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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Is Mounjaro Safe to Take? Comparing Mounjaro and Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management medications approved in the UK, and safety profiles are established through clinical trials. The key question isn't which is safer overall, but which may be more suitable for your individual health situation. Both require a clinical assessment before prescription. This guide compares the safety, mechanisms, and side effects of both medications to help you understand the differences.
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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.
Clinical Trial Safety Data and How They Work Differently
Mounjaro (tirzepatide) uses a dual GIP and GLP-1 receptor agonist mechanism, meaning it activates two natural hormone pathways to help regulate appetite and blood sugar. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. The SURMOUNT clinical trials for Mounjaro and the STEP trials for Wegovy both showed safety profiles managed with dose titration — starting at lower doses and increasing gradually over weeks.
In SURMOUNT-1, Mounjaro at 15 mg produced average weight loss of around 20.9% of body weight. In STEP-1, Wegovy at 2.4 mg produced approximately clinically studied weight loss. Both showed gastrointestinal side effects (nausea, diarrhoea, constipation) as the most common adverse events, typically mild to moderate and often improving over time. Individual responses vary significantly, and a prescriber will assess which medication's side effect profile may suit you better based on your medical history and current medications.
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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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Which Medication Is Right for You? A Prescriber Decision
Safety isn't just about clinical trial data — it's about whether a medication is appropriate for your individual health situation. Both Mounjaro and Wegovy are prescription-only and require a UK-licensed prescriber to assess your eligibility and medical history. Your prescriber will review your BMI, any weight-related health conditions, and any existing medications (oral contraceptives, diabetes drugs, etc.) before deciding which is safer and more suitable for you.
Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication, then discuss the results with your GP or a licensed prescriber. They can provide personalised guidance on safety, side effect management, and which option aligns with your health goals. Both have proven safety records when used as prescribed and with appropriate clinical monitoring.
- Mounjaro KwikPen: Once-weekly injection, maximum dose 15 mg
- Wegovy: Once-weekly injection, maximum dose 2.4 mg
- Both require: Clinical assessment, prescription, and dose titration starting at lower doses
- Both used with: Reduced-calorie diet and increased physical activity
- 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?
Both have established safety profiles from clinical trials. "Safety" depends on individual factors — medical history, medications, and side effect tolerance. A prescriber assesses which is safer and more suitable for you personally. Neither is universally "safer" — it's about individual fit.
What are the most common side effects of both medications?
Both commonly cause nausea, diarrhoea, vomiting, and constipation, especially when starting or increasing doses. These are usually mild to moderate and often improve over time. Serious side effects are rare. Always report persistent symptoms to your prescriber.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber — there is no direct automatic conversion. Mounjaro and Wegovy use different mechanisms and dosing schedules. Your prescriber will advise on any transition and timing between stopping one and starting another.
Do either Mounjaro or Wegovy interact with oral contraceptives?
Yes — both may reduce oral contraceptive absorption. Women taking oral contraceptives should use an additional barrier method or switch to a non-oral contraceptive for 4 weeks after starting either medication and 4 weeks after each dose increase. Discuss alternatives with your GP.
How long do I need to take Mounjaro or Wegovy?
Duration depends on individual response and health goals. Some people maintain weight loss long-term on the medication; others stop after reaching their goal. This is discussed with your prescriber. Weight regain is possible if stopped without lifestyle support — treatment works best alongside lasting dietary and activity changes.
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.