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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Mounjaro Dangerous Side Effects vs Wegovy
Mounjaro and Wegovy are both prescription-only weight loss injections licensed in the UK, but they work differently and carry different risk profiles. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Both can cause serious side effects, and understanding the differences is important before discussing either medication with your GP or a licensed prescriber. This guide compares the dangerous side effects of both medications factually, using clinical trial and regulatory data from the UK.
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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.
Serious Side Effects: SURMOUNT vs STEP Trial Data
Mounjaro (SURMOUNT-1 trial) participants reported gastrointestinal side effects in the majority of cases — nausea affected over 25% of those on the 15 mg dose, vomiting over 10%, and pancreatitis (inflammation of the pancreas) was a rare but serious concern flagged by the MHRA. The most dangerous side effects requiring urgent medical attention include severe, persistent abdominal pain radiating to the back, which may signal acute pancreatitis, and serious allergic reactions including swelling of the face, lips, or throat.
Wegovy (STEP-1 trial) showed similar gastrointestinal profiles — nausea was common, affecting around 25% of participants on the 2.4 mg dose, and vomiting occurred in approximately 10%. Rare serious risks include gallbladder problems, pancreatitis, and diabetic retinopathy complications in people with existing diabetes.
Both medications slow gastric emptying, which is why nausea and vomiting are common during dose escalation. However, the dual mechanism of Mounjaro (GIP + GLP-1) versus Wegovy's single GLP-1 mechanism means gastrointestinal effects may persist slightly longer with Mounjaro in some patients, though individual responses vary significantly.
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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 Has Greater Risk — And Why Your Prescriber Decides
Neither medication is inherently "more dangerous" — the risks are comparable, but the likelihood of individual side effects varies by person. Someone with a history of gallbladder disease may face higher risk with either medication. Someone with a family history of pancreatitis should discuss this with their prescriber before starting either treatment. The tirzepatide (Mounjaro) kwikpen delivers a fixed dose weekly, whilst Wegovy escalates more gradually — this may mean Mounjaro reaches therapeutic effect faster but could increase early nausea risk.
Use our free BMI calculator and GLP-1 weight loss calculator above to understand your personal situation, then discuss the results with a licensed prescriber. They will assess your individual risk factors, medical history, and which medication is safer for you. Both require a full clinical assessment before prescription.
- 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.
What are the most dangerous side effects of Mounjaro?
Severe abdominal pain spreading to the back (possible pancreatitis), serious allergic reactions, gallbladder problems, and changes in vision in people with diabetes. Seek immediate medical attention if you experience any of these. Nausea and vomiting are very common but usually manageable and reduce over time.
Is Wegovy safer than Mounjaro?
Neither is definitively "safer" — both carry similar serious risks. The STEP and SURMOUNT trials show comparable side effect profiles. Safety depends entirely on your individual health history, medications, and risk factors. Your prescriber will assess which is appropriate for you.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways (dual action), whilst Wegovy (semaglutide) activates GLP-1 only. This means Mounjaro may work faster but could cause slightly different gastrointestinal effects. Both require careful dose titration and medical supervision.
Can I switch from Wegovy to Mounjaro if I have side effects?
Possibly, but never switch without medical guidance. Some side effects reduce with time as your body adjusts. Your prescriber may adjust the dose, recommend a break, or suggest switching after careful assessment. Always speak to a licensed prescriber before making any changes.
Are Mounjaro side effects permanent?
Most Mounjaro side effects are temporary and reduce as your body adjusts during dose titration. Nausea, vomiting, and diarrhoea typically improve after the first few weeks. However, rare serious effects like pancreatitis require immediate medical attention. Individual experiences vary — discuss your concerns with your prescriber.
Ready to take control of your weight loss journey?
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