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 — UK Health Information
Whilst most people taking Mounjaro experience only mild to moderate nausea or digestive changes, serious side effects can occur and require urgent medical attention. This guide explains the severe side effects associated with tirzepatide, when to contact your GP, and what warning signs should never be ignored. Understanding these risks is essential before starting Mounjaro, whether prescribed through the NHS or privately. Use our GLP-1 weight loss calculator to explore your options alongside this important safety information.
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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 That Require Urgent Attention
The most serious side effects of Mounjaro require immediate medical attention and should not be delayed:
- Severe abdominal pain spreading to the back — This may indicate pancreatitis (inflammation of the pancreas). Pancreatitis is a medical emergency. Seek urgent care immediately if you experience sudden, severe stomach pain that radiates to your back, especially accompanied by vomiting or fever.
- Severe allergic reaction — Signs include swelling of the face, lips, tongue, or throat, difficulty breathing, or severe rash. These indicate anaphylaxis and require emergency treatment (call 999 or go to A&E immediately).
- Gallbladder problems — Severe pain in the upper right stomach area, yellowing of skin or eyes, or dark urine may indicate gallstones or gallbladder inflammation. These require urgent assessment.
- Vision changes — Sudden blurred vision, vision loss, or eye pain (particularly if you have diabetes) may indicate serious eye complications and require urgent optometry or A&E assessment.
- Rapid or severe heart rate changes — If you experience chest pain, shortness of breath, fainting, or a significantly elevated or irregular heartbeat, seek emergency care.
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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.
Common vs. Serious Side Effects — How to Tell the Difference
Understanding which side effects are expected and which are serious helps you respond appropriately. Very common side effects — nausea, diarrhoea, constipation, and indigestion — typically occur during dose escalation and often improve within days or weeks. These can usually be managed at home with GP guidance. However, if nausea or vomiting becomes so severe that you cannot keep fluids or food down, or if diarrhoea is unrelenting and causing severe dehydration, contact your GP. Never ignore persistent severe pain, vision changes, or signs of allergic reaction. When in doubt, contact your GP or NHS 111 (available 24/7) rather than waiting. If you feel in immediate danger, call 999.
- 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 should I do if I experience severe abdominal pain whilst taking Mounjaro?
Severe abdominal pain, especially if it radiates to your back, may indicate pancreatitis and is a medical emergency. Call 999 or go to A&E immediately. Do not wait or try to manage it at home. Stop taking Mounjaro and seek urgent medical assessment. Inform healthcare staff that you are taking tirzepatide, as this will help guide your care.
Can Mounjaro cause serious allergic reactions?
Serious allergic reactions are rare but possible with Mounjaro. Signs include swelling of the face, lips, tongue, or throat, difficulty breathing, severe itching, or severe rash. These are medical emergencies requiring immediate treatment. Call 999 or go to A&E. Always inform your prescriber of any known allergies to tirzepatide or its ingredients before starting treatment.
Are vision changes a known side effect of Mounjaro?
Vision changes can occur, particularly in people with type 2 diabetes or existing eye conditions. Changes may relate to blood sugar shifts or, rarely, complications like retinopathy. Any sudden blurred vision, vision loss, or eye pain requires urgent assessment from an optometrist or A&E. If you have diabetes, inform your eye care provider that you are taking Mounjaro.
How do I know if my nausea is just a normal side effect or something serious?
Normal Mounjaro nausea is usually mild to moderate and occurs especially during dose escalation, often improving within days. Serious nausea causes severe vomiting preventing food or fluid intake, leading to dehydration or weight loss beyond expected levels. If you cannot keep anything down, feel severely dehydrated, or vomit persists despite rest, contact your GP. Dehydration is a genuine medical concern requiring assessment.
Should I stop taking Mounjaro if I develop severe side effects?
If you experience severe side effects, contact your prescriber or GP immediately before your next dose. For genuine emergencies (severe pain, allergic reaction, chest pain), call 999 or go to A&E first, seek treatment, then inform your prescriber. Do not assume you must stop; your GP may advise continuing at the same dose, adjusting the dose, or stopping. Medical guidance is essential for safe management.
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.