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 vs Mounjaro for Weight Loss
Both semaglutide (Wegovy) and Mounjaro (tirzepatide) are prescription-only weekly injection weight loss medications licensed in the UK. They work differently and deliver different results in clinical trials. Understanding how they compare can help you have an informed conversation with your prescriber about which might be suitable for your individual situation. Both require a clinical assessment before being prescribed.
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 and Mounjaro Work Differently
Semaglutide (brand name Wegovy for weight loss) is a GLP-1 receptor agonist — it activates a single natural hormone pathway to reduce appetite, support feeling fuller after smaller meals, and slow gastric emptying. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously, which is why some research suggests it may offer stronger appetite suppression.
- Semaglutide: GLP-1 agonist only
- Mounjaro: dual GIP + GLP-1 agonist
- Both are once-weekly injections
- Both require a reduced-calorie diet and increased physical activity to work effectively
In clinical trials, this dual mechanism may explain why Mounjaro showed slightly higher average weight loss than semaglutide — though individual results vary significantly.
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.
Trial Results and Doses — What the Evidence Shows
The SURMOUNT-1 trial (Mounjaro) showed average weight loss of approximately 20.9% of body weight at the highest dose (15 mg) over 72 weeks. The STEP-1 trial (semaglutide) showed approximately 14.9% average weight loss at 2.4 mg over 68 weeks. Semaglutide starts at 0.25 mg and escalates to a maximum of 2.4 mg. Mounjaro starts at 2.5 mg and escalates to 15 mg. Both medications are individualised — your prescriber will determine the right dose for you based on your response and tolerability. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and calculate your personal BMI using our BMI calculator to understand your baseline.
- 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 Wegovy?
Yes. Wegovy is the brand name for semaglutide when prescribed for weight loss. Ozempic is also semaglutide, but at a lower dose (up to 2 mg) and approved only for type 2 diabetes — it is not licensed for weight management in the UK.
Is Mounjaro better than semaglutide for weight loss?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss due to its dual mechanism, but individual results vary considerably. The "better" choice depends on your personal health history, tolerability, and what your prescriber assesses as most suitable for you. Both require clinical assessment.
Can I switch from semaglutide to Mounjaro?
Yes, people sometimes switch between medications. There is an approximate dose equivalence (5 mg Mounjaro ≈ 0.5 mg semaglutide), but any switch must be managed by your prescriber. Do not switch without clinical guidance.
What are the main side effects of both medications?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain — especially when starting or increasing doses. These often improve over time. Less common are serious side effects like severe pancreatitis. Always report unexpected symptoms to your prescriber immediately.
How do I know which medication is right for me?
Your GP or prescriber will assess your individual health, weight, BMI, any comorbidities, previous medication responses, and personal preferences. Both medications require eligibility assessment — typically BMI ≥30 or ≥27 with weight-related health conditions. Your prescriber will recommend the most suitable option for your circumstances.
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.