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 Is Semaglutide? A Clear UK Guide
Semaglutide is a prescription-only weight loss injection licensed in the UK as Wegovy. It's a GLP-1 receptor agonist — a medication that activates natural hormones in your body to help reduce appetite and support weight management. Unlike Mounjaro, which uses a dual mechanism, semaglutide works through a single hormone pathway. If you're considering weight loss treatment, understanding what semaglutide is and how it differs from other available options can help you have an informed conversation with your prescriber about what's right for your individual health needs.
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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 Semaglutide Works vs Mounjaro — Key Differences
Semaglutide is a GLP-1 receptor agonist — it activates the GLP-1 hormone pathway, which signals fullness to your brain, slows how food moves through your stomach, and helps regulate blood sugar. Mounjaro (tirzepatide), by contrast, is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously.
- Semaglutide mechanism: Single GLP-1 pathway activation
- Mounjaro mechanism: Dual GIP + GLP-1 pathway activation
- Semaglutide clinical evidence: STEP-1 trial showed average weight loss of ~15% body weight at 68 weeks
- Mounjaro clinical evidence: SURMOUNT-1 trial showed average weight loss of ~21% body weight at 72 weeks
Both require a reduced-calorie diet and increased physical activity for best results. Individual outcomes vary considerably, and prescribers will assess which medication suits your personal health profile.
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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.
Semaglutide Dosing, Access & Suitability in the UK
Semaglutide (Wegovy) is dosed from 0.25 mg up to 2.4 mg, taken once weekly. In the UK, you can access semaglutide through NHS specialist weight management services (tier 3/4) or privately through a UK-licensed prescriber following a clinical assessment. You're eligible if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes or hypertension. Whether semaglutide or Mounjaro is more suitable depends on your medical history, other medications, and individual response — your prescriber will discuss this with you. Both are prescription-only and require ongoing medical support.
- 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 semaglutide, but Ozempic (approved for type 2 diabetes, doses up to 2 mg) is not licensed for weight loss in the UK. Wegovy is the licensed weight management version at higher doses up to 2.4 mg. They should not be used interchangeably — only use the form prescribed to you.
How does semaglutide cause weight loss?
Semaglutide mimics the GLP-1 hormone, reducing hunger signals, making you feel fuller on smaller portions, and slowing gastric emptying. It also helps regulate blood sugar. These effects combine to support a calorie deficit when paired with dietary changes and activity.
What's the difference between semaglutide and Mounjaro?
Semaglutide activates one hormone pathway (GLP-1 only). Mounjaro activates two (GIP and GLP-1). Clinical trials suggest Mounjaro may produce slightly more weight loss on average, but individual results vary. Your prescriber will recommend the best option for you based on your health profile.
How long does semaglutide take to work?
Most people notice reduced appetite within 1–2 weeks, but significant weight loss typically becomes apparent after 4–8 weeks. Results continue over months. Progress varies between individuals — use our free progress tracker to log your journey alongside your health team.
Can I get semaglutide on the NHS in the UK?
Yes, through specialist tier 3 or 4 weight management services following a GP referral, though availability and waiting times vary by region. Private prescription is also available through UK-licensed online prescribers if you meet eligibility criteria. Your GP or prescriber will discuss your options.
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.