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 Serious Side Effects — UK Health Information
Mounjaro (tirzepatide) is a prescription-only weight management injection that works as a dual GIP and GLP-1 receptor agonist. While most people experience mild, manageable side effects like nausea or constipation, some individuals may develop serious side effects that require urgent medical attention. This guide explains which side effects are serious, warning signs to watch for, and when to contact your GP or seek emergency care. Understanding these risks is important before starting any prescription weight loss treatment.
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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.
Serious Side Effects That Require Urgent Medical Attention
While serious side effects from Mounjaro are uncommon, they can occur and require immediate medical attention. Severe, persistent stomach or abdominal pain — especially pain radiating to your back — may indicate pancreatitis (inflammation of the pancreas) and requires emergency assessment. Signs of a serious allergic reaction include swelling of your face, lips, tongue, or throat, difficulty breathing, or severe skin rash. These require immediate emergency care.
Other serious side effects to watch for include gallbladder problems (severe pain in the upper right stomach area, yellowing of skin or eyes, dark urine), changes in vision (especially if you have diabetes, as this may indicate diabetic retinopathy complications), and severe dehydration from persistent vomiting or diarrhoea. Additionally, unusual heart rate changes (very rapid or very slow heartbeat, chest pain, or fainting) warrant urgent evaluation. If you experience any of these, contact your GP immediately or attend A&E if symptoms are severe.
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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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Distinguishing Serious from Common Side Effects
Mounjaro commonly causes nausea, diarrhoea, constipation, vomiting, and stomach pain in more than 1 in 10 people. These are usually temporary and improve as your body adjusts or after dose adjustments. However, if these common symptoms become severe or persistent — for example, vomiting that prevents you from keeping food or fluids down for more than a few hours, or abdominal pain that doesn't improve — they may signal a serious condition requiring medical assessment.
Before starting Mounjaro, discuss your full medical history with your prescriber, including any previous pancreatitis, gallbladder disease, or diabetic retinopathy. Your GP can provide personalised guidance on what symptoms warrant contacting them. You can also use our free BMI calculator and GLP-1 weight loss calculator above to understand your eligibility and potential outcomes alongside professional medical assessment.
- 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 pancreatitis a common side effect of Mounjaro?
No, pancreatitis is uncommon with Mounjaro. However, severe abdominal pain radiating to the back is a warning sign of possible pancreatitis and requires immediate medical evaluation. Contact your GP or A&E straight away if you develop this symptom.
What should I do if I experience severe stomach pain after my Mounjaro injection?
Seek immediate medical attention. Severe, persistent stomach pain — especially if it spreads to your back — may indicate pancreatitis. Call your GP, NHS 111, or attend A&E. Do not wait to see if it improves on its own.
Can Mounjaro cause gallbladder problems?
Gallbladder issues are not listed as a common side effect, but gradual weight management from any treatment can increase gallstone formation risk. If you experience severe pain in the upper right stomach area, yellowing of skin or eyes, or dark urine, contact your GP promptly.
How is Mounjaro different from Wegovy in terms of serious side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 medications with similar serious side effect profiles. The main difference is Mounjaro's dual GIP and GLP-1 action. Both require prescription and clinical assessment to identify individual risks.
Should I stop taking Mounjaro if I experience any side effect?
No — do not stop suddenly without medical advice. Contact your prescriber about any side effects. Common side effects often improve with time or dose adjustments. Only stop if directed by your GP or if you experience a serious side effect requiring emergency care.
Ready to take control of your weight loss journey?
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