The UK's most complete GLP-1 guide
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- 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 — How These Weight Loss Injections Compare
If you've searched for information about weight loss injections, you may have encountered mentions of both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are prescription-only medications licensed in the UK for weight management, but they work through different mechanisms and show different results in clinical trials. This guide compares the two fairly, looking at how they differ and what the evidence shows, so you can understand what a prescriber might consider when assessing your individual needs.
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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. This means it activates two separate hormone pathways in the body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). The dual action may help regulate appetite more broadly and support feeling fuller after smaller meals. In the SURMOUNT-1 clinical trial, Mounjaro showed an average weight loss of approximately 15–21% of body weight, depending on dose, over 72 weeks alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—it activates a single hormone pathway. The STEP-1 clinical trial showed an average weight loss of approximately 14.9% body weight at 68 weeks, also alongside lifestyle changes. Both medications slow gastric emptying (how quickly food leaves the stomach), though this effect tends to reduce over time.
Both are given as once-weekly injections and both require a prescription and clinical assessment before use.
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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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Eligibility, Doses, and Individual Suitability
In the UK, both Mounjaro and Wegovy are licensed for adults with a BMI of 30 kg/m² or above, or BMI 27–29.9 kg/m² with weight-related health conditions such as high blood pressure, type 2 diabetes, or cardiovascular disease. Lower thresholds may apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. Mounjaro doses range from 2.5 mg to 15 mg once weekly, whilst Wegovy doses range from 0.25 mg to 2.4 mg. A prescriber will assess your medical history, other medications, and individual health needs to determine which—if either—is suitable for you. Neither medication is right for everyone, and individual results vary significantly.
Use our free GLP-1 weight loss calculator to explore potential outcomes, or check your BMI using the calculator on this page to understand your starting point.
- 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 Mounjaro more effective than Wegovy?
Clinical trial data suggests Mounjaro showed slightly higher average weight loss (15–21%) compared to Wegovy (14.9%), but individual results vary significantly. "More effective" depends on your personal response, medical history, and lifestyle. A prescriber will assess which is suitable for you individually—it's not simply about choosing the one with higher trial averages.
Can you switch from Wegovy to Mounjaro?
Yes, but only under prescriber guidance. There is no official conversion chart between the two—approximate equivalences exist (e.g., Wegovy 1.7 mg roughly equates to Mounjaro 10 mg), but any switch must be managed carefully by your healthcare provider to ensure safe transition and appropriate dosing.
What's the difference between GLP-1 and GIP/GLP-1?
GLP-1 is a single hormone pathway that reduces appetite. GIP/GLP-1 (as in Mounjaro) activates two pathways together, potentially offering a broader appetite-regulating effect. Both mechanisms are evidence-based, but the dual approach in Mounjaro may suit some people better than others—your prescriber will advise.
How do I access Mounjaro or Wegovy in the UK?
Both are prescription-only. On the NHS, you'll typically be referred to a specialist weight management service (tier 3) by your GP, though availability varies by region and waiting lists can be lengthy. Privately, they're available through GPhC-registered online pharmacies following a clinical assessment by a UK-licensed prescriber.
Are there side effects with both medications?
Yes. Both commonly cause nausea, diarrhoea, vomiting, and constipation, especially during dose escalation. Side effects typically improve over time. Serious side effects are rare but possible (severe stomach pain, gallbladder issues, allergic reactions). Always discuss side effect risk with your prescriber before starting either medication.
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