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 — What You Need to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed by the MHRA for use in the UK. While most people tolerate it well, serious side effects can occur and require urgent medical attention. This guide covers the serious adverse effects identified in clinical trials and NHS prescribing guidance, what symptoms to watch for, and when to contact your GP or seek emergency care.
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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 Requiring Immediate Medical Attention
Several serious side effects of Mounjaro require prompt medical assessment or emergency care. Pancreatitis (inflammation of the pancreas) is the most serious — seek immediate medical attention if you experience severe, persistent stomach pain that radiates to your back, especially with nausea or vomiting. Symptoms may include upper abdominal pain, fever, and rapid heart rate. Gallbladder problems including cholecystitis and cholelithiasis have been reported; contact your GP urgently if you have severe pain in the upper right abdomen, particularly after fatty meals. Severe allergic reactions are rare but serious — seek emergency care immediately if you experience swelling of the face, lips, tongue, or throat, severe rash, or difficulty breathing. Changes in vision, particularly blurred vision or vision loss, may indicate diabetic retinopathy complications — inform your GP or eye care provider immediately. Severe dehydration from persistent vomiting or diarrhoea can be dangerous, especially in older adults or those taking certain 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.
When to Contact Your GP vs Seeking Emergency Care
Contact your GP urgently (same day if possible) if you experience new or worsening symptoms that concern you, persistent severe nausea or vomiting lasting more than a few hours, signs of gallbladder problems, or any vision changes. Call 999 or go to A&E immediately if you have severe abdominal pain with back radiation, signs of severe allergic reaction, chest pain or severe shortness of breath, or signs of severe dehydration (extreme dizziness, confusion, rapid heart rate). NHS 111 is available 24/7 for non-emergency health advice — call if you're unsure whether your symptoms need urgent assessment. Your prescriber should have provided guidance on how to report side effects; inform them promptly if serious effects occur. Keep your GP updated on your Mounjaro KwikPen use, especially if you're on other medications, as drug interactions can increase risk.
- 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 side effects of Mounjaro?
Common side effects (nausea, diarrhoea, constipation, stomach pain) occur in more than 1 in 10 people and typically improve over time as your body adjusts. Serious side effects are less common but medically significant and require urgent evaluation — pancreatitis, severe allergic reaction, and gallbladder problems are examples. Always report any new or worsening symptoms to your GP.
Is pancreatitis a common side effect of Mounjaro in the UK?
Pancreatitis is rare but serious. Clinical trials and NHS monitoring data show it occurs in a small percentage of users. Risk may be higher in people with a personal or family history of pancreatitis. Seek emergency care immediately if you have severe upper abdominal pain radiating to your back with nausea — do not delay.
Can Mounjaro cause permanent vision problems?
Vision changes with Mounjaro are typically related to blood sugar fluctuations or worsening of existing diabetic retinopathy — not direct damage to the eye. If you have diabetes, inform your eye care provider that you're taking Mounjaro and attend regular eye checks. Report any vision changes to your GP immediately.
What should I do if I experience severe vomiting on Mounjaro?
Persistent severe vomiting can lead to dehydration and electrolyte imbalance. Drink small sips of water or oral rehydration solution. If vomiting continues for more than a few hours, you cannot keep fluids down, or you feel dizzy or confused, contact your GP or call NHS 111 urgently — severe dehydration requires medical assessment.
Should I stop taking Mounjaro if I develop serious side effects?
Do not stop suddenly without medical guidance — discuss with your GP or prescriber immediately. Depending on the side effect, your prescriber may recommend pausing doses, dose reduction, or permanent discontinuation. Emergency situations (severe allergic reaction, severe pancreatitis) require immediate medical care first — then contact your prescriber once stabilised.
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.