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 — How It Works for Weight Loss
Semaglutide is a prescription-only GLP-1 receptor agonist medication used for weight management in the UK under the brand name Wegovy. It works by mimicking a natural hormone that regulates appetite and blood sugar control. If you're considering weight loss medication or comparing options, understanding how semaglutide works — and how it differs from other injections like Mounjaro — can help you have an informed conversation with your GP or prescriber. This guide explains the mechanism, compares it to alternatives, and outlines 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 vs Other Weight Loss Medications
Semaglutide is a GLP-1 receptor agonist, meaning it activates the GLP-1 hormone pathway in your body. This natural hormone is produced in the gut after eating and signals to your brain that you're full, which may help reduce appetite and food cravings. Semaglutide slows how quickly food leaves your stomach and helps regulate blood sugar levels.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways instead of one. This dual mechanism may explain why clinical trials showed slightly higher average weight loss with Mounjaro compared to semaglutide, though individual results vary significantly. Both medications are once-weekly injections prescribed for adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes.
The STEP trials (semaglutide) showed average weight loss of approximately 14.9% of body weight, while SURMOUNT trials (Mounjaro) showed average weight loss of 15–20.9% depending on dose. These are averages — your actual results depend on your individual health, adherence to a reduced-calorie diet, and physical activity level.
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.
Which Medication Might Be Right for You
Both semaglutide and Mounjaro require a prescription from a UK-licensed prescriber following a clinical assessment. Your GP or prescriber will consider your BMI, weight-related health conditions, current medications, and any contraindications before recommending either medication.
Neither medication is suitable during pregnancy or breastfeeding. Both may reduce the effectiveness of oral contraceptives, so additional contraception is recommended for 4 weeks after starting or dose increases. Try our free GLP-1 weight loss calculator to estimate potential weight loss at different doses, and use our BMI calculator to check whether you meet eligibility criteria. Only a prescriber can assess your individual suitability, but these tools provide helpful context for your conversation with your GP.
- 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 is the difference between semaglutide and Mounjaro?
Semaglutide (Wegovy) is a GLP-1 receptor agonist that activates one hormone pathway. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that activates two pathways. Clinical trials suggest Mounjaro may result in slightly higher average weight loss, but individual results vary. Both are once-weekly injections licensed in the UK for weight management.
Is semaglutide the same as Ozempic?
Both Ozempic and Wegovy contain semaglutide, but they are different products. Ozempic is licensed for type 2 diabetes (doses up to 2mg). Wegovy is licensed specifically for weight management (doses up to 2.4mg). Using Ozempic for weight loss is off-label in the UK. Always use the medication prescribed to you by your prescriber.
How does semaglutide cause weight loss?
Semaglutide mimics GLP-1, a natural gut hormone. It reduces appetite, increases feelings of fullness, slows stomach emptying, and helps regulate blood sugar. These effects together may support weight loss when combined with a reduced-calorie diet and increased physical activity.
How long does semaglutide take to work?
Most people notice appetite changes within the first 2–4 weeks of starting semaglutide. Weight loss typically becomes measurable after 4–8 weeks. Doses are increased gradually every 4 weeks until reaching your maintenance dose. Full effects are usually seen after 16–20 weeks.
Can I get semaglutide (Wegovy) on the NHS in the UK?
NHS access to semaglutide for weight management is through specialist tier 3 weight management services, usually following GP referral. Availability varies significantly by region and waiting lists can be long. Private prescription options are also available through UK-licensed online prescribers following a clinical assessment.
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.