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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Which Is Safer: Mounjaro or Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, and neither is inherently "safer" than the other—safety depends on individual health factors. Both have been subject to rigorous clinical trials and carry similar common side effects, though their mechanisms differ. Your prescriber will assess which is more suitable for you based on your medical history, current medications, and health conditions. This guide explains the safety profiles, trial data, and how prescribers choose between them.
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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—it activates two separate hormone pathways. This dual mechanism may contribute to its efficacy in clinical trials, where participants lost an average of 15–21% of body weight on the maximum dose over 72 weeks in the SURMOUNT-1 trial. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. In the STEP-1 clinical trial, participants on Wegovy lost approximately 14.9% of body weight at 68 weeks on the maximum 2.4 mg dose.
Both slow gastric emptying and reduce appetite, but the dual mechanism of Mounjaro may explain why some trial participants achieved slightly higher average weight loss. However, individual responses vary significantly—some people respond better to one medication than the other, which is why prescriber assessment is essential.
- Mounjaro KwikPen: Once weekly, doses from 2.5 mg to 15 mg
- Wegovy: Once weekly, doses from 0.25 mg to 2.4 mg
- Both require gradual dose titration over weeks
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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.
Safety, Side Effects, and Prescriber Decision
Both medications carry similar common side effects—nausea, diarrhoea, vomiting, constipation, and stomach pain are very common with both. Serious adverse events (pancreatitis, gallbladder problems, severe allergic reactions) are rare with both. There is no clinical evidence that one is inherently safer than the other; both have undergone rigorous safety monitoring, and Mounjaro is subject to additional MHRA monitoring (Black Triangle ▼ status).
Your prescriber will consider: your medical history, any kidney or liver conditions, whether you take other medications (both can affect oral contraceptive absorption), your response to previous GLP-1 medications if applicable, and your personal preference. Neither medication is suitable if you are pregnant, breastfeeding, or have a personal or family history of medullary thyroid cancer. Both require a clinical assessment before prescription—use our free GLP-1 weight loss calculator to understand potential outcomes, and discuss eligibility with your GP or prescriber.
- 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 safer than Wegovy?
No—both have similar safety profiles and were tested in rigorous clinical trials. "Safer" depends on your individual health. Mounjaro uses a dual mechanism (GIP+GLP-1), Wegovy a single mechanism (GLP-1 only). Your prescriber will assess which suits you based on medical history, current medications, and health conditions.
What is the difference in weight loss between Mounjaro and Wegovy?
Clinical trials showed Mounjaro participants lost an average of 15–21% body weight, while Wegovy participants lost approximately 14.9%. However, individual results vary significantly—some people lose more on one medication than the other. Your prescriber can discuss expected outcomes during your assessment.
Do Mounjaro and Wegovy have different side effects?
Both share common side effects: nausea, diarrhoea, vomiting, constipation, and stomach pain. Serious side effects (pancreatitis, gallbladder issues) are rare with both. Side effect severity and duration vary individually. Most people find side effects settle as their body adjusts.
Can I switch from Wegovy to Mounjaro or vice versa?
Potentially, yes—but any medication switch must be managed by your prescriber. There is no official conversion chart between doses. Your prescriber will assess whether switching is appropriate for you and manage the transition safely if it is recommended.
How do I know which injection is right for me?
A UK-licensed prescriber will assess your individual factors: medical history, medications, kidney/liver function, previous GLP-1 response, and personal preference. Both require a clinical assessment before prescription. Neither is universally "better"—it depends on your specific health profile.
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.