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 in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK for adults with obesity or overweight with health conditions. Whilst most people tolerate it well, serious side effects can occur and require urgent medical attention. This guide explains what serious side effects to watch for, when to contact your GP, and how the NHS and MHRA monitor safety. Understanding the risks helps you use Mounjaro safely and confidently.
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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.
What Are the Serious Side Effects of Mounjaro?
Serious side effects of Mounjaro are uncommon but require immediate medical attention. The most important serious side effects to be aware of include:
- Pancreatitis (inflammation of the pancreas) — severe pain in the upper stomach spreading to the back, nausea, and vomiting. This is rare but potentially life-threatening. Seek emergency care immediately.
- Serious allergic reactions — swelling of the face, lips, tongue, or throat; difficulty breathing; severe rash. These require emergency medical attention.
- Diabetic retinopathy complications — changes in vision, blurred sight, or vision loss in people with existing diabetes. Report any vision changes to your GP urgently.
- Gallbladder problems — severe pain in the upper right stomach area, fever, or yellowing of skin or eyes, which may indicate gallstones or cholecystitis.
- Thyroid tumours — whilst animal studies raised concerns, human evidence remains limited. Any neck lump, hoarseness, or difficulty swallowing should be reported to your GP.
The MHRA (UK Medicines and Healthcare Products Regulatory Authority) continuously monitors Mounjaro safety through its Black Triangle (▼) post-marketing surveillance system. Report any suspected serious side effects to your prescriber or via the MHRA Yellow Card scheme.
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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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When to Contact Your GP or Seek Emergency Care
Contact your GP urgently (same day) if you experience: severe stomach pain not relieved by paracetamol, persistent vomiting lasting more than a few hours, signs of dehydration (dark urine, extreme thirst, dizziness), yellowing of skin or eyes, or unusual changes in bowel habits lasting more than a few days.
Seek emergency care (call 999 or go to A&E) if you have: severe, sudden stomach pain spreading to the back with vomiting, difficulty breathing or swallowing, severe allergic reaction symptoms, chest pain, or fainting. NHS guidance emphasises that when in doubt, it is safer to seek medical advice. Your prescriber should have discussed these warning signs before starting Mounjaro treatment through tier 3 specialist services or private prescription.
- 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 in the UK?
No. Pancreatitis is a rare serious side effect of Mounjaro. However, because it is potentially life-threatening, you must seek emergency care immediately if you develop severe upper stomach pain spreading to the back, especially with nausea and vomiting. The MHRA monitors all reported cases closely.
What is the MHRA Yellow Card scheme and how do I report a serious side effect?
The MHRA Yellow Card scheme allows patients and healthcare professionals to report suspected side effects of medicines in the UK. You can report online at yellowcard.mhra.gov.uk or via the Yellow Card app. Reports help the MHRA identify safety patterns and protect future patients. Your report remains confidential.
Can Mounjaro cause vision problems if I don't have diabetes?
Serious vision changes are most concerning in people with existing diabetes. However, any significant change in vision should be reported to your GP promptly, regardless of diabetes status. Mounjaro can affect blood sugar levels, which in people with diabetes may temporarily worsen eye complications. Your GP can advise you specifically.
What is the difference between common and serious side effects of Mounjaro?
Common side effects (nausea, diarrhoea, constipation, stomach pain) are mild to moderate and usually improve within a few weeks as your body adjusts. Serious side effects (pancreatitis, severe allergic reactions, gallbladder problems) are rare but require urgent medical attention. Your prescriber should discuss both before starting treatment.
How does the NHS monitor Mounjaro safety in the UK?
The MHRA monitors Mounjaro using its Black Triangle (▼) system for medicines requiring additional post-marketing surveillance. NHS specialist services also track patient outcomes and safety data. Your prescriber will ask about side effects at follow-up appointments. Report any concerns to your GP, prescriber, or via the MHRA Yellow Card scheme.
Ready to take control of your weight loss journey?
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