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
How Do weight loss injections Work — A UK Health Explanation
weight loss injections—the colloquial term for GLP-1 and GIP/GLP-1 weight loss injections—work by mimicking natural hormones that regulate appetite and blood sugar. These prescription-only medicines, including Wegovy (semaglutide) and Mounjaro (tirzepatide), may help reduce hunger, increase feelings of fullness, and support sustainable weight management alongside lifestyle changes. This guide explains the science, eligibility in the UK, and what the clinical evidence shows about how these injections function in your body.
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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 in Your Body
weight loss injections are GLP-1 receptor agonist or GIP/GLP-1 receptor agonist medications. GLP-1 is a natural hormone your gut produces after eating—it tells your brain you're full, slows how quickly food leaves your stomach, and helps regulate blood sugar.
Prescription weight loss injections amplify these signals. When you inject Wegovy or Mounjaro, the medication:
- Signals to your brain that you're satisfied after smaller meals
- Reduces cravings and desire to snack between meals
- Slows gastric emptying (food passes through your stomach more slowly)
- Helps regulate blood sugar levels
- May increase fat burning and energy use
The NHS recognises these as legitimate prescription treatments for weight management. Clinical trials, including the STEP-1 trial for Wegovy (semaglutide), showed participants lost an average of around 15% of their body weight over 68 weeks—though results vary significantly between individuals and depend on diet and activity changes.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Eligibility and NHS Access in the UK
In the UK, prescription weight loss injections are available for adults with a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds may apply due to higher cardiovascular risk at lower BMI levels.
Access is available through NHS specialist weight management services (tier 3) or private online prescribing with a UK-licensed prescriber. Use our free BMI calculator above to check your starting point, and our GLP-1 weight loss calculator to project potential outcomes at different doses. You must have a clinical assessment before prescription—your GP or prescriber will evaluate whether this treatment is suitable for you.
- 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 "weight loss injections" and Ozempic?
Both Wegovy and Ozempic contain semaglutide, but they're different. Ozempic (up to 2mg) is licensed for type 2 diabetes. Wegovy (up to 2.4mg) is the licensed weight management version in the UK. Using Ozempic for weight loss is off-label—Wegovy is the approved choice for weight management. Always use the correct, prescribed medication.
How long does it take to see weight loss results with weight loss injections?
Clinical trials showed weight loss typically begins within 2–4 weeks, with gradual progression over months. The STEP-1 trial measured results at 68 weeks. Faster results don't necessarily mean better outcomes—sustainable weight loss of 0.5–1kg per week is the NHS recommendation. Individual results vary based on diet, activity, and dose.
Do I have to take weight loss injections forever to keep weight off?
This is unclear. Clinical trials are ongoing. Some people regain weight after stopping—others maintain loss. Your prescriber will discuss long-term plans. Weight management is most successful when combined with lasting lifestyle changes (diet and exercise), not medication alone. Discuss your personal goals with your GP.
Can I get weight loss injections on the NHS?
Yes, but availability varies significantly by region and waiting lists can be very long. You need referral to a specialist tier 3 weight management service. Private online prescribing is faster and doesn't require NHS referral, but costs around £150–£320 monthly depending on dose. Ask your GP about local NHS availability first.
What happens if I miss a dose of my weight loss injections injection?
If you miss a dose within 4 days of when you should have taken it, inject as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next one on your regular scheduled day. Never take a double dose. Always check your patient information leaflet or contact your prescriber for specific advice.
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.