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 medications that may help reduce appetite and support sustainable weight loss alongside diet and exercise. In the UK, two main types are licensed for weight management: GLP-1 receptor agonists (like Wegovy) and dual GIP/GLP-1 agonists (like Mounjaro). These are not cosmetic injections—they are clinically prescribed medicines that work by mimicking natural hormones. Access in the UK is through NHS specialist weight management services or private online prescribing following a clinical assessment. Individual results vary considerably, and they work best alongside lifestyle changes.
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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 medications that activate natural hormone pathways in your body. GLP-1 receptor agonists like Wegovy (semaglutide) signal to your brain that you are full, reduce appetite and food cravings, and slow how quickly food leaves your stomach. Dual GIP/GLP-1 agonists like Mounjaro (tirzepatide) activate two hormone pathways, offering potentially greater appetite reduction.
Clinical trials show significant results: in the STEP-1 trial, Wegovy helped participants lose an average of approximately 15% of their body weight over 68 weeks alongside a reduced-calorie diet and increased physical activity. Mounjaro showed average weight loss of 15–21% across trials. However, results vary between individuals—some people lose more, some less, and some do not respond as expected. These injections are not insulin and are suitable for people without diabetes.
Try our free GLP-1 weight loss calculator above to estimate potential weight loss at different doses, and use the BMI calculator to understand your current health category.
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 Access: NHS and Private Routes
NHS access is through specialist tier 3 weight management services. You typically need a GP referral, but availability varies significantly by region—some areas have long waiting lists or limited services. NHS eligibility usually requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes or high blood pressure.
Private access is available through GPhC-registered online pharmacies without an NHS referral. A UK-licensed prescriber conducts an online clinical assessment to confirm suitability. Costs typically range from £129–£320 per month depending on dose and provider. Prescription-only status applies to all weight loss injections—they cannot be purchased without a clinical assessment and prescription. Your GP or prescriber will assess whether this medication is right for your individual health situation.
- 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 Wegovy and Mounjaro?
Wegovy (semaglutide) activates one hormone pathway (GLP-1) and is a once-weekly injection. Mounjaro (tirzepatide) activates two pathways (GIP and GLP-1), also once-weekly. Mounjaro showed slightly higher average weight loss in trials (15–21% vs ~15%), but both are effective. Individual response varies. Your prescriber will recommend the most suitable option.
Is this the same as Ozempic?
No. Ozempic also contains semaglutide but is licensed for type 2 diabetes at lower doses (up to 2mg). Wegovy is the licensed weight management version at higher doses (up to 2.4mg). Using Ozempic for weight loss would be off-label. Always ask your prescriber which medication they recommend.
How quickly will I lose weight?
Weight loss typically begins within the first 4 weeks but varies between individuals. The NHS recommends losing 0.5–1kg per week for sustainable results. Clinical trials measured weight loss over 68 weeks, so this is a longer-term commitment. Lifestyle changes (diet and exercise) are essential alongside medication for best results.
Are there any serious side effects I should know about?
Common side effects include nausea, diarrhoea, constipation, and stomach pain—these often reduce over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. Always report new or severe symptoms to your GP immediately. A prescriber will assess your suitability during the clinical assessment.
Can I get weight loss injections on the NHS in my area?
NHS availability depends on your local integrated care board (ICB). Some regions offer specialist weight management services; others have limited access or long waiting lists. Contact your GP to ask if your area has a tier 3 weight management service and what the referral process is. Private online prescribing is an alternative if NHS services aren't available locally.
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.