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 — How It Compares to Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed and approved for use in the UK, meaning both have undergone rigorous safety assessment by the MHRA. The key difference is their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 receptor agonist only. This guide compares their safety profiles based on clinical trial data so you can understand how they differ. Both require a clinical assessment before prescription, and your prescriber will determine which is more suitable for your individual health needs.
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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.
Safety Profiles — Clinical Trial Evidence
Both Mounjaro and Wegovy have been studied extensively in clinical trials. Mounjaro (tirzepatide) was tested in the SURMOUNT trials, which followed over 4,500 participants for up to 72 weeks. Wegovy (semaglutide) was evaluated in the STEP trials with similar scale and duration. Both trials showed comparable safety profiles overall.
The most common side effects of both medications are gastrointestinal: nausea, diarrhoea, vomiting, and constipation. These typically occur during dose escalation and often improve over time as the body adjusts. Serious adverse events were rare in both trials.
- Mounjaro: nausea reported in ~25–30% of participants; most cases mild to moderate
- Wegovy: nausea reported in ~25% of participants; similar severity pattern
- Both: gastrointestinal side effects reduced significantly after 4–8 weeks on stable doses
The MHRA monitors both medications closely — Mounjaro carries a Black Triangle (▼) designation, indicating enhanced monitoring. This is standard for newer weight management medications and does not indicate a safety concern; it means the MHRA is actively gathering additional safety data.
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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.
Individual Suitability — Which is Right for You?
Neither Mounjaro nor Wegovy is inherently "safer" than the other — safety depends on your individual health profile, medications, and medical history. Your prescriber will assess which is more appropriate for you during a clinical consultation.
Factors your prescriber considers:
- Your current medications and potential interactions
- Any history of pancreatitis, gallbladder problems, or thyroid cancer
- Whether you have type 2 diabetes or prediabetes
- Pregnancy status or plans to become pregnant
- Any allergies to tirzepatide or semaglutide
Use our free GLP-1 weight loss calculator above to explore potential weight loss outcomes with either medication — this can help guide your conversation with your prescriber. Both are prescription-only and require a clinical assessment to confirm suitability.
- 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 based on clinical trial data. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is GLP-1 only, but this difference relates to effectiveness, not safety. Both are MHRA-approved and carry comparable risks. Your prescriber will determine which suits your individual health needs.
What are the main side effects of both medications?
The most common side effects are gastrointestinal: nausea, diarrhoea, vomiting, and constipation. These typically occur during dose escalation and often improve within 4–8 weeks. Serious side effects are rare. Both medications carry similar side effect profiles overall.
Why does Mounjaro have a Black Triangle designation?
The Black Triangle (▼) means the MHRA is actively monitoring Mounjaro for safety data. This is standard for newer medications and does not indicate a safety problem — it reflects the MHRA's commitment to gather additional real-world evidence as more people use the medication.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber. There is no official established conversion chart between doses. Your prescriber will assess whether a switch is appropriate and determine the safest starting dose on the new medication.
Are there any health conditions that make one medication safer than the other?
Yes. If you have a history of pancreatitis, gallbladder disease, severe gastroparesis, or a personal history of thyroid cancer, your prescriber may recommend one over the other — or advise against both. Always disclose your full medical history during your clinical assessment.
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.