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
Semaglutide Injections For Weight Loss — How They Work
Semaglutide injections for weight loss are a prescription-only treatment licensed in the UK under the brand name Wegovy. Semaglutide is a GLP-1 receptor agonist — a weekly injection that may help reduce appetite and support weight management alongside diet and exercise. Many people in the UK compare semaglutide with tirzepatide (Mounjaro), as both are licensed weight loss injections. Understanding how each works, their clinical evidence, and individual suitability can help you make an informed decision with your GP or prescriber.
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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 Works vs Mounjaro — Mechanism and Trial Data
Semaglutide (Wegovy) is a GLP-1 receptor agonist — it activates a single hormone pathway that signals fullness to the brain, reduces appetite, and slows stomach emptying. In the STEP-1 clinical trial, participants taking semaglutide 2.4 mg (the maximum dose) lost an average of approximately 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) works differently — it's a dual GIP and GLP-1 receptor agonist, activating two hormone pathways simultaneously. In the SURMOUNT-1 trial, tirzepatide 15 mg achieved average weight loss of approximately 20.9% of body weight. The dual mechanism may contribute to greater average weight loss, though individual results vary significantly.
- Semaglutide: GLP-1 only, once-weekly injection, doses 0.25 mg to 2.4 mg
- Mounjaro: GIP + GLP-1, once-weekly injection, doses 2.5 mg to 15 mg
- Both require a clinical assessment before prescription
- Both are used alongside lifestyle changes, not as standalone treatment
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 Is Right For You — Access and Suitability in the UK
Neither semaglutide nor Mounjaro is automatically "better" — suitability depends on your individual health, medical history, and prescriber assessment. Both are available in the UK through NHS specialist tier 3 weight management services or private online prescribing services following a clinical assessment. Your GP or prescriber will review your eligibility (BMI 30+, or 27+ with weight-related health conditions), existing medications, and health conditions to recommend the most appropriate option.
Use our free GLP-1 weight loss calculator to explore potential outcomes with different medications and doses. Track your progress with our progress tracker if you begin treatment. Individual responses to these medications vary — some people may respond better to semaglutide, others to Mounjaro. A prescriber can also help if you need to switch between medications.
- 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 they are different products. Ozempic is licensed for type 2 diabetes (doses up to 2 mg). Wegovy is the licensed weight management version (doses up to 2.4 mg). Using Ozempic for weight loss would be off-label and is not recommended. Ask your prescriber about Wegovy specifically for weight management in the UK.
What are the main side effects of semaglutide injections?
Very common side effects include nausea, diarrhoea, vomiting, and constipation — especially when starting or increasing doses. Most side effects improve over time. Seek urgent medical attention for severe stomach pain (possible pancreatitis), gallbladder problems, or serious allergic reactions. Discuss any concerns with your prescriber.
How much weight can you lose with semaglutide?
Clinical trials showed average weight loss of approximately 14.9% of body weight on semaglutide 2.4 mg over 68 weeks, alongside diet and exercise. Individual results vary significantly — some people lose more, some less. Weight loss depends on adherence, diet quality, activity level, and individual metabolism. Results are not assured.
Can I get semaglutide on the NHS in the UK?
Yes, semaglutide (Wegovy) is available through NHS tier 3 specialist weight management services, though availability and waiting times vary significantly by region. Private access is available through GPhC-registered online services following a clinical assessment. Ask your GP about NHS options in your area or explore private prescribing if you prefer faster access.
How does semaglutide compare to Mounjaro for weight loss?
Semaglutide (GLP-1 only) showed average weight loss of ~14.9% in trials; Mounjaro (dual GIP+GLP-1) showed ~20.9%. Mounjaro's dual mechanism may offer greater average loss, but individual responses vary. Neither is universally "better" — your prescriber will recommend based on your health profile, existing medications, and suitability. Some people respond better to one than the other.
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.