injections are once-weekly (Mounjaro and Wegovy) or once-daily (Saxenda), making them easier to use than daily pills. Use our free GLP-1 weight loss calculator to explore how different doses may affect weight loss projections.
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.
UK Eligibility and How to Access
In the UK, weight loss injections are prescription-only and require a clinical assessment. You may be eligible if you have a BMI of 30 or above, or BMI of 27–29.9 with weight-related conditions like type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease. If you are of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds (from 27.5) may apply.
NHS access: Referral to tier 3 specialist weight management services; availability and waiting times vary by region. Private access: Online consultation with a UK-licensed prescriber; prescription dispensed by GPhC-registered pharmacies. A prescriber will assess your personal health, medications, and suitability before issuing a prescription. Check your BMI using our free BMI calculator above to understand your starting point.
- 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 the difference between weight loss injections and Ozempic?
Both Wegovy and Ozempic contain semaglutide, but they are different medicines. Ozempic is approved for type 2 diabetes (doses up to 2 mg) and is not licensed for weight loss in the UK. Wegovy is specifically licensed for weight management (dose up to 2.4 mg). Using Ozempic for weight loss would be off-label. Always use the medicine prescribed for you.
How quickly do weight loss injections work?
Weight loss typically begins within the first 2–4 weeks, but most significant loss occurs after 8–12 weeks as your body adjusts and your dose increases. Clinical trials showed average weight loss of 15–21% over 68–72 weeks. Individual results vary based on starting weight, diet, exercise, and adherence.
Are weight loss injections safe?
Weight loss injections are licensed medicines with established safety profiles. Common side effects include nausea, diarrhoea, and stomach pain, typically mild and improving over time. Serious side effects are rare but include pancreatitis and gallbladder problems. A prescriber will assess whether they are safe for your personal health before prescribing.
Can I get weight loss injections on the NHS?
Yes. The NHS commissions tirzepatide (Mounjaro) and semaglutide (Wegovy) through tier 3 specialist weight management services. You need a referral from your GP. Access, waiting times, and availability vary significantly by region and ICB (integrated care board).
What happens when you stop taking weight loss injections?
Weight may return to previous levels if diet and exercise habits do not change. These medications support weight management but are not permanent solutions. Most people continue injections long-term if weight loss is maintained and benefits continue. Discuss stopping plans with your prescriber.
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.