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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Should I Go On Mounjaro? Mounjaro vs Wegovy Comparison
Deciding whether to go on Mounjaro depends on your individual health, BMI, and medical history — both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK, but they work differently and suit different people. This guide compares the two medications factually so you can understand the key differences before discussing options with your GP or a licensed prescriber.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in your body. This dual mechanism may help regulate appetite more effectively than single-mechanism medications. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating one hormone pathway. Both slow how quickly food leaves your stomach and increase feelings of fullness after meals.
In clinical trials, Mounjaro users lost an average of 15–21% of body weight depending on dose, whilst Wegovy users lost approximately 15% on average. However, individual results vary significantly — these are averages, not assures. Mounjaro's dual mechanism may explain slightly higher average weight loss in SURMOUNT trials compared to STEP trials for Wegovy, though both are effective for many people.
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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
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Which Might Be Right For You?
Both medications require a UK prescription following a clinical assessment — a prescriber will assess your BMI, medical history, and any weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease) to determine suitability. Eligibility thresholds are the same: BMI 30 or above, or BMI 27–29.9 with weight-related conditions. Use our free BMI calculator above to check your personal BMI before discussing with a healthcare professional.
Side effect profiles are similar — nausea, diarrhoea, and constipation are common early on — though individual tolerance varies. Your prescriber will help choose based on your health profile, preferences, and access route (NHS specialist services or private prescription). Neither is assured to work; both require reduced-calorie diet and increased physical activity alongside treatment.
Eligibility, Access, and Next StepsAccess in the UK differs by route: NHS specialist tier 3 weight management services can prescribe either medication following referral from your GP, though waiting lists vary significantly by region. Private access is available through licensed UK prescribers via online clinical assessment, without NHS referral. You'll start on the lowest dose with careful dose titration — Mounjaro's KwikPen begins at 2.5 mg weekly, Wegovy at 0.25 mg weekly — adjusted every 4 weeks as tolerated. Try our free GLP-1 weight loss calculator to explore projected weight loss at different doses and see how results might compare.
- 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 for weight loss?
Clinical trial data shows Mounjaro users lost slightly more weight on average (15–21% vs 15%), likely due to its dual GIP+GLP-1 mechanism. However, individual results vary significantly. Effectiveness depends on your body, lifestyle adherence, and dose tolerability. Your prescriber will advise which suits your situation.
What are the main side effects of both medications?
Both commonly cause nausea, diarrhoea, vomiting, and constipation, especially at the start. Mounjaro may cause additional injection site reactions. Side effects often reduce over time as your body adjusts. Serious side effects are rare but include pancreatitis symptoms and severe allergic reactions — seek urgent medical attention if these occur.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible but must be managed by your prescriber. There's no official NHS conversion chart — approximate equivalences exist, but any switch requires clinical oversight. Do not attempt to switch doses independently; inform your prescriber if you wish to change medications.
Will the NHS prescribe Mounjaro or Wegovy?
Yes, both are available through NHS specialist weight management (tier 3) services following GP referral. However, availability and waiting lists vary significantly by region. Many people access these medications via private prescription more quickly. Discuss both routes with your GP.
Do I need to take these injections forever?
These are ongoing treatments — weight often returns if you stop. Your prescriber will advise on treatment duration based on your progress, tolerability, and health goals. Some people use them for 6–12 months, others long-term. This decision is made between you and your healthcare team.
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.