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 Complications — A UK Health Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection that works as a dual GIP and GLP-1 receptor agonist. While most people tolerate it well, it can cause complications in some cases. Understanding what serious complications may occur, how to recognise them, and when to contact your GP is essential for safe use. This guide covers the important complications you should be aware of, based on clinical evidence and UK safety data.
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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 Complications to Watch For
Most complications from Mounjaro are mild gastrointestinal side effects like nausea and diarrhoea, which typically ease over time. However, some serious complications require urgent medical attention. The most significant is acute pancreatitis — severe, persistent pain in the upper abdomen spreading to the back, often with nausea and vomiting. If you experience this, seek emergency care immediately.
- Gallbladder problems: gradual weight management can increase gallstone risk. Pain in the upper right abdomen, nausea, or yellowing of skin/eyes needs urgent assessment.
- Severe dehydration: Persistent vomiting or diarrhoea can lead to dehydration. Drink fluids regularly and contact your GP if symptoms persist.
- Allergic reactions: Swelling of the face, lips, tongue, or throat requires emergency care — call 999.
- Diabetic complications: If you have diabetes, Mounjaro may affect your blood sugar. Your prescriber should adjust other diabetes medications accordingly.
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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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Who Is at Higher Risk of Complications?
Certain individuals face higher complication risks from Mounjaro and require careful monitoring. Those with a personal or family history of thyroid cancer, medullary thyroid carcinoma (MTC), or multiple endocrine neoplasia syndrome type 2 (MEN2) should not use Mounjaro — a prescriber will screen for this before issuing a tirzepatide UK prescription.
People with severe gastroparesis or chronic pancreatitis should not take Mounjaro. Those taking sulphonylureas or insulin for diabetes need dose adjustments to avoid dangerously low blood sugar. Pregnant women and those breastfeeding should not use it. Your prescriber will assess your individual risk at your initial clinical assessment before starting 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 most serious complication of Mounjaro?
Acute pancreatitis is the most serious potential complication — characterised by severe, persistent upper abdominal pain spreading to the back, often with nausea and vomiting. This is a medical emergency requiring immediate hospital care. Contact emergency services (999) if you experience these symptoms. Your GP should have warned you about this risk before you started tirzepatide treatment.
Can Mounjaro cause gallstones?
gradual weight management from any cause — including Mounjaro — can increase gallstone risk. Gallstones form when bile crystallises, often triggered by quick fat loss. Symptoms include upper right abdominal pain, nausea, and yellowing of skin or eyes. Not everyone develops gallstones, but if you experience these symptoms, contact your GP for assessment. Ultrasound imaging can confirm diagnosis.
What should I do if I experience severe nausea or vomiting on Mounjaro?
Mild nausea is common when starting Mounjaro, especially in the first few weeks, and usually improves. However, severe or persistent vomiting lasting more than a few hours requires medical attention — contact your GP or NHS 111. Severe vomiting can lead to dehydration. Your prescriber may adjust your dose or discuss whether Mounjaro is appropriate for you.
Can Mounjaro affect my diabetes medications?
Yes — if you take insulin or sulphonylureas (diabetes medications that lower blood sugar), Mounjaro may cause dangerously low blood sugar. Your prescriber must be aware of all diabetes medications before starting tirzepatide. Doses of insulin and sulphonylureas often need reducing. Never adjust diabetes medication yourself — always contact your prescriber first.
Is Mounjaro safe during pregnancy?
No — Mounjaro is not recommended during pregnancy or if you are planning to become pregnant. Animal studies suggest potential risks to the developing baby. If you become pregnant while taking Mounjaro, stop using it and contact your GP immediately for guidance. Women using oral contraceptives should add a barrier method or switch to non-oral contraception while taking tirzepatide.
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