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 Pen 10mg vs Wegovy — Key Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and produce different clinical outcomes. If you're considering either medication, understanding how they compare helps you have an informed conversation with your prescriber. Both require a clinical assessment before prescription and work best alongside a reduced-calorie diet and increased physical activity. This guide breaks down the factual differences so you can understand what your GP or prescriber might recommend based on your individual health.
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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 — Different Mechanisms
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. The GIP pathway is newer and may enhance the appetite-suppressing effects of the GLP-1 pathway. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. In clinical trials, this dual mechanism in Mounjaro resulted in numerically higher average weight loss: SURMOUNT-1 showed approximately clinically studied weight loss at the maximum 15mg dose, compared to approximately 14.9% in the STEP-1 trial for Wegovy at 2.4mg. However, individual results vary significantly. Both medications slow gastric emptying and may reduce appetite and food cravings. The key difference is that Mounjaro targets two hormonal pathways, whereas Wegovy targets one—this is reflected in their different dosing schedules and escalation patterns. Your prescriber will consider whether the dual mechanism or single mechanism approach is more suitable for your specific circumstances.
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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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Doses, Side Effects, and Individual Suitability
Mounjaro pen 10mg is an intermediate maintenance dose in the titration schedule (doses range from 2.5mg to 15mg weekly). Wegovy reaches its maximum at 2.4mg weekly. Both are injected once weekly. Common side effects of both medications include nausea, diarrhoea, vomiting, and constipation—usually improving over time as your body adjusts. The choice between them is not about which is "better," but which is more appropriate for you. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Your GP or prescriber will assess your medical history, current medications, and health goals to recommend the most suitable option. Both medications require ongoing clinical oversight and should be combined with lifestyle changes for best results. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
- 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 10mg more effective than Wegovy?
Mounjaro showed higher average weight loss (20.9% at 15mg) than Wegovy (14.9% at 2.4mg) in their respective clinical trials. However, individual results vary significantly, and Mounjaro 10mg is not the maximum dose. Effectiveness depends on your body, adherence, diet, and activity level. Your prescriber will recommend the medication most suitable for your health profile.
Which has worse side effects, Mounjaro or Wegovy?
Both have similar common side effects: nausea, diarrhoea, vomiting, and constipation. These typically improve after a few weeks as your body adjusts. Serious side effects are rare but include pancreatitis and gallbladder issues. Your prescriber will discuss individual risk factors with you during assessment.
Can I switch from Wegovy to Mounjaro?
Yes, it is possible to switch between medications, but this must be managed by your prescriber. There is no official conversion chart—your prescriber will assess your response and recommend an appropriate starting dose on the new medication. Never switch without clinical guidance.
How long does it take to reach Mounjaro 10mg?
Mounjaro uses a dose titration schedule: 2.5mg weeks 1–4, then 5mg weeks 5–8, then 7.5mg weeks 9–12, then 10mg weeks 13–16. You spend a minimum of 4 weeks on each dose. Reaching 10mg typically takes 16–20 weeks. Your prescriber may adjust this timeline based on your response.
Do I need a UK prescription for both Mounjaro and Wegovy?
Yes, both are prescription-only medicines in the UK. Access is available through NHS specialist weight management services (tier 3) or private online prescribing following a clinical assessment. MyWeightCheck does not supply medication—it provides information only. Always consult your GP or a licensed prescriber.
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