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 Does Semaglutide Work For Weight Loss?
Semaglutide is a prescription-only medication that works by mimicking a natural hormone in your body called GLP-1. When taken as a weekly injection, semaglutide helps reduce appetite, control food cravings, and support feeling fuller after eating smaller meals. The medication slows how quickly food leaves your stomach and signals your brain that you are satisfied, making it easier to eat less without constant hunger. Wegovy is the licensed semaglutide formulation specifically approved in the UK for weight management in adults. Understanding how this weekly weight loss injection works can help you decide if it might be right for your health goals alongside lifestyle changes.
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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 Affects Your Appetite and Digestion
Semaglutide is a GLP-1 receptor agonist — it works by activating receptors in your brain and gut that control hunger and fullness. GLP-1 (glucagon-like peptide-1) is a natural hormone your body already produces after eating. Semaglutide amplifies this effect, making your brain receive stronger signals that you are full, which reduces appetite and food cravings.
The medication also slows gastric emptying — the rate at which food moves from your stomach into your small intestine. This means you feel satisfied for longer after eating, making it easier to stick to a reduced-calorie diet. Additionally, semaglutide may help regulate how your body uses and stores fat, supporting more effective weight loss when combined with dietary changes and increased physical activity.
Unlike insulin, semaglutide does not directly lower blood sugar — it works through natural hormone pathways already present in your body.
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.
Semaglutide Weight Loss Results and NHS Context
In the STEP-1 clinical trial, participants taking semaglutide 2.4mg (the maximum dose in Wegovy) lost an average of approximately 14.9% of their body weight over 68 weeks — though individual results vary significantly. Some people lose more, others less, depending on genetics, adherence to diet and exercise, and individual body composition.
In the UK, semaglutide is available through NHS specialist weight management services (tier 3) or via private prescription following an online clinical assessment. Access to Wegovy through the NHS depends on your BMI, weight-related health conditions, and regional availability — your GP can advise on eligibility. Our free GLP-1 weight loss calculator can help estimate potential outcomes at different doses. Weight loss medications work best alongside sustained lifestyle changes, not instead of them.
Common Questions About How Semaglutide Works- 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.
How quickly does semaglutide start working for weight loss?
Most people notice reduced appetite and food cravings within the first 1–2 weeks of starting semaglutide. However, significant weight loss typically becomes measurable after 4–8 weeks. Your prescriber will increase your dose gradually over 16+ weeks, with the full appetite-suppressing effect usually reached at the maintenance dose (2.4mg for Wegovy).
Is semaglutide the same as Ozempic?
Both Wegovy and Ozempic contain semaglutide, but they are different medicines. Ozempic is licensed for type 2 diabetes management (up to 2mg dose) and is not approved for weight loss in the UK. Wegovy is the licensed weight management version, with a higher maximum dose of 2.4mg. Using Ozempic off-label for weight loss would not be recommended without medical supervision.
Does semaglutide work differently than other weight loss injections?
Semaglutide (Wegovy) is a single GLP-1 receptor agonist. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, activating two hormone pathways rather than one. Both help reduce appetite, but Mounjaro's dual mechanism may produce slightly different results. Clinical trials show similar average weight loss, though individual outcomes vary. Your prescriber will discuss which is more suitable for you.
What happens to semaglutide in your body?
When you inject semaglutide once weekly, it enters your bloodstream and travels throughout your body, activating GLP-1 receptors in your brain (particularly the appetite centre), your gut, and your pancreas. It reaches peak levels 1–3 days after injection and remains active throughout the week. Your body gradually breaks it down, which is why weekly injections are needed rather than daily doses.
Will I lose weight if I do not change my diet or exercise?
Semaglutide may help reduce your appetite and food intake, but weight loss results are best when combined with a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss alongside dietary changes and lifestyle support. Medication alone is less effective than medication plus lifestyle change. Your GP or prescriber will discuss realistic expectations and the importance of supporting the medication with healthy habits.
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.