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
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Mounjaro vs Wegovy: Understanding Half-Life and How They Work
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight loss injections licensed in the UK, but they work differently in your body. A key difference is their half-life — how long the medication remains active. This affects dosing schedules and how your body responds. Understanding these differences can help you and your GP or prescriber discuss which option might be more suitable for your individual needs. Both require a clinical assessment before prescription, and results vary significantly between individuals.
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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 Half-Life Differs Between Mounjaro and Wegovy
Mounjaro (tirzepatide) has a half-life of approximately 5 days, meaning half the medication is cleared from your body every 5 days. This longer half-life supports a once-weekly injection schedule, with a more gradual build-up and clearance in your system. Wegovy (semaglutide) has a shorter half-life of around 7 days, though its effects persist because it binds to tissue receptors over time.
The more significant difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways simultaneously, whilst Wegovy is a GLP-1-only agonist. In clinical trials, Mounjaro showed slightly higher average weight loss (approximately 15–21% of body weight at 15mg) compared to Wegovy (approximately 14.9% at 2.4mg), though individual results vary considerably. Both medications are used alongside reduced-calorie diet and increased physical activity for best results. Neither medication is suitable during pregnancy or whilst breastfeeding.
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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.
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Which Is Right for You? Individual Assessment Matters
There is no universally "better" medication — suitability depends on your individual health profile, medical history, other medications, and how your body responds. Your GP or licensed prescriber will assess factors including your BMI, any weight-related health conditions (such as type 2 diabetes or high blood pressure), allergies, and whether you've tried other weight management approaches.
Use our free GLP-1 weight loss calculator above to explore projected weight loss outcomes at different doses. Both medications require ongoing medical monitoring and regular check-ins with your prescriber. Access in the UK is through NHS specialist tier 3 weight management services (with potential waiting lists) or private online prescribing following clinical assessment. Individual results vary — some people respond better to one medication than the other.
- 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.
Does Mounjaro's longer half-life mean it works better than Wegovy?
Not necessarily. Half-life affects dosing frequency and how the medication builds up in your system, but it doesn't directly determine weight loss effectiveness. Clinical trials show both can produce significant results — on average, Mounjaro showed slightly higher weight loss, but individual responses vary considerably. Suitability depends on your personal health profile, not half-life alone.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching between medications is possible and sometimes done in clinical practice. However, any switch must be managed by your prescriber. There's no official dose conversion chart — your prescriber will assess the most appropriate starting dose on the new medication. Wegovy 2.4mg is approximately equivalent to Mounjaro 10–15mg, but this is not a fixed rule.
Why is Mounjaro given once weekly if it only has a 5-day half-life?
Because half-life doesn't mean the medication stops working after that time — it means half the dose is cleared. After 5 days, approximately 50% remains active. By day 7 (when you take the next dose), enough medication is still present to maintain therapeutic effect. Additionally, Mounjaro binds to tissue receptors, prolonging its action beyond simple half-life suggests.
Are there differences in side effects between Mounjaro and Wegovy?
Both share similar common side effects: nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. Individual tolerability varies. Some people experience fewer side effects on one medication than the other. Your prescriber will monitor you and can adjust your dose or consider alternatives if side effects are problematic.
How do I access Mounjaro or Wegovy on the NHS in the UK?
Both medications are available through NHS specialist tier 3 weight management services, typically via referral from your GP. However, availability and waiting times vary significantly by region and integrated care board (ICB). Private access is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber.
Ready to take control of your weight loss journey?
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