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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Semaglutide Weight Loss Injections: How They Work in the UK
Semaglutide is a GLP-1 receptor agonist that may help support weight loss when combined with diet and exercise. In the UK, it's available as Wegovy — a prescription-only weekly injection licensed for weight management in adults. Wegovy is different from Ozempic, which contains the same active ingredient but is licensed for type 2 diabetes, not weight loss. This guide explains how semaglutide works, who can access it on the NHS and privately, and what the evidence shows about weight loss results. Try our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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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.
How Semaglutide Weight Loss Works
Semaglutide is a GLP-1 receptor agonist — it mimics a natural hormone your gut produces after eating. This hormone sends signals to your brain that help regulate appetite, reduce food cravings, and create a feeling of fullness after smaller meals. Semaglutide also slows how quickly food leaves your stomach, which helps you feel satisfied for longer.
Unlike insulin or other diabetes medications, semaglutide doesn't work by lowering blood sugar directly. Instead, it works on appetite control centres in the brain and adjusts how your digestive system processes food. It's injected once weekly under the skin — typically in the stomach, thigh, or upper arm.
Clinical trial results: In the STEP-1 trial, adults taking semaglutide 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity. Results vary significantly between individuals.
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: Semaglutide (Wegovy) is available through NHS specialist tier 3 and tier 4 weight management services. You'll need a referral from your GP. Availability varies significantly by region — waiting lists can be months or years in some areas. NHS access requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea.
Private access: Available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber. No NHS referral needed. Costs typically range from £129–£280 per month depending on dose. Your prescriber will assess suitability based on your health, medications, and medical history.
Use our BMI calculator to check your starting point, or our weight converter to track progress in kg, stone, or lbs.
- 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.
Is semaglutide the same as Ozempic?
Both contain the same active ingredient, but they're licensed for different uses. Ozempic (up to 2 mg) is licensed for type 2 diabetes. Wegovy (up to 2.4 mg) is the licensed weight management version. Using Ozempic for weight loss is off-label in the UK. Only Wegovy is officially approved for weight loss.
What are the semaglutide doses for weight loss?
Semaglutide starts at 0.25 mg weekly and increases gradually: 0.5 mg (weeks 5–8), 1 mg (weeks 9–12), 1.7 mg (weeks 13–16), then 2.4 mg (week 17 onwards). Each dose level is held for a minimum of 4 weeks before increasing. Maximum dose is 2.4 mg weekly.
What side effects does semaglutide cause?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain — especially when starting or increasing doses. These often improve over time. Less common: headache, fatigue, dizziness, indigestion. Seek immediate medical attention for severe stomach pain (possible pancreatitis) or allergic reactions.
How much weight will I lose on semaglutide?
Clinical trials showed average weight loss of approximately 15% of body weight. Individual results vary widely — some people lose more, some less. Weight loss depends on your starting weight, diet, exercise, genetics, and how your body responds. Results are not assured. Your prescriber can discuss realistic expectations.
Can I get semaglutide on the NHS in the UK?
Yes, through specialist weight management services following a GP referral. Availability varies by region and ICB. Eligibility requires BMI 30+ or 27–29.9 with weight-related health conditions. Private access is available without NHS referral through online prescribing services.
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.