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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Severe Side Effects Of Mounjaro — What You Need To Know
Mounjaro (tirzepatide) is a prescription-only GLP-1 receptor agonist approved for weight management in the UK. Like all medications, it can cause serious side effects in some people, though these are uncommon. This guide explains the severe adverse effects you should be aware of, how they compare to Wegovy, and when to seek medical attention. Understanding these risks helps you make an informed decision with your prescriber during your clinical assessment. Our free GLP-1 weight loss calculator can also help you explore what to expect during treatment.
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Calculate Your BMI Instantly
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.
Severe Side Effects Of Mounjaro vs Wegovy — Clinical Evidence
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) activates GLP-1 alone. This mechanistic difference may influence side effect profiles, though both carry similar serious risks.
Severe side effects requiring urgent medical attention include:
- Acute pancreatitis: Severe upper stomach pain spreading to the back, nausea, and vomiting. Seek emergency care immediately. Both medications carry this risk, though it is rare.
- Serious allergic reactions: Swelling of face, lips, tongue, or throat; difficulty breathing; severe rash. Call 999 or go to A&E.
- Gallbladder complications: Severe pain in the upper right abdomen, jaundice, dark urine. Requires urgent medical evaluation.
- Retinal complications: Vision changes or blurred vision (particularly if you have diabetes). Contact your eye care team immediately.
- Severe dehydration: Persistent vomiting and diarrhoea leading to dizziness, weakness, or fast heart rate.
SURMOUNT trial data showed that severe adverse events were rare in both Mounjaro and Wegovy groups, but individual risk varies based on personal health history and concurrent 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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Who Is At Higher Risk And Prescriber Considerations
Certain groups face increased risk of severe Mounjaro side effects and require careful prescriber assessment: people with a personal or family history of pancreatitis, those with diabetic retinopathy, individuals with gallbladder disease, and anyone taking medications that affect gastric function.
Your prescriber will take a full medical history before issuing a UK prescription. If you experience warning signs—persistent stomach pain, vision changes, severe allergic symptoms, or signs of dehydration—contact your GP or NHS 111 immediately rather than waiting for your next appointment. Both Mounjaro and Wegovy require ongoing clinical monitoring, and your healthcare provider will decide which is most suitable based on your individual health profile and risk factors.
Use our BMI calculator to confirm your current health category, and discuss all medication options thoroughly with your 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.
What is the most serious side effect of Mounjaro?
The most concerning rare side effect is acute pancreatitis, presenting as severe upper abdominal pain radiating to the back, persistent vomiting, and nausea. This requires emergency medical attention. Seek immediate care at A&E or call 999 if you experience these symptoms. Serious allergic reactions and gallbladder complications are also rare but serious.
How do Mounjaro and Wegovy severe side effects compare?
Both medications carry similar risks of pancreatitis, allergic reactions, and gallbladder issues. Mounjaro's dual GIP+GLP-1 mechanism means gastrointestinal side effects may differ slightly in profile, but severe events are uncommon with both. Clinical trial data showed rare serious adverse events in both SURMOUNT and STEP trials.
Can Mounjaro cause vision problems?
Yes, vision changes or blurred vision can occur, particularly in people with existing diabetes or diabetic retinopathy. These changes may indicate worsening of existing retinal conditions. Contact your eye care team or GP immediately if you notice vision changes. This is one reason your prescriber assesses your full medical history before issuing a UK prescription.
What should I do if I experience severe stomach pain on Mounjaro?
Severe upper abdominal pain spreading to the back may indicate pancreatitis and is a medical emergency. Call 999, go to A&E, or contact NHS 111 immediately. Do not delay seeking help. Describe your symptoms clearly and mention you are taking Mounjaro. Even if you are unsure, medical professionals should evaluate severe stomach pain promptly.
Is Mounjaro safe if I have a history of gallbladder problems?
A personal or family history of gallbladder disease increases your risk of Mounjaro-related complications. Your prescriber must know this before issuing a prescription. Many people with gallbladder history can safely take Mounjaro, but your GP or specialist will weigh risks and benefits individually and may recommend additional monitoring or an alternative medication like Wegovy.
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.