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 Injection For Weight Loss: How It Works
Semaglutide is a prescription-only weekly weight loss injection available in the UK under the brand name Wegovy. It's a GLP-1 receptor agonist that may help reduce appetite and support weight management alongside a reduced-calorie diet and increased physical activity. If you're considering a weight loss injection, you'll often hear semaglutide compared with tirzepatide (Mounjaro), another dual-action option. This guide explains the differences, trial outcomes, and how to assess which might suit your individual circumstances.
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.
Semaglutide vs Mounjaro: Key Mechanism and Trial Differences
Mechanism: Semaglutide works as a single GLP-1 receptor agonist, activating one hormone pathway to reduce hunger and improve satiety. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, engaging two pathways simultaneously — this is the key difference between them.
Clinical trial results: In the STEP-1 trial, semaglutide 2.4 mg achieved an average weight loss of approximately 14.9% of body weight over 68 weeks. Mounjaro's SURMOUNT-1 trial showed average weight loss of approximately 20.9% at the maximum 15 mg dose over 72 weeks. However, individual results vary significantly, and trial participants also followed a reduced-calorie diet and increased physical activity. Neither medication assures specific weight loss outcomes.
Dosing: Semaglutide escalates from 0.25 mg to a maximum of 2.4 mg weekly. Mounjaro ranges from 2.5 mg to 15 mg weekly, offering more dose options if escalation is needed.
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 Is Right For You? Assessment and Suitability
Both semaglutide and tirzepatide require a clinical assessment before prescription. Your eligibility depends on your BMI, weight-related health conditions, and individual health profile. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses — this can help inform your discussion with your prescriber. Your GP or a licensed prescriber will assess which medication may be most suitable based on your medical history, other medications, and health goals. Neither is universally "better" — suitability is personal. Both are prescription-only, available through NHS specialist weight management services (with variable waiting times) or private prescribing following an online assessment.
- 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 main difference between semaglutide and Mounjaro?
Semaglutide (Wegovy) is a single GLP-1 receptor agonist, while Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways. This dual mechanism in Mounjaro may contribute to its slightly higher average weight loss in clinical trials, though individual results vary considerably.
How much weight loss can I expect with semaglutide 2.4 mg?
In the STEP-1 clinical trial, semaglutide 2.4 mg resulted in an average weight loss of approximately 14.9% of body weight over 68 weeks. However, outcomes depend on individual factors, adherence to a reduced-calorie diet, physical activity, and genetics. Always discuss realistic expectations with your prescriber.
Can I switch from semaglutide to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist (e.g. semaglutide 2.4 mg roughly corresponds to tirzepatide 15 mg), but your prescriber will determine the appropriate approach based on your individual response and health needs.
How do I access semaglutide (Wegovy) in the UK?
Semaglutide is available through NHS specialist tier 3 weight management services (often with lengthy waiting lists and regional variation) or via private prescribing through a UK-licensed prescriber following an online clinical assessment. Both routes require a valid prescription from a UK healthcare professional.
Are there serious side effects I should be aware of with semaglutide?
Common side effects include nausea, diarrhoea, vomiting, and constipation — typically mild and temporary. Serious but rare risks include pancreatitis (severe stomach pain) and gallbladder complications. Your prescriber will screen for contraindications and discuss all risks before starting 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.