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
What Does Semaglutide Do?
Semaglutide is a prescription-only GLP-1 receptor agonist injection licensed in the UK for weight management in adults. It works by mimicking a natural gut hormone that regulates appetite, helping you feel fuller after smaller meals and reducing food cravings. When combined with a reduced-calorie diet and increased physical activity, semaglutide may help support sustainable weight loss. In the UK, semaglutide is available as Wegovy (the weight management formulation) or Ozempic (for type 2 diabetes). This guide explains how it works, UK eligibility, and what to expect.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Works in Your Body
Semaglutide activates the GLP-1 receptor — this is a natural hormone pathway in your gut that signals fullness to your brain. When you take a GLP-1 receptor agonist injection, it amplifies these natural signals, helping you feel satisfied with smaller portions and reducing cravings between meals.
- Appetite regulation: Signals to the brain that you are full, even with less food
- Gastric emptying: Slows how quickly food leaves your stomach, extending feelings of fullness
- Blood sugar control: Helps regulate glucose responses after eating, reducing energy crashes that trigger hunger
- Cravings reduction: May reduce desire for high-calorie or sugary foods
Unlike insulin or other diabetes medications, semaglutide does not directly lower blood sugar — it works through appetite and satiety mechanisms. Clinical trials (the STEP programme) demonstrated that semaglutide, when used as a weekly weight loss injection, helped participants lose an average of approximately 15% of their body weight alongside lifestyle changes. Individual results vary considerably.
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 vs Ozempic — Key Differences in the UK
A common source of confusion: both Wegovy and Ozempic contain semaglutide, but they are licensed for different purposes in the UK. Ozempic is approved for type 2 diabetes management (doses up to 2mg) and uses semaglutide to help control blood sugar. Wegovy is the licensed weight management formulation (doses up to 2.4mg) and is approved specifically for weight loss in adults with a BMI of 30 or above, or 27 or above with weight-related health conditions.
Using Ozempic for weight loss is off-label in the UK, meaning it is not its licensed indication. If you are interested in prescription weight loss treatment, your GP or prescriber will assess whether Wegovy (the licensed semaglutide weight loss version) is appropriate for you following a clinical assessment. Try our free GLP-1 weight loss calculator to explore potential outcomes.
- 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 is semaglutide different from other weight loss injections?
Semaglutide is a GLP-1 receptor agonist — it works through one hormone pathway. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, activating two pathways. Both are prescription-only, once-weekly injections licensed in the UK. Clinical trial results suggest both support meaningful weight loss, but individual response varies. Your prescriber will advise which is appropriate for you.
How long does it take for semaglutide to start working?
Most people notice reduced appetite within 1–2 weeks of starting semaglutide, though some take longer. Weight loss typically becomes noticeable after 4–8 weeks. Semaglutide is taken as a once-weekly injection, with doses increased gradually every 4 weeks to minimise side effects. Full effects are usually seen after 12–16 weeks at an effective maintenance dose.
What are the side effects of semaglutide?
Common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — particularly during dose increases. Most improve as your body adjusts. Serious side effects are rare but include severe abdominal pain (possible pancreatitis) or allergic reactions. Always report unusual symptoms to your GP or prescriber.
Is semaglutide available on the NHS in the UK?
Semaglutide (Wegovy) is available on the NHS through specialist tier 3 weight management services, following a GP referral. Availability and waiting times vary significantly by region. Private access is also available through GPhC-registered online prescribers following a clinical assessment. Costs typically range from £129–£200 per month depending on dose.
Can I stop taking semaglutide once I reach my target weight?
Semaglutide is typically used as a long-term treatment. Weight regain can occur if you stop taking it without maintaining dietary and exercise changes. Your prescriber will discuss a personalised plan, including whether to continue, adjust dose, or transition to maintenance strategies. Use our free progress tracker to monitor your weight loss journey throughout treatment. ```
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.