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 Severe Side Effects — What You Need to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection available in the UK through NHS specialist services and private prescribing. While most people tolerate it well, severe side effects can occur and require immediate medical attention. Understanding what constitutes a serious reaction—and when to contact your GP—is essential for safe use. This guide explains the severe side effects of Mounjaro in a UK context, drawing on clinical trial data and MHRA safety monitoring.
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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.
Severe Side Effects of Mounjaro — When to Seek Immediate Help
Severe side effects of Mounjaro are rare but serious and require urgent medical attention. The most critical warnings relate to pancreatitis, gallbladder complications, and serious allergic reactions.
- Severe stomach pain spreading to the back: This may indicate acute pancreatitis. Stop treatment and seek emergency medical care immediately (call 999 or attend A&E). This is one of the most serious potential complications.
- Serious allergic reactions: Swelling of the face, lips, tongue, or throat; difficulty breathing; or rash affecting a large area of your body. Call 999 immediately.
- Severe gallbladder problems: Intense pain in the upper right stomach area, particularly after eating fatty foods, may indicate cholecystitis or gallstones. Contact your GP urgently or attend A&E if pain is severe.
- Changes in vision: Sudden blurred vision, flashing lights, or vision loss—particularly in people with diabetes—may signal diabetic retinopathy complications. Contact your eye specialist or GP urgently.
- Signs of dehydration: Severe vomiting and diarrhoea lasting more than a few hours can lead to dehydration. Seek medical advice if you cannot keep fluids down.
The MHRA monitors Mounjaro safety closely (it carries a Black Triangle ▼ designation for additional monitoring). If you experience any severe symptoms, stop injecting and contact your GP or NHS 111 service immediately.
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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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How Severe Side Effects Differ From Common Side Effects
Most people experience common side effects like nausea, diarrhoea, or constipation, which typically settle within days or weeks as your body adjusts. These are managed through dose adjustments and dietary changes. Severe side effects, by contrast, develop suddenly and pose a genuine health risk.
If you experience persistent nausea preventing food or fluid intake, vomiting lasting more than a few hours, or abdominal pain that doesn't resolve, contact your GP or NHS 111. Don't wait for symptoms to worsen. When prescribed Mounjaro through NHS tier 3 services or private prescribers in the UK, you should receive clear written guidance on emergency symptoms and how to contact your healthcare team. Always keep this information to hand.
- 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 difference between common and severe Mounjaro side effects?
Common side effects (nausea, diarrhoea, constipation) are mild to moderate and usually settle within days or weeks. Severe side effects (acute pancreatitis, serious allergic reactions, severe gallbladder pain) require immediate medical attention and may result in hospitalisation. Severe effects are rare but serious.
What should I do if I have severe stomach pain while on Mounjaro?
Severe stomach pain spreading to your back may indicate pancreatitis, a medical emergency. Stop your Mounjaro injection immediately and call 999 or attend A&E without delay. Do not delay seeking help—pancreatitis requires urgent hospital assessment and treatment.
Is Mounjaro safe to use in the UK?
Mounjaro is approved by the MHRA for weight management in the UK and has been used safely by thousands of people. It carries a Black Triangle (▼) designation for additional monitoring. Serious side effects are rare, but they do occur. Your GP or prescriber will assess your suitability before prescribing and monitor you during treatment.
How can I check if I am eligible for Mounjaro on the NHS?
NHS prescribing of tirzepatide is managed through tier 3 specialist weight management services, not standard GP prescribing. Your GP can refer you if you have a BMI of 30 or above (or 27 with weight-related health conditions). However, availability varies significantly by region. Your local Integrated Care Board (ICB) determines access.
Can I use our GLP-1 weight loss calculator to estimate outcomes if prescribed Mounjaro?
Yes. Our free GLP-1 weight loss calculator can help you project potential weight loss at different Mounjaro doses based on clinical trial data. Remember that individual results vary significantly, and outcomes depend on adherence to dose titration, dietary changes, and physical activity. A prescriber will discuss realistic expectations with you.
Ready to take control of your weight loss journey?
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