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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What Are The Risks Of Mounjaro vs Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK. While both can support weight loss, they work differently and have different risk profiles. Understanding the potential side effects and safety considerations of each medication helps you and your prescriber make an informed decision about which, if either, is suitable for you. Individual risks vary based on your health history and current medications.
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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 Differ in Mechanism and Safety Profile
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways, which may explain why clinical trials showed slightly higher average weight loss (~20.9% at the maximum 15mg dose) compared to Wegovy. Wegovy (semaglutide) is a GLP-1-only receptor agonist, with average weight loss around 14.9% at the maximum 2.4mg dose in clinical trials.
The key difference in risk relates to this dual mechanism: because Mounjaro activates two pathways, some users report stronger gastrointestinal side effects, particularly nausea and vomiting, especially during the dose titration phase. Wegovy users also experience these side effects, but clinical data suggests they may be slightly less intense on average, though individual experiences vary significantly.
Both medications are subject to additional MHRA monitoring (Black Triangle ▼ status), meaning safety data is actively reviewed. Neither medication should be used if you have a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2 (MEN2).
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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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Common Side Effects and Risk Considerations for Both Medications
Very common side effects (affecting more than 1 in 10 people): nausea, diarrhoea, vomiting, constipation, and stomach pain occur with both medications. Nausea tends to be most severe when starting or increasing the dose and often improves over weeks. Gastrointestinal side effects can lead to dehydration and electrolyte imbalances if you do not maintain adequate fluid intake.
Serious but rare risks: Both carry a small risk of pancreatitis (severe stomach pain spreading to the back), gallbladder problems, and diabetic retinopathy complications (changes in vision) if you have existing diabetes. gradual weight management can also trigger or worsen gallstones. Your prescriber will assess your individual risk during the clinical assessment. Use our free BMI calculator and GLP-1 weight loss calculator to understand your baseline health context before your appointment.
- 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 riskier than Wegovy?
No clear evidence shows one is definitively riskier than the other. Mounjaro may cause slightly stronger initial nausea due to its dual mechanism, but Wegovy carries the same serious risk profile (pancreatitis, gallbladder issues). Individual risk depends on your health history, not just the medication. A prescriber must assess your suitability.
Can I switch from Mounjaro to Wegovy if I experience side effects?
Possibly, but only under prescriber guidance. Some people tolerate one better than the other. However, switching requires a clinical decision and washout period. Your GP or weight management specialist will advise whether switching is appropriate or whether dose adjustment or lifestyle changes might help.
What GLP-1 side effects require urgent medical attention?
Seek immediate medical help for severe stomach pain (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), changes in heart rate, or vision changes. These are rare but serious. Contact 999 or go to A&E if you experience these.
Do both Mounjaro and Wegovy require the same clinical assessment?
Yes. Both are prescription-only and require assessment by a UK-licensed prescriber via NHS specialist services or private online consultation. Eligibility criteria are the same: BMI ≥30, or BMI 27–29.9 with weight-related health conditions. Your medical history and current medications must be reviewed.
Why do some people get worse side effects than others?
Individual response depends on genetics, gut health, underlying digestive conditions, other medications, and how quickly you increase the dose. Slower dose titration may reduce side effects. Your prescriber will tailor your dosing schedule, and our GLP-1 weight loss calculator can help you understand expected timelines.
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.