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 — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and have distinct eligibility pathways. This guide explains the key differences between these two medications, their UK prescribing context, NHS access routes, and how to understand which might be relevant to your situation. Both are used alongside diet and physical activity changes, and individual results vary significantly. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways simultaneously. This dual mechanism may help reduce appetite more effectively in clinical trials compared to single-mechanism medications. The SURMOUNT-1 trial showed average weight loss of approximately 15–21% of body weight at the highest dose.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—it activates a single hormone pathway. The STEP-1 clinical trial demonstrated average weight loss of approximately 14.9% of body weight. Both medications slow gastric emptying and signal fullness to the brain, but Mounjaro's dual action may provide additional metabolic effects. Individual responses vary; some people respond better to one medication than the other. Neither is assured to work, and results depend on adherence, diet, and lifestyle changes alongside the injection.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS Access and UK Prescribing Pathways
In the UK, both Mounjaro and Wegovy are available through NHS specialist tier 3 weight management services following GP referral, though availability varies significantly by region and waiting lists can be substantial. NICE has approved both medications for use within NHS services for eligible adults.
Eligibility for both: BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, dyslipidaemia, sleep apnoea, cardiovascular disease). South Asian, Chinese, Black African, and Black Caribbean adults may have lower BMI thresholds due to higher health risks at lower weights. Private access is available through GPhC-registered online prescribers without NHS referral. Your GP can advise on NHS tier 3 services in your area; a licensed prescriber can assess private eligibility after an online clinical 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's the main difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1 receptor agonist only—one pathway. Clinical trials suggest Mounjaro may produce slightly higher average weight loss, but individual results vary significantly.
Which one is more available on the NHS in the UK?
Both are NICE-approved for NHS tier 3 weight management services, but regional availability is inconsistent. Waiting lists are often lengthy. Your GP can refer you to your local NHS tier 3 service. Neither is routinely prescribed in primary care—specialist assessment is required.
What are the starting doses for each medication?
Mounjaro starts at 2.5 mg weekly, escalating to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg, escalating to a maximum of 2.4 mg weekly. Mounjaro doses are significantly higher in milligrams but both are once-weekly injections. Dose escalation requires a minimum of 4 weeks at each level.
Are there differences in side effects between Mounjaro and Wegovy?
Both cause similar common side effects: nausea, diarrhoea, vomiting, constipation, and stomach pain. Most side effects improve over time as the body adjusts. Serious side effects (pancreatitis, gallbladder problems) are rare but require urgent medical attention. Always report new or severe symptoms to your prescriber.
Can I switch from Wegovy to Mounjaro or vice versa on the NHS?
Switching between medications must be managed by your NHS specialist or prescriber. There is no official conversion chart between tirzepatide and semaglutide doses. A prescriber will assess your current response and health status before recommending any switch. Never stop one medication and start another without medical guidance.
Ready to take control of your weight loss journey?
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