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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What Are the Serious Side Effects of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that may help with weight loss when used alongside diet and exercise. Like all medications, it can cause side effects. While most side effects are mild to moderate, there are some serious side effects you need to be aware of. This guide explains what they are, how to recognise them, and when to seek urgent medical attention. Always discuss any concerns with your GP or prescriber before starting or during treatment.
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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 That Need Urgent Medical Attention
While serious side effects of Mounjaro are uncommon, you should seek urgent medical help immediately if you experience:
- Severe, persistent stomach pain spreading to the back — this may indicate acute pancreatitis (pancreas inflammation). Stop using Mounjaro and contact your GP or A&E immediately.
- Severe abdominal pain in the upper right area — this could signal gallbladder disease or gallstones. Mounjaro may increase gallstone risk by reducing gallbladder contractions.
- Signs of serious allergic reaction — swelling of face, lips, tongue, or throat; difficulty breathing; severe rash. Call 999 immediately.
- Sudden vision changes — in people with type 2 diabetes, Mounjaro may worsen diabetic eye complications (retinopathy). Urgent eye assessment needed.
- Rapid or severe heart rate changes — unusual dizziness, fainting, or chest discomfort warrant immediate evaluation.
These serious reactions are rare but require urgent assessment. Do not delay seeking help if you have any of these symptoms.
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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.
When to Contact Your GP vs Seek Emergency Care
Contact your GP urgently (same day if possible) if you have persistent nausea, vomiting, or severe constipation lasting more than a few days — these can indicate serious digestive complications or dehydration.
Call 999 or go to A&E immediately for severe abdominal pain, signs of allergic reaction, difficulty breathing, loss of consciousness, or symptoms suggesting pancreatitis or gallbladder emergency.
Contact your prescriber if you develop new symptoms or are unsure whether your symptoms need urgent attention. Keep their emergency contact details handy. Your GP can assess whether Mounjaro is still appropriate for you if you experience serious side effects during treatment.
- 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 serious Mounjaro side effects?
Common side effects (nausea, diarrhoea, constipation) are mild to moderate and usually improve as your body adjusts to the dose. Serious side effects like severe abdominal pain, allergic reactions, or vision changes require urgent medical attention and may mean you need to stop treatment. Always report any new symptoms to your GP.
Can Mounjaro cause pancreatitis?
Yes, acute pancreatitis is a rare but serious side effect of GLP-1 and GIP/GLP-1 medications including Mounjaro. It presents as severe stomach pain spreading to the back, often with nausea and vomiting. If you experience these symptoms, stop Mounjaro and seek emergency medical attention immediately. Your GP can arrange testing if needed.
Does Mounjaro increase the risk of gallstones?
Mounjaro may increase gallstone risk by slowing gallbladder emptying. Symptoms include sharp pain in the upper right abdomen, particularly after eating fatty foods. While gallstones don't always cause problems, severe pain suggests gallbladder disease requiring urgent assessment. This is one reason why a prescriber assesses your suitability before starting treatment.
Is it safe to take Mounjaro if I have type 2 diabetes?
Mounjaro can be prescribed to people with type 2 diabetes, but you need careful monitoring. The medication may worsen diabetic eye disease in some people. If you have diabetes, inform your prescriber and eye care specialist. You may need more frequent eye checks. Diabetic medication doses may also need adjustment — your GP will advise.
What should I do if I think I'm having a serious side effect?
Stop taking Mounjaro and contact your GP, call 111, or go to A&E depending on severity. Severe abdominal pain, allergic reactions, vision changes, and breathing difficulties are emergencies — call 999. Keep your prescriber's contact details handy. Document your symptoms so you can describe them accurately to healthcare staff.
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.